2Notes on the Generic Block Layer Rewrite in Linux 2.5
   5.. note::
   7        It seems that there are lot of outdated stuff here. This seems
   8        to be written somewhat as a task list. Yet, eventually, something
   9        here might still be useful.
  11Notes Written on Jan 15, 2002:
  13        - Jens Axboe <>
  14        - Suparna Bhattacharya <>
  16Last Updated May 2, 2002
  18September 2003: Updated I/O Scheduler portions
  19        - Nick Piggin <>
  24These are some notes describing some aspects of the 2.5 block layer in the
  25context of the bio rewrite. The idea is to bring out some of the key
  26changes and a glimpse of the rationale behind those changes.
  28Please mail corrections & suggestions to
  332.5 bio rewrite:
  34        - Jens Axboe <>
  36Many aspects of the generic block layer redesign were driven by and evolved
  37over discussions, prior patches and the collective experience of several
  38people. See sections 8 and 9 for a list of some related references.
  40The following people helped with review comments and inputs for this
  43        - Christoph Hellwig <>
  44        - Arjan van de Ven <>
  45        - Randy Dunlap <>
  46        - Andre Hedrick <>
  48The following people helped with fixes/contributions to the bio patches
  49while it was still work-in-progress:
  51        - David S. Miller <>
  54.. Description of Contents:
  56   1. Scope for tuning of logic to various needs
  57     1.1 Tuning based on device or low level driver capabilities
  58        - Per-queue parameters
  59        - Highmem I/O support
  60        - I/O scheduler modularization
  61     1.2 Tuning based on high level requirements/capabilities
  62        1.2.1 Request Priority/Latency
  63     1.3 Direct access/bypass to lower layers for diagnostics and special
  64         device operations
  65        1.3.1 Pre-built commands
  66   2. New flexible and generic but minimalist i/o structure or descriptor
  67      (instead of using buffer heads at the i/o layer)
  68     2.1 Requirements/Goals addressed
  69     2.2 The bio struct in detail (multi-page io unit)
  70     2.3 Changes in the request structure
  71   3. Using bios
  72     3.1 Setup/teardown (allocation, splitting)
  73     3.2 Generic bio helper routines
  74       3.2.1 Traversing segments and completion units in a request
  75       3.2.2 Setting up DMA scatterlists
  76       3.2.3 I/O completion
  77       3.2.4 Implications for drivers that do not interpret bios (don't handle
  78          multiple segments)
  79     3.3 I/O submission
  80   4. The I/O scheduler
  81   5. Scalability related changes
  82     5.1 Granular locking: Removal of io_request_lock
  83     5.2 Prepare for transition to 64 bit sector_t
  84   6. Other Changes/Implications
  85     6.1 Partition re-mapping handled by the generic block layer
  86   7. A few tips on migration of older drivers
  87   8. A list of prior/related/impacted patches/ideas
  88   9. Other References/Discussion Threads
  91Bio Notes
  94Let us discuss the changes in the context of how some overall goals for the
  95block layer are addressed.
  971. Scope for tuning the generic logic to satisfy various requirements
 100The block layer design supports adaptable abstractions to handle common
 101processing with the ability to tune the logic to an appropriate extent
 102depending on the nature of the device and the requirements of the caller.
 103One of the objectives of the rewrite was to increase the degree of tunability
 104and to enable higher level code to utilize underlying device/driver
 105capabilities to the maximum extent for better i/o performance. This is
 106important especially in the light of ever improving hardware capabilities
 107and application/middleware software designed to take advantage of these
 1101.1 Tuning based on low level device / driver capabilities
 113Sophisticated devices with large built-in caches, intelligent i/o scheduling
 114optimizations, high memory DMA support, etc may find some of the
 115generic processing an overhead, while for less capable devices the
 116generic functionality is essential for performance or correctness reasons.
 117Knowledge of some of the capabilities or parameters of the device should be
 118used at the generic block layer to take the right decisions on
 119behalf of the driver.
 121How is this achieved ?
 123Tuning at a per-queue level:
 125i. Per-queue limits/values exported to the generic layer by the driver
 127Various parameters that the generic i/o scheduler logic uses are set at
 128a per-queue level (e.g maximum request size, maximum number of segments in
 129a scatter-gather list, logical block size)
 131Some parameters that were earlier available as global arrays indexed by
 132major/minor are now directly associated with the queue. Some of these may
 133move into the block device structure in the future. Some characteristics
 134have been incorporated into a queue flags field rather than separate fields
 135in themselves.  There are blk_queue_xxx functions to set the parameters,
 136rather than update the fields directly
 138Some new queue property settings:
 140        blk_queue_bounce_limit(q, u64 dma_address)
 141                Enable I/O to highmem pages, dma_address being the
 142                limit. No highmem default.
 144        blk_queue_max_sectors(q, max_sectors)
 145                Sets two variables that limit the size of the request.
 147                - The request queue's max_sectors, which is a soft size in
 148                  units of 512 byte sectors, and could be dynamically varied
 149                  by the core kernel.
 151                - The request queue's max_hw_sectors, which is a hard limit
 152                  and reflects the maximum size request a driver can handle
 153                  in units of 512 byte sectors.
 155                The default for both max_sectors and max_hw_sectors is
 156                255. The upper limit of max_sectors is 1024.
 158        blk_queue_max_phys_segments(q, max_segments)
 159                Maximum physical segments you can handle in a request. 128
 160                default (driver limit). (See 3.2.2)
 162        blk_queue_max_hw_segments(q, max_segments)
 163                Maximum dma segments the hardware can handle in a request. 128
 164                default (host adapter limit, after dma remapping).
