linux/Documentation/nommu-mmap.txt
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   1                         =============================
   2                         NO-MMU MEMORY MAPPING SUPPORT
   3                         =============================
   4
   5The kernel has limited support for memory mapping under no-MMU conditions, such
   6as are used in uClinux environments. From the userspace point of view, memory
   7mapping is made use of in conjunction with the mmap() system call, the shmat()
   8call and the execve() system call. From the kernel's point of view, execve()
   9mapping is actually performed by the binfmt drivers, which call back into the
  10mmap() routines to do the actual work.
  11
  12Memory mapping behaviour also involves the way fork(), vfork(), clone() and
  13ptrace() work. Under uClinux there is no fork(), and clone() must be supplied
  14the CLONE_VM flag.
  15
  16The behaviour is similar between the MMU and no-MMU cases, but not identical;
  17and it's also much more restricted in the latter case:
  18
  19 (*) Anonymous mapping, MAP_PRIVATE
  20
  21        In the MMU case: VM regions backed by arbitrary pages; copy-on-write
  22        across fork.
  23
  24        In the no-MMU case: VM regions backed by arbitrary contiguous runs of
  25        pages.
  26
  27 (*) Anonymous mapping, MAP_SHARED
  28
  29        These behave very much like private mappings, except that they're
  30        shared across fork() or clone() without CLONE_VM in the MMU case. Since
  31        the no-MMU case doesn't support these, behaviour is identical to
  32        MAP_PRIVATE there.
  33
  34 (*) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, !PROT_WRITE
  35
  36        In the MMU case: VM regions backed by pages read from file; changes to
  37        the underlying file are reflected in the mapping; copied across fork.
  38
  39        In the no-MMU case:
  40
  41         - If one exists, the kernel will re-use an existing mapping to the
  42           same segment of the same file if that has compatible permissions,
  43           even if this was created by another process.
  44
  45         - If possible, the file mapping will be directly on the backing device
  46           if the backing device has the BDI_CAP_MAP_DIRECT capability and
  47           appropriate mapping protection capabilities. Ramfs, romfs, cramfs
  48           and mtd might all permit this.
  49
  50         - If the backing device device can't or won't permit direct sharing,
  51           but does have the BDI_CAP_MAP_COPY capability, then a copy of the
  52           appropriate bit of the file will be read into a contiguous bit of
  53           memory and any extraneous space beyond the EOF will be cleared
  54
  55         - Writes to the file do not affect the mapping; writes to the mapping
  56           are visible in other processes (no MMU protection), but should not
  57           happen.
  58
  59 (*) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, PROT_WRITE
  60
  61        In the MMU case: like the non-PROT_WRITE case, except that the pages in
  62        question get copied before the write actually happens. From that point
  63        on writes to the file underneath that page no longer get reflected into
  64        the mapping's backing pages. The page is then backed by swap instead.
  65
  66        In the no-MMU case: works much like the non-PROT_WRITE case, except
  67        that a copy is always taken and never shared.
  68
  69 (*) Regular file / blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
  70
  71        In the MMU case: VM regions backed by pages read from file; changes to
  72        pages written back to file; writes to file reflected into pages backing
  73        mapping; shared across fork.
  74
  75        In the no-MMU case: not supported.
  76
  77 (*) Memory backed regular file, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
  78
  79        In the MMU case: As for ordinary regular files.
  80
  81        In the no-MMU case: The filesystem providing the memory-backed file
  82        (such as ramfs or tmpfs) may choose to honour an open, truncate, mmap
  83        sequence by providing a contiguous sequence of pages to map. In that
  84        case, a shared-writable memory mapping will be possible. It will work
  85        as for the MMU case. If the filesystem does not provide any such
  86        support, then the mapping request will be denied.
  87
  88 (*) Memory backed blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
  89
  90        In the MMU case: As for ordinary regular files.
  91
  92        In the no-MMU case: As for memory backed regular files, but the
  93        blockdev must be able to provide a contiguous run of pages without
  94        truncate being called. The ramdisk driver could do this if it allocated
  95        all its memory as a contiguous array upfront.
  96
  97 (*) Memory backed chardev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
  98
  99        In the MMU case: As for ordinary regular files.
 100
 101        In the no-MMU case: The character device driver may choose to honour
 102        the mmap() by providing direct access to the underlying device if it
 103        provides memory or quasi-memory that can be accessed directly. Examples
 104        of such are frame buffers and flash devices. If the driver does not
 105        provide any such support, then the mapping request will be denied.
 106
 107
 108============================
 109FURTHER NOTES ON NO-MMU MMAP
 110============================
 111
 112 (*) A request for a private mapping of a file may return a buffer that is not
 113     page-aligned.  This is because XIP may take place, and the data may not be
 114     paged aligned in the backing store.
 115
 116 (*) A request for an anonymous mapping will always be page aligned.  If
 117     possible the size of the request should be a power of two otherwise some
 118     of the space may be wasted as the kernel must allocate a power-of-2
 119     granule but will only discard the excess if appropriately configured as
 120     this has an effect on fragmentation.
 121
 122 (*) A list of all the private copy and anonymous mappings on the system is
 123     visible through /proc/maps in no-MMU mode.
 124
 125 (*) A list of all the mappings in use by a process is visible through
 126     /proc/<pid>/maps in no-MMU mode.
 127
 128 (*) Supplying MAP_FIXED or a requesting a particular mapping address will
 129     result in an error.
