1pagemap, from the userspace perspective
   4pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
   5userspace programs to examine the page tables and related information by
   6reading files in /proc.
   8There are three components to pagemap:
  10 * /proc/pid/pagemap.  This file lets a userspace process find out which
  11   physical frame each virtual page is mapped to.  It contains one 64-bit
  12   value for each virtual page, containing the following data (from
  13   fs/proc/task_mmu.c, above pagemap_read):
  15    * Bits 0-54  page frame number (PFN) if present
  16    * Bits 0-4   swap type if swapped
  17    * Bits 5-54  swap offset if swapped
  18    * Bits 55-60 page shift (page size = 1<<page shift)
  19    * Bit  61    page is file-page or shared-anon
  20    * Bit  62    page swapped
  21    * Bit  63    page present
  23   If the page is not present but in swap, then the PFN contains an
  24   encoding of the swap file number and the page's offset into the
  25   swap. Unmapped pages return a null PFN. This allows determining
  26   precisely which pages are mapped (or in swap) and comparing mapped
  27   pages between processes.
  29   Efficient users of this interface will use /proc/pid/maps to
  30   determine which areas of memory are actually mapped and llseek to
  31   skip over unmapped regions.
  33 * /proc/kpagecount.  This file contains a 64-bit count of the number of
  34   times each page is mapped, indexed by PFN.
  36 * /proc/kpageflags.  This file contains a 64-bit set of flags for each
  37   page, indexed by PFN.
  39   The flags are (from fs/proc/page.c, above kpageflags_read):
  41     0. LOCKED
  42     1. ERROR
  43     2. REFERENCED
  44     3. UPTODATE
  45     4. DIRTY
  46     5. LRU
  47     6. ACTIVE
  48     7. SLAB
  49     8. WRITEBACK
  50     9. RECLAIM
  51    10. BUDDY
  52    11. MMAP
  53    12. ANON
  54    13. SWAPCACHE
  55    14. SWAPBACKED
  56    15. COMPOUND_HEAD
  57    16. COMPOUND_TAIL
  58    16. HUGE
  59    18. UNEVICTABLE
  60    19. HWPOISON
  61    20. NOPAGE
  62    21. KSM
  63    22. THP
  65Short descriptions to the page flags:
  67 0. LOCKED
  68    page is being locked for exclusive access, eg. by undergoing read/write IO
  70 7. SLAB
  71    page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator
  72    When compound page is used, SLUB/SLQB will only set this flag on the head
  73    page; SLOB will not flag it at all.
  7510. BUDDY
  76    a free memory block managed by the buddy system allocator
  77    The buddy system organizes free memory in blocks of various orders.
  78    An order N block has 2^N physically contiguous pages, with the BUDDY flag
  79    set for and _only_ for the first page.
  83    A compound page with order N consists of 2^N physically contiguous pages.
  84    A compound page with order 2 takes the form of "HTTT", where H donates its
  85    head page and T donates its tail page(s).  The major consumers of compound
  86    pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc.
  87    memory allocators and various device drivers. However in this interface,
  88    only huge/giga pages are made visible to end users.
  8917. HUGE
  90    this is an integral part of a HugeTLB page
  9219. HWPOISON
  93    hardware detected memory corruption on this page: don't touch the data!
  9520. NOPAGE
  96    no page frame exists at the requested address
  9821. KSM
  99    identical memory pages dynamically shared between one or more processes
 10122. THP
 102    contiguous pages which construct transparent hugepages
 104    [IO related page flags]
 105 1. ERROR     IO error occurred
 106 3. UPTODATE  page has up-to-date data
 107              ie. for file backed page: (in-memory data revision >= on-disk one)
 108 4. DIRTY     page has been written to, hence contains new data
 109              ie. for file backed page: (in-memory data revision >  on-disk one)
 110 8. WRITEBACK page is being synced to disk
 112    [LRU related page flags]
 113 5. LRU         page is in one of the LRU lists
 114 6. ACTIVE      page is in the active LRU list
 11518. UNEVICTABLE page is in the unevictable (non-)LRU list
 116                It is somehow pinned and not a candidate for LRU page reclaims,
 117                eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments
 118 2. REFERENCED  page has been referenced since last LRU list enqueue/requeue
 119 9. RECLAIM     page will be reclaimed soon after its pageout IO completed
 12011. MMAP        a memory mapped page
 12112. ANON        a memory mapped page that is not part of a file
 12213. SWAPCACHE   page is mapped to swap space, ie. has an associated swap entry
 12314. SWAPBACKED  page is backed by swap/RAM
 125The page-types tool in this directory can be used to query the above flags.
 127Using pagemap to do something useful:
 129The general procedure for using pagemap to find out about a process' memory
 130usage goes like this:
 132 1. Read /proc/pid/maps to determine which parts of the memory space are
 133    mapped to what.
 134 2. Select the maps you are interested in -- all of them, or a particular
 135    library, or the stack or the heap, etc.
 136 3. Open /proc/pid/pagemap and seek to the pages you would like to examine.
 137 4. Read a u64 for each page from pagemap.
 138 5. Open /proc/kpagecount and/or /proc/kpageflags.  For each PFN you just
 139    read, seek to that entry in the file, and read the data you want.
 141For example, to find the "unique set size" (USS), which is the amount of
 142memory that a process is using that is not shared with any other process,
 143you can go through every map in the process, find the PFNs, look those up
 144in kpagecount, and tally up the number of pages that are only referenced
 147Other notes:
 149Reading from any of the files will return -EINVAL if you are not starting
 150the read on an 8-byte boundary (e.g., if you seeked an odd number of bytes
 151into the file), or if the size of the read is not a multiple of 8 bytes.