 165                (See 3.2.2)
 167        blk_queue_max_segment_size(q, max_seg_size)
 168                Maximum size of a clustered segment, 64kB default.
 170        blk_queue_logical_block_size(q, logical_block_size)
 171                Lowest possible sector size that the hardware can operate
 172                on, 512 bytes default.
 174New queue flags:
 176        - QUEUE_FLAG_CLUSTER (see 3.2.2)
 177        - QUEUE_FLAG_QUEUED (see 3.2.4)
 180ii. High-mem i/o capabilities are now considered the default
 182The generic bounce buffer logic, present in 2.4, where the block layer would
 183by default copyin/out i/o requests on high-memory buffers to low-memory buffers
 184assuming that the driver wouldn't be able to handle it directly, has been
 185changed in 2.5. The bounce logic is now applied only for memory ranges
 186for which the device cannot handle i/o. A driver can specify this by
 187setting the queue bounce limit for the request queue for the device
 188(blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out
 189where a device is capable of handling high memory i/o.
 191In order to enable high-memory i/o where the device is capable of supporting
 192it, the pci dma mapping routines and associated data structures have now been
 193modified to accomplish a direct page -> bus translation, without requiring
 194a virtual address mapping (unlike the earlier scheme of virtual address
 195-> bus translation). So this works uniformly for high-memory pages (which
 196do not have a corresponding kernel virtual address space mapping) and
 197low-memory pages.
 199Note: Please refer to Documentation/core-api/dma-api-howto.rst for a discussion
 200on PCI high mem DMA aspects and mapping of scatter gather lists, and support
 201for 64 bit PCI.
 203Special handling is required only for cases where i/o needs to happen on
 204pages at physical memory addresses beyond what the device can support. In these
 205cases, a bounce bio representing a buffer from the supported memory range
 206is used for performing the i/o with copyin/copyout as needed depending on
 207the type of the operation.  For example, in case of a read operation, the
 208data read has to be copied to the original buffer on i/o completion, so a
 209callback routine is set up to do this, while for write, the data is copied
 210from the original buffer to the bounce buffer prior to issuing the
 211operation. Since an original buffer may be in a high memory area that's not
 212mapped in kernel virtual addr, a kmap operation may be required for
 213performing the copy, and special care may be needed in the completion path
 214as it may not be in irq context. Special care is also required (by way of
 215GFP flags) when allocating bounce buffers, to avoid certain highmem
 216deadlock possibilities.
 218It is also possible that a bounce buffer may be allocated from high-memory
 219area that's not mapped in kernel virtual addr, but within the range that the
 220device can use directly; so the bounce page may need to be kmapped during
 221copy operations. [Note: This does not hold in the current implementation,
 224There are some situations when pages from high memory may need to
 225be kmapped, even if bounce buffers are not necessary. For example a device
 226may need to abort DMA operations and revert to PIO for the transfer, in
 227which case a virtual mapping of the page is required. For SCSI it is also
 228done in some scenarios where the low level driver cannot be trusted to
 229handle a single sg entry correctly. The driver is expected to perform the
 230kmaps as needed on such occasions as appropriate. A driver could also use
 231the blk_queue_bounce() routine on its own to bounce highmem i/o to low
 232memory for specific requests if so desired.
 234iii. The i/o scheduler algorithm itself can be replaced/set as appropriate
 236As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular
 237queue or pick from (copy) existing generic schedulers and replace/override
 238certain portions of it. The 2.5 rewrite provides improved modularization
 239of the i/o scheduler. There are more pluggable callbacks, e.g for init,
 240add request, extract request, which makes it possible to abstract specific
 241i/o scheduling algorithm aspects and details outside of the generic loop.
 242It also makes it possible to completely hide the implementation details of
 243the i/o scheduler from block drivers.
 245I/O scheduler wrappers are to be used instead of accessing the queue directly.
 246See section 4. The I/O scheduler for details.
 2481.2 Tuning Based on High level code capabilities
 251i. Application capabilities for raw i/o
 253This comes from some of the high-performance database/middleware
 254requirements where an application prefers to make its own i/o scheduling
 255decisions based on an understanding of the access patterns and i/o
 258ii. High performance filesystems or other higher level kernel code's
 261Kernel components like filesystems could also take their own i/o scheduling
 262decisions for optimizing performance. Journalling filesystems may need
 263some control over i/o ordering.
 265What kind of support exists at the generic block layer for this ?
 267The flags and rw fields in the bio structure can be used for some tuning
 268from above e.g indicating that an i/o is just a readahead request, or priority
 269settings (currently unused). As far as user applications are concerned they
 270would need an additional mechanism either via open flags or ioctls, or some
 271other upper level mechanism to communicate such settings to block.
 2731.2.1 Request Priority/Latency
 276Todo/Under discussion::
 278  Arjan's proposed request priority scheme allows higher levels some broad
 279  control (high/med/low) over the priority  of an i/o request vs other pending
 280  requests in the queue. For example it allows reads for bringing in an
 281  executable page on demand to be given a higher priority over pending write
 282  requests which haven't aged too much on the queue. Potentially this priority
 283  could even be exposed to applications in some manner, providing higher level
 284  tunability. Time based aging avoids starvation of lower priority
 285  requests. Some bits in the bi_opf flags field in the bio structure are
 286  intended to be used for this priority information.
 2891.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode)
 292(e.g Diagnostics, Systems Management)
 294There are situations where high-level code needs to have direct access to
 295the low level device capabilities or requires the ability to issue commands
 296to the device bypassing some of the intermediate i/o layers.
 297These could, for example, be special control commands issued through ioctl
 298interfaces, or could be raw read/write commands that stress the drive's
 299capabilities for certain kinds of fitness tests. Having direct interfaces at
 300multiple levels without having to pass through upper layers makes
 301it possible to perform bottom up validation of the i/o path, layer by
 302layer, starting from the media.