 130
 131 (*) Files mapped privately usually have to have a read method provided by the
 132     driver or filesystem so that the contents can be read into the memory
 133     allocated if mmap() chooses not to map the backing device directly. An
 134     error will result if they don't. This is most likely to be encountered
 135     with character device files, pipes, fifos and sockets.
 136
 137
 138==========================
 139INTERPROCESS SHARED MEMORY
 140==========================
 141
 142Both SYSV IPC SHM shared memory and POSIX shared memory is supported in NOMMU
 143mode.  The former through the usual mechanism, the latter through files created
 144on ramfs or tmpfs mounts.
 145
 146
 147=======
 148FUTEXES
 149=======
 150
 151Futexes are supported in NOMMU mode if the arch supports them.  An error will
 152be given if an address passed to the futex system call lies outside the
 153mappings made by a process or if the mapping in which the address lies does not
 154support futexes (such as an I/O chardev mapping).
 155
 156
 157=============
 158NO-MMU MREMAP
 159=============
 160
 161The mremap() function is partially supported.  It may change the size of a
 162mapping, and may move it[*] if MREMAP_MAYMOVE is specified and if the new size
 163of the mapping exceeds the size of the slab object currently occupied by the
 164memory to which the mapping refers, or if a smaller slab object could be used.
 165
 166MREMAP_FIXED is not supported, though it is ignored if there's no change of
 167address and the object does not need to be moved.
 168
 169Shared mappings may not be moved.  Shareable mappings may not be moved either,
 170even if they are not currently shared.
 171
 172The mremap() function must be given an exact match for base address and size of
 173a previously mapped object.  It may not be used to create holes in existing
 174mappings, move parts of existing mappings or resize parts of mappings.  It must
 175act on a complete mapping.
 176
 177[*] Not currently supported.
 178
 179
 180============================================
 181PROVIDING SHAREABLE CHARACTER DEVICE SUPPORT
 182============================================
 183
 184To provide shareable character device support, a driver must provide a
 185file->f_op->get_unmapped_area() operation. The mmap() routines will call this
 186to get a proposed address for the mapping. This may return an error if it
 187doesn't wish to honour the mapping because it's too long, at a weird offset,
 188under some unsupported combination of flags or whatever.
 189
 190The driver should also provide backing device information with capabilities set
 191to indicate the permitted types of mapping on such devices. The default is
 192assumed to be readable and writable, not executable, and only shareable
 193directly (can't be copied).
 194
 195The file->f_op->mmap() operation will be called to actually inaugurate the
 196mapping. It can be rejected at that point. Returning the ENOSYS error will
 197cause the mapping to be copied instead if BDI_CAP_MAP_COPY is specified.
 198
 199The vm_ops->close() routine will be invoked when the last mapping on a chardev
 200is removed. An existing mapping will be shared, partially or not, if possible
 201without notifying the driver.
 202
 203It is permitted also for the file->f_op->get_unmapped_area() operation to
 204return -ENOSYS. This will be taken to mean that this operation just doesn't
 205want to handle it, despite the fact it's got an operation. For instance, it
 206might try directing the call to a secondary driver which turns out not to
 207implement it. Such is the case for the framebuffer driver which attempts to
 208direct the call to the device-specific driver. Under such circumstances, the
 209mapping request will be rejected if BDI_CAP_MAP_COPY is not specified, and a
 210copy mapped otherwise.
 211
 212IMPORTANT NOTE:
 213
 214        Some types of device may present a different appearance to anyone
 215        looking at them in certain modes. Flash chips can be like this; for
 216        instance if they're in programming or erase mode, you might see the
 217        status reflected in the mapping, instead of the data.
 218
 219        In such a case, care must be taken lest userspace see a shared or a
 220        private mapping showing such information when the driver is busy
 221        controlling the device. Remember especially: private executable
 222        mappings may still be mapped directly off the device under some
 223        circumstances!
 224
 225
 226==============================================
 227PROVIDING SHAREABLE MEMORY-BACKED FILE SUPPORT
 228==============================================
 229
 230Provision of shared mappings on memory backed files is similar to the provision
 231of support for shared mapped character devices. The main difference is that the
 232filesystem providing the service will probably allocate a contiguous collection
 233of pages and permit mappings to be made on that.
 234
 235It is recommended that a truncate operation applied to such a file that
 236increases the file size, if that file is empty, be taken as a request to gather
 237enough pages to honour a mapping. This is required to support POSIX shared
 238memory.
 239
 240Memory backed devices are indicated by the mapping's backing device info having
 241the memory_backed flag set.
 242
 243
 244========================================
 245PROVIDING SHAREABLE BLOCK DEVICE SUPPORT
 246========================================
 247
 248Provision of shared mappings on block device files is exactly the same as for
 249character devices. If there isn't a real device underneath, then the driver
 250should allocate sufficient contiguous memory to honour any supported mapping.
 251
 252
 253=================================
 254ADJUSTING PAGE TRIMMING BEHAVIOUR
 255=================================
 256
 257NOMMU mmap automatically rounds up to the nearest power-of-2 number of pages
 258when performing an allocation.  This can have adverse effects on memory
 259fragmentation, and as such, is left configurable.  The default behaviour is to
 260aggressively trim allocations and discard any excess pages back in to the page
 261allocator.  In order to retain finer-grained control over fragmentation, this
 262behaviour can either be disabled completely, or bumped up to a higher page
 263watermark where trimming begins.
 264
 265Page trimming behaviour is configurable via the sysctl `vm.nr_trim_pages'.
 266
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