 304The normal i/o submission interfaces, e.g submit_bio, could be bypassed
 305for specially crafted requests which such ioctl or diagnostics
 306interfaces would typically use, and the elevator add_request routine
 307can instead be used to directly insert such requests in the queue or preferably
 308the blk_do_rq routine can be used to place the request on the queue and
 309wait for completion. Alternatively, sometimes the caller might just
 310invoke a lower level driver specific interface with the request as a
 313If the request is a means for passing on special information associated with
 314the command, then such information is associated with the request->special
 315field (rather than misuse the request->buffer field which is meant for the
 316request data buffer's virtual mapping).
 318For passing request data, the caller must build up a bio descriptor
 319representing the concerned memory buffer if the underlying driver interprets
 320bio segments or uses the block layer end*request* functions for i/o
 321completion. Alternatively one could directly use the request->buffer field to
 322specify the virtual address of the buffer, if the driver expects buffer
 323addresses passed in this way and ignores bio entries for the request type
 324involved. In the latter case, the driver would modify and manage the
 325request->buffer, request->sector and request->nr_sectors or
 326request->current_nr_sectors fields itself rather than using the block layer
 327end_request or end_that_request_first completion interfaces.
 328(See 2.3 or Documentation/block/request.rst for a brief explanation of
 329the request structure fields)
 333  [TBD: end_that_request_last should be usable even in this case;
 334  Perhaps an end_that_direct_request_first routine could be implemented to make
 335  handling direct requests easier for such drivers; Also for drivers that
 336  expect bios, a helper function could be provided for setting up a bio
 337  corresponding to a data buffer]
 339  <JENS: I dont understand the above, why is end_that_request_first() not
 340  usable? Or _last for that matter. I must be missing something>
 342  <SUP: What I meant here was that if the request doesn't have a bio, then
 343   end_that_request_first doesn't modify nr_sectors or current_nr_sectors,
 344   and hence can't be used for advancing request state settings on the
 345   completion of partial transfers. The driver has to modify these fields
 346   directly by hand.
 347   This is because end_that_request_first only iterates over the bio list,
 348   and always returns 0 if there are none associated with the request.
 349   _last works OK in this case, and is not a problem, as I mentioned earlier
 350  >
 3521.3.1 Pre-built Commands
 355A request can be created with a pre-built custom command  to be sent directly
 356to the device. The cmd block in the request structure has room for filling
 357in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for
 358command pre-building, and the type of the request is now indicated
 359through rq->flags instead of via rq->cmd)
 361The request structure flags can be set up to indicate the type of request
 362in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC:
 363packet command issued via blk_do_rq, REQ_SPECIAL: special request).
 365It can help to pre-build device commands for requests in advance.
 366Drivers can now specify a request prepare function (q->prep_rq_fn) that the
 367block layer would invoke to pre-build device commands for a given request,
 368or perform other preparatory processing for the request. This is routine is
 369called by elv_next_request(), i.e. typically just before servicing a request.
 370(The prepare function would not be called for requests that have RQF_DONTPREP
 374  Pre-building could possibly even be done early, i.e before placing the
 375  request on the queue, rather than construct the command on the fly in the
 376  driver while servicing the request queue when it may affect latencies in
 377  interrupt context or responsiveness in general. One way to add early
 378  pre-building would be to do it whenever we fail to merge on a request.
 379  Now REQ_NOMERGE is set in the request flags to skip this one in the future,
 380  which means that it will not change before we feed it to the device. So
 381  the pre-builder hook can be invoked there.
 3842. Flexible and generic but minimalist i/o structure/descriptor
 3872.1 Reason for a new structure and requirements addressed
 390Prior to 2.5, buffer heads were used as the unit of i/o at the generic block
 391layer, and the low level request structure was associated with a chain of
 392buffer heads for a contiguous i/o request. This led to certain inefficiencies
 393when it came to large i/o requests and readv/writev style operations, as it
 394forced such requests to be broken up into small chunks before being passed
 395on to the generic block layer, only to be merged by the i/o scheduler
 396when the underlying device was capable of handling the i/o in one shot.
 397Also, using the buffer head as an i/o structure for i/os that didn't originate
 398from the buffer cache unnecessarily added to the weight of the descriptors
 399which were generated for each such chunk.
 401The following were some of the goals and expectations considered in the
 402redesign of the block i/o data structure in 2.5.
 4041.  Should be appropriate as a descriptor for both raw and buffered i/o  -
 405    avoid cache related fields which are irrelevant in the direct/page i/o path,
 406    or filesystem block size alignment restrictions which may not be relevant
 407    for raw i/o.
 4082.  Ability to represent high-memory buffers (which do not have a virtual
 409    address mapping in kernel address space).
 4103.  Ability to represent large i/os w/o unnecessarily breaking them up (i.e
 411    greater than PAGE_SIZE chunks in one shot)
 4124.  At the same time, ability to retain independent identity of i/os from
 413    different sources or i/o units requiring individual completion (e.g. for
 414    latency reasons)
 4155.  Ability to represent an i/o involving multiple physical memory segments
 416    (including non-page aligned page fragments, as specified via readv/writev)
 417    without unnecessarily breaking it up, if the underlying device is capable of
 418    handling it.
 4196.  Preferably should be based on a memory descriptor structure that can be
 420    passed around different types of subsystems or layers, maybe even
 421    networking, without duplication or extra copies of data/descriptor fields
 422    themselves in the process
 4237.  Ability to handle the possibility of splits/merges as the structure passes
 424    through layered drivers (lvm, md, evms), with minimal overhead.
 426The solution was to define a new structure (bio)  for the block layer,
 427instead of using the buffer head structure (bh) directly, the idea being
 428avoidance of some associated baggage and limitations. The bio structure
 429is uniformly used for all i/o at the block layer ; it forms a part of the
 430bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are
 431mapped to bio structures.
 4332.2 The bio struct
 436The bio structure uses a vector representation pointing to an array of tuples
 437of <page, offset, len> to describe the i/o buffer, and has various other
 438fields describing i/o parameters and state that needs to be maintained for
 439performing the i/o.
 441Notice that this representation means that a bio has no virtual address
 442mapping at all (unlike buffer heads).
 446  struct bio_vec {
 447       struct page     *bv_page;
 448       unsigned short  bv_len;
 449       unsigned short  bv_offset;
 450  };
 452  /*
 453   * main unit of I/O for the block layer and lower layers (ie drivers)
 454   */
 455  struct bio {
 456       struct bio          *bi_next;    /* request queue link */
 457       struct block_device *bi_bdev;    /* target device */
 458       unsigned long       bi_flags;    /* status, command, etc */
 459       unsigned long       bi_opf;       /* low bits: r/w, high: priority */
 461       unsigned int     bi_vcnt;     /* how may bio_vec's */
 462       struct bvec_iter bi_iter;        /* current index into bio_vec array */
 464       unsigned int     bi_size;     /* total size in bytes */
 465       unsigned short   bi_hw_segments; /* segments after DMA remapping */
 466       unsigned int     bi_max;      /* max bio_vecs we can hold
 467                                        used as index into pool */
 468       struct bio_vec   *bi_io_vec;  /* the actual vec list */
 469       bio_end_io_t     *bi_end_io;  /* bi_end_io (bio) */
 470       atomic_t         bi_cnt;      /* pin count: free when it hits zero */
 471       void             *bi_private;
 472  };
 474With this multipage bio design:
 476- Large i/os can be sent down in one go using a bio_vec list consisting
 477  of an array of <page, offset, len> fragments (similar to the way fragments
 478  are represented in the zero-copy network code)
 479- Splitting of an i/o request across multiple devices (as in the case of
 480  lvm or raid) is achieved by cloning the bio (where the clone points to
 481  the same bi_io_vec array, but with the index and size accordingly modified)
 482- A linked list of bios is used as before for unrelated merges [#]_ - this
 483  avoids reallocs and makes independent completions easier to handle.
 484- Code that traverses the req list can find all the segments of a bio
 485  by using rq_for_each_segment.  This handles the fact that a request
 486  has multiple bios, each of which can have multiple segments.
 487- Drivers which can't process a large bio in one shot can use the bi_iter
 488  field to keep track of the next bio_vec entry to process.
 489  (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE)
 490  [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying
 491  bi_offset an len fields]
 493.. [#]
 495        unrelated merges -- a request ends up containing two or more bios that
 496        didn't originate from the same place.
 498bi_end_io() i/o callback gets called on i/o completion of the entire bio.
 500At a lower level, drivers build a scatter gather list from the merged bios.
 501The scatter gather list is in the form of an array of <page, offset, len>
 502entries with their corresponding dma address mappings filled in at the
 503appropriate time. As an optimization, contiguous physical pages can be
 504covered by a single entry where <page> refers to the first page and <len>
 505covers the range of pages (up to 16 contiguous pages could be covered this
 506way). There is a helper routine (blk_rq_map_sg) which drivers can use to build
 507the sg list.
 509Note: Right now the only user of bios with more than one page is ll_rw_kio,
 510which in turn means that only raw I/O uses it (direct i/o may not work
 511right now). The intent however is to enable clustering of pages etc to
 512become possible. The pagebuf abstraction layer from SGI also uses multi-page
 513bios, but that is currently not included in the stock development kernels.
 514The same is true of Andrew Morton's work-in-progress multipage bio writeout
 515and readahead patches.
 5172.3 Changes in the Request Structure
 520The request structure is the structure that gets passed down to low level
 521drivers. The block layer make_request function builds up a request structure,
 522places it on the queue and invokes the drivers request_fn. The driver makes
 523use of block layer helper routine elv_next_request to pull the next request
 524off the queue. Control or diagnostic functions might bypass block and directly
 525invoke underlying driver entry points passing in a specially constructed
 526request structure.
 528Only some relevant fields (mainly those which changed or may be referred
 529to in some of the discussion here) are listed below, not necessarily in
 530the order in which they occur in the structure (see include/linux/blkdev.h)
 531Refer to Documentation/block/request.rst for details about all the request
 532structure fields and a quick reference about the layers which are
 533supposed to use or modify those fields::
 535  struct request {
 536        struct list_head queuelist;  /* Not meant to be directly accessed by
 537                                        the driver.
 538                                        Used by q->elv_next_request_fn
 539                                        rq->queue is gone
 540                                        */
 541        .
 542        .
 543        unsigned char cmd[16]; /* prebuilt command data block */
 544        unsigned long flags;   /* also includes earlier rq->cmd settings */
 545        .
 546        .
 547        sector_t sector; /* this field is now of type sector_t instead of int
 548                            preparation for 64 bit sectors */
 549        .
 550        .
 552        /* Number of scatter-gather DMA addr+len pairs after
 553         * physical address coalescing is performed.
 554         */
 555        unsigned short nr_phys_segments;
 557        /* Number of scatter-gather addr+len pairs after
 558         * physical and DMA remapping hardware coalescing is performed.
 559         * This is the number of scatter-gather entries the driver
 560         * will actually have to deal with after DMA mapping is done.
 561         */
 562        unsigned short nr_hw_segments;
 564        /* Various sector counts */
 565        unsigned long nr_sectors;  /* no. of sectors left: driver modifiable */
 566        unsigned long hard_nr_sectors;  /* block internal copy of above */
 567        unsigned int current_nr_sectors; /* no. of sectors left in the
 568                                           current segment:driver modifiable */
 569        unsigned long hard_cur_sectors; /* block internal copy of the above */
 570        .
 571        .
 572        int tag;        /* command tag associated with request */
 573        void *special;  /* same as before */
 574        char *buffer;   /* valid only for low memory buffers up to
 575                         current_nr_sectors */
 576        .
 577        .
 578        struct bio *bio, *biotail;  /* bio list instead of bh */
 579        struct request_list *rl;
 580  }
 582See the req_ops and req_flag_bits definitions for an explanation of the various
 583flags available. Some bits are used by the block layer or i/o scheduler.
 585The behaviour of the various sector counts are almost the same as before,
 586except that since we have multi-segment bios, current_nr_sectors refers
 587to the numbers of sectors in the current segment being processed which could
 588be one of the many segments in the current bio (i.e i/o completion unit).
 589The nr_sectors value refers to the total number of sectors in the whole
 590request that remain to be transferred (no change). The purpose of the
 591hard_xxx values is for block to remember these counts every time it hands
 592over the request to the driver. These values are updated by block on
 593end_that_request_first, i.e. every time the driver completes a part of the
 594transfer and invokes block end*request helpers to mark this. The
 595driver should not modify these values. The block layer sets up the
 596nr_sectors and current_nr_sectors fields (based on the corresponding
 597hard_xxx values and the number of bytes transferred) and updates it on
 598every transfer that invokes end_that_request_first. It does the same for the
 599buffer, bio, bio->bi_iter fields too.
 601The buffer field is just a virtual address mapping of the current segment
 602of the i/o buffer in cases where the buffer resides in low-memory. For high
 603memory i/o, this field is not valid and must not be used by drivers.
 605Code that sets up its own request structures and passes them down to
 606a driver needs to be careful about interoperation with the block layer helper
 607functions which the driver uses. (Section 1.3)
 6093. Using bios
 6123.1 Setup/Teardown
 615There are routines for managing the allocation, and reference counting, and
 616freeing of bios (bio_alloc, bio_get, bio_put).
 618This makes use of Ingo Molnar's mempool implementation, which enables
 619subsystems like bio to maintain their own reserve memory pools for guaranteed
 620deadlock-free allocations during extreme VM load. For example, the VM
 621subsystem makes use of the block layer to writeout dirty pages in order to be
 622able to free up memory space, a case which needs careful handling. The
 623allocation logic draws from the preallocated emergency reserve in situations
 624where it cannot allocate through normal means. If the pool is empty and it
 625can wait, then it would trigger action that would help free up memory or
 626replenish the pool (without deadlocking) and wait for availability in the pool.
 627If it is in IRQ context, and hence not in a position to do this, allocation
 628could fail if the pool is empty. In general mempool always first tries to
 629perform allocation without having to wait, even if it means digging into the
 630pool as long it is not less that 50% full.
 632On a free, memory is released to the pool or directly freed depending on
 633the current availability in the pool. The mempool interface lets the
 634subsystem specify the routines to be used for normal alloc and free. In the
 635case of bio, these routines make use of the standard slab allocator.
 637The caller of bio_alloc is expected to taken certain steps to avoid
 638deadlocks, e.g. avoid trying to allocate more memory from the pool while
 639already holding memory obtained from the pool.
 643  [TBD: This is a potential issue, though a rare possibility
 644   in the bounce bio allocation that happens in the current code, since
 645   it ends up allocating a second bio from the same pool while
 646   holding the original bio ]
 648Memory allocated from the pool should be released back within a limited
 649amount of time (in the case of bio, that would be after the i/o is completed).
 650This ensures that if part of the pool has been used up, some work (in this
 651case i/o) must already be in progress and memory would be available when it
 652is over. If allocating from multiple pools in the same code path, the order
 653or hierarchy of allocation needs to be consistent, just the way one deals
 654with multiple locks.
 656The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc())
 657for a non-clone bio. There are the 6 pools setup for different size biovecs,
 658so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the
 659given size from these slabs.
 661The bio_get() routine may be used to hold an extra reference on a bio prior
 662to i/o submission, if the bio fields are likely to be accessed after the
 663i/o is issued (since the bio may otherwise get freed in case i/o completion
 664happens in the meantime).
 666The bio_clone_fast() routine may be used to duplicate a bio, where the clone
 667shares the bio_vec_list with the original bio (i.e. both point to the
 668same bio_vec_list). This would typically be used for splitting i/o requests
 669in lvm or md.
 6713.2 Generic bio helper Routines
 6743.2.1 Traversing segments and completion units in a request
 677The macro rq_for_each_segment() should be used for traversing the bios
 678in the request list (drivers should avoid directly trying to do it
 679themselves). Using these helpers should also make it easier to cope
 680with block changes in the future.
 684        struct req_iterator iter;
 685        rq_for_each_segment(bio_vec, rq, iter)
 686                /* bio_vec is now current segment */
 688I/O completion callbacks are per-bio rather than per-segment, so drivers
 689that traverse bio chains on completion need to keep that in mind. Drivers
 690which don't make a distinction between segments and completion units would
 691need to be reorganized to support multi-segment bios.
 6933.2.2 Setting up DMA scatterlists
 696The blk_rq_map_sg() helper routine would be used for setting up scatter
 697gather lists from a request, so a driver need not do it on its own.
 699        nr_segments = blk_rq_map_sg(q, rq, scatterlist);
 701The helper routine provides a level of abstraction which makes it easier
 702to modify the internals of request to scatterlist conversion down the line
 703without breaking drivers. The blk_rq_map_sg routine takes care of several
 704things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER
 705is set) and correct segment accounting to avoid exceeding the limits which
 706the i/o hardware can handle, based on various queue properties.
 708- Prevents a clustered segment from crossing a 4GB mem boundary
 709- Avoids building segments that would exceed the number of physical
 710  memory segments that the driver can handle (phys_segments) and the
 711  number that the underlying hardware can handle at once, accounting for
 712  DMA remapping (hw_segments)  (i.e. IOMMU aware limits).
 714Routines which the low level driver can use to set up the segment limits:
 716blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of
 717hw data segments in a request (i.e. the maximum number of address/length
 718pairs the host adapter can actually hand to the device at once)
 720blk_queue_max_phys_segments() : Sets an upper limit on the maximum number
 721of physical data segments in a request (i.e. the largest sized scatter list
 722a driver could handle)
 7243.2.3 I/O completion
 727The existing generic block layer helper routines end_request,
 728end_that_request_first and end_that_request_last can be used for i/o
 729completion (and setting things up so the rest of the i/o or the next
 730request can be kicked of) as before. With the introduction of multi-page
 731bio support, end_that_request_first requires an additional argument indicating
 732the number of sectors completed.
 7343.2.4 Implications for drivers that do not interpret bios
 737(don't handle multiple segments)
 739Drivers that do not interpret bios e.g those which do not handle multiple
 740segments and do not support i/o into high memory addresses (require bounce
 741buffers) and expect only virtually mapped buffers, can access the rq->buffer
 742field. As before the driver should use current_nr_sectors to determine the
 743size of remaining data in the current segment (that is the maximum it can
 744transfer in one go unless it interprets segments), and rely on the block layer
 745end_request, or end_that_request_first/last to take care of all accounting
 746and transparent mapping of the next bio segment when a segment boundary
 747is crossed on completion of a transfer. (The end*request* functions should
 748be used if only if the request has come down from block/bio path, not for
 749direct access requests which only specify rq->buffer without a valid rq->bio)
 7513.3 I/O Submission
 754The routine submit_bio() is used to submit a single io. Higher level i/o
 755routines make use of this:
 757(a) Buffered i/o:
 759The routine submit_bh() invokes submit_bio() on a bio corresponding to the
 760bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before.
 762(b) Kiobuf i/o (for raw/direct i/o):
 764The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and
 765maps the array to one or more multi-page bios, issuing submit_bio() to
 766perform the i/o on each of these.
 768The embedded bh array in the kiobuf structure has been removed and no
 769preallocation of bios is done for kiobufs. [The intent is to remove the
 770blocks array as well, but it's currently in there to kludge around direct i/o.]
 771Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc.
 775 A single kiobuf structure is assumed to correspond to a contiguous range
 776 of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec.
 777 So right now it wouldn't work for direct i/o on non-contiguous blocks.
 778 This is to be resolved.  The eventual direction is to replace kiobuf
 779 by kvec's.
 781 Badari Pulavarty has a patch to implement direct i/o correctly using
 782 bio and kvec.
 785(c) Page i/o:
 787Todo/Under discussion:
 789 Andrew Morton's multi-page bio patches attempt to issue multi-page
 790 writeouts (and reads) from the page cache, by directly building up
 791 large bios for submission completely bypassing the usage of buffer
 792 heads. This work is still in progress.
 794 Christoph Hellwig had some code that uses bios for page-io (rather than
 795 bh). This isn't included in bio as yet. Christoph was also working on a
 796 design for representing virtual/real extents as an entity and modifying
 797 some of the address space ops interfaces to utilize this abstraction rather
 798 than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf
 799 abstraction, but intended to be as lightweight as possible).
 801(d) Direct access i/o:
 803Direct access requests that do not contain bios would be submitted differently
 804as discussed earlier in section 1.3.
 808  Kvec i/o:
 810  Ben LaHaise's aio code uses a slightly different structure instead
 811  of kiobufs, called a kvec_cb. This contains an array of <page, offset, len>
 812  tuples (very much like the networking code), together with a callback function
 813  and data pointer. This is embedded into a brw_cb structure when passed
 814  to brw_kvec_async().
 816  Now it should be possible to directly map these kvecs to a bio. Just as while
 817  cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec
 818  array pointer to point to the veclet array in kvecs.
 820  TBD: In order for this to work, some changes are needed in the way multi-page
 821  bios are handled today. The values of the tuples in such a vector passed in
 822  from higher level code should not be modified by the block layer in the course
 823  of its request processing, since that would make it hard for the higher layer
 824  to continue to use the vector descriptor (kvec) after i/o completes. Instead,
 825  all such transient state should either be maintained in the request structure,
 826  and passed on in some way to the endio completion routine.
 8294. The I/O scheduler
 832I/O scheduler, a.k.a. elevator, is implemented in two layers.  Generic dispatch
 833queue and specific I/O schedulers.  Unless stated otherwise, elevator is used
 834to refer to both parts and I/O scheduler to specific I/O schedulers.
 836Block layer implements generic dispatch queue in `block/*.c`.
 837The generic dispatch queue is responsible for requeueing, handling non-fs
 838requests and all other subtleties.
 840Specific I/O schedulers are responsible for ordering normal filesystem
 841requests.  They can also choose to delay certain requests to improve
 842throughput or whatever purpose.  As the plural form indicates, there are
 843multiple I/O schedulers.  They can be built as modules but at least one should
 844be built inside the kernel.  Each queue can choose different one and can also
 845change to another one dynamically.
 847A block layer call to the i/o scheduler follows the convention elv_xxx(). This
 848calls elevator_xxx_fn in the elevator switch (block/elevator.c). Oh, xxx
 849and xxx might not match exactly, but use your imagination. If an elevator
 850doesn't implement a function, the switch does nothing or some minimal house
 851keeping work.
 8534.1. I/O scheduler API
 856The functions an elevator may implement are: (* are mandatory)
 858=============================== ================================================
 859elevator_merge_fn               called to query requests for merge with a bio
 861elevator_merge_req_fn           called when two requests get merged. the one
 862                                which gets merged into the other one will be
 863                                never seen by I/O scheduler again. IOW, after
 864                                being merged, the request is gone.
 866elevator_merged_fn              called when a request in the scheduler has been
 867                                involved in a merge. It is used in the deadline
 868                                scheduler for example, to reposition the request
 869                                if its sorting order has changed.
 871elevator_allow_merge_fn         called whenever the block layer determines
 872                                that a bio can be merged into an existing
 873                                request safely. The io scheduler may still
 874                                want to stop a merge at this point if it
 875                                results in some sort of conflict internally,
 876                                this hook allows it to do that. Note however
 877                                that two *requests* can still be merged at later
 878                                time. Currently the io scheduler has no way to
 879                                prevent that. It can only learn about the fact
 880                                from elevator_merge_req_fn callback.
 882elevator_dispatch_fn*           fills the dispatch queue with ready requests.
 883                                I/O schedulers are free to postpone requests by
 884                                not filling the dispatch queue unless @force
 885                                is non-zero.  Once dispatched, I/O schedulers
 886                                are not allowed to manipulate the requests -
 887                                they belong to generic dispatch queue.
 889elevator_add_req_fn*            called to add a new request into the scheduler
 892elevator_latter_req_fn          These return the request before or after the
 893                                one specified in disk sort order. Used by the
 894                                block layer to find merge possibilities.
 896elevator_completed_req_fn       called when a request is completed.
 899elevator_put_req_fn             Must be used to allocate and free any elevator
 900                                specific storage for a request.
 902elevator_activate_req_fn        Called when device driver first sees a request.
 903                                I/O schedulers can use this callback to
 904                                determine when actual execution of a request
 905                                starts.
 906elevator_deactivate_req_fn      Called when device driver decides to delay
 907                                a request by requeueing it.
 910elevator_exit_fn                Allocate and free any elevator specific storage
 911                                for a queue.
 912=============================== ================================================
 9144.2 Request flows seen by I/O schedulers
 917All requests seen by I/O schedulers strictly follow one of the following three
 920 set_req_fn ->
 922 i.   add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn ->
 923      (deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn
 924 ii.  add_req_fn -> (merged_fn ->)* -> merge_req_fn
 925 iii. [none]
 927 -> put_req_fn
 9294.3 I/O scheduler implementation
 932The generic i/o scheduler algorithm attempts to sort/merge/batch requests for
 933optimal disk scan and request servicing performance (based on generic
 934principles and device capabilities), optimized for:
 936i.   improved throughput
 937ii.  improved latency
 938iii. better utilization of h/w & CPU time
 942i. Binary tree
 943AS and deadline i/o schedulers use red black binary trees for disk position
 944sorting and searching, and a fifo linked list for time-based searching. This
 945gives good scalability and good availability of information. Requests are
 946almost always dispatched in disk sort order, so a cache is kept of the next
 947request in sort order to prevent binary tree lookups.
 949This arrangement is not a generic block layer characteristic however, so
 950elevators may implement queues as they please.
 952ii. Merge hash
 953AS and deadline use a hash table indexed by the last sector of a request. This
 954enables merging code to quickly look up "back merge" candidates, even when
 955multiple I/O streams are being performed at once on one disk.
 957"Front merges", a new request being merged at the front of an existing request,
 958are far less common than "back merges" due to the nature of most I/O patterns.
 959Front merges are handled by the binary trees in AS and deadline schedulers.
 961iii. Plugging the queue to batch requests in anticipation of opportunities for
 962     merge/sort optimizations
 964Plugging is an approach that the current i/o scheduling algorithm resorts to so
 965that it collects up enough requests in the queue to be able to take
 966advantage of the sorting/merging logic in the elevator. If the
 967queue is empty when a request comes in, then it plugs the request queue
 968(sort of like plugging the bath tub of a vessel to get fluid to build up)
 969till it fills up with a few more requests, before starting to service
 970the requests. This provides an opportunity to merge/sort the requests before
 971passing them down to the device. There are various conditions when the queue is
 972unplugged (to open up the flow again), either through a scheduled task or
 973could be on demand. For example wait_on_buffer sets the unplugging going
 974through sync_buffer() running blk_run_address_space(mapping). Or the caller
 975can do it explicity through blk_unplug(bdev). So in the read case,
 976the queue gets explicitly unplugged as part of waiting for completion on that
 980  This is kind of controversial territory, as it's not clear if plugging is
 981  always the right thing to do. Devices typically have their own queues,
 982  and allowing a big queue to build up in software, while letting the device be
 983  idle for a while may not always make sense. The trick is to handle the fine
 984  balance between when to plug and when to open up. Also now that we have
 985  multi-page bios being queued in one shot, we may not need to wait to merge
 986  a big request from the broken up pieces coming by.
 9884.4 I/O contexts
 991I/O contexts provide a dynamically allocated per process data area. They may
 992be used in I/O schedulers, and in the block layer (could be used for IO statis,
 993priorities for example). See `*io_context` in block/ll_rw_blk.c, and as-iosched.c
 994for an example of usage in an i/o scheduler.
 9975. Scalability related changes
10005.1 Granular Locking: io_request_lock replaced by a per-queue lock
1003The global io_request_lock has been removed as of 2.5, to avoid
1004the scalability bottleneck it was causing, and has been replaced by more
1005granular locking. The request queue structure has a pointer to the
1006lock to be used for that queue. As a result, locking can now be
1007per-queue, with a provision for sharing a lock across queues if
1008necessary (e.g the scsi layer sets the queue lock pointers to the
1009corresponding adapter lock, which results in a per host locking
1010granularity). The locking semantics are the same, i.e. locking is
1011still imposed by the block layer, grabbing the lock before
1012request_fn execution which it means that lots of older drivers
1013should still be SMP safe. Drivers are free to drop the queue
1014lock themselves, if required. Drivers that explicitly used the
1015io_request_lock for serialization need to be modified accordingly.
1016Usually it's as easy as adding a global lock::
1018        static DEFINE_SPINLOCK(my_driver_lock);
1020and passing the address to that lock to blk_init_queue().
10225.2 64 bit sector numbers (sector_t prepares for 64 bit support)
1025The sector number used in the bio structure has been changed to sector_t,
1026which could be defined as 64 bit in preparation for 64 bit sector support.
10286. Other Changes/Implications
10316.1 Partition re-mapping handled by the generic block layer
1034In 2.5 some of the gendisk/partition related code has been reorganized.
1035Now the generic block layer performs partition-remapping early and thus
1036provides drivers with a sector number relative to whole device, rather than
1037having to take partition number into account in order to arrive at the true
1038sector number. The routine blk_partition_remap() is invoked by
1039submit_bio_noacct even before invoking the queue specific ->submit_bio,
1040so the i/o scheduler also gets to operate on whole disk sector numbers. This
1041should typically not require changes to block drivers, it just never gets
1042to invoke its own partition sector offset calculations since all bios
1043sent are offset from the beginning of the device.
10467. A Few Tips on Migration of older drivers
1049Old-style drivers that just use CURRENT and ignores clustered requests,
1050may not need much change.  The generic layer will automatically handle
1051clustered requests, multi-page bios, etc for the driver.
1053For a low performance driver or hardware that is PIO driven or just doesn't
1054support scatter-gather changes should be minimal too.
1056The following are some points to keep in mind when converting old drivers
1057to bio.
1059Drivers should use elv_next_request to pick up requests and are no longer
1060supposed to handle looping directly over the request list.
1061(struct request->queue has been removed)
1063Now end_that_request_first takes an additional number_of_sectors argument.
1064It used to handle always just the first buffer_head in a request, now
1065it will loop and handle as many sectors (on a bio-segment granularity)
1066as specified.
1068Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the
1069right thing to use is bio_endio(bio) instead.
1071If the driver is dropping the io_request_lock from its request_fn strategy,
1072then it just needs to replace that with q->queue_lock instead.
1074As described in Sec 1.1, drivers can set max sector size, max segment size
1075etc per queue now. Drivers that used to define their own merge functions i
1076to handle things like this can now just use the blk_queue_* functions at
1077blk_init_queue time.
1079Drivers no longer have to map a {partition, sector offset} into the
1080correct absolute location anymore, this is done by the block layer, so
1081where a driver received a request ala this before::
1083        rq->rq_dev = mk_kdev(3, 5);     /* /dev/hda5 */
1084        rq->sector = 0;                 /* first sector on hda5 */
1086it will now see::
1088        rq->rq_dev = mk_kdev(3, 0);     /* /dev/hda */
1089        rq->sector = 123128;            /* offset from start of disk */
1091As mentioned, there is no virtual mapping of a bio. For DMA, this is
1092not a problem as the driver probably never will need a virtual mapping.
1093Instead it needs a bus mapping (dma_map_page for a single segment or
1094use dma_map_sg for scatter gather) to be able to ship it to the driver. For
1095PIO drivers (or drivers that need to revert to PIO transfer once in a
1096while (IDE for example)), where the CPU is doing the actual data
1097transfer a virtual mapping is needed. If the driver supports highmem I/O,
1098(Sec 1.1, (ii) ) it needs to use kmap_atomic or similar to temporarily map
1099a bio into the virtual address space.
11028. Prior/Related/Impacted patches
11058.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp)
1108- orig kiobuf & raw i/o patches (now in 2.4 tree)
1109- direct kiobuf based i/o to devices (no intermediate bh's)
1110- page i/o using kiobuf
1111- kiobuf splitting for lvm (mkp)
1112- elevator support for kiobuf request merging (axboe)
11148.2. Zero-copy networking (Dave Miller)
11178.3. SGI XFS - pagebuf patches - use of kiobufs
11198.4. Multi-page pioent patch for bio (Christoph Hellwig)
11218.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11
11238.6. Async i/o implementation patch (Ben LaHaise)
11258.7. EVMS layering design (IBM EVMS team)
11278.8. Larger page cache size patch (Ben LaHaise) and Large page size (Daniel Phillips)
1130    => larger contiguous physical memory buffers
11328.9. VM reservations patch (Ben LaHaise)
11348.10. Write clustering patches ? (Marcelo/Quintela/Riel ?)
11368.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+
11388.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar, Badari)
11408.13  Priority based i/o scheduler - prepatches (Arjan van de Ven)
11428.14  IDE Taskfile i/o patch (Andre Hedrick)
11448.15  Multi-page writeout and readahead patches (Andrew Morton)
11468.16  Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy)
11499. Other References
11529.1 The Splice I/O Model
1155Larry McVoy (and subsequent discussions on lkml, and Linus' comments - Jan 2001
11579.2 Discussions about kiobuf and bh design
1160On lkml between sct, linus, alan et al - Feb-March 2001 (many of the
1161initial thoughts that led to bio were brought up in this discussion thread)
11639.3 Discussions on mempool on lkml - Dec 2001.