1pagemap, from the userspace perspective
  2---------------------------------------
  3
  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.
  7
  8There are three components to pagemap:
  9
 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):
 14
 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    reserved for future use
 20    * Bit  62    page swapped
 21    * Bit  63    page present
 22
 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.
 28
 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.
 32
 33 * /proc/kpagecount.  This file contains a 64-bit count of the number of
 34   times each page is mapped, indexed by PFN.
 35
 36 * /proc/kpageflags.  This file contains a 64-bit set of flags for each
 37   page, indexed by PFN.
 38
 39   The flags are (from fs/proc/page.c, above kpageflags_read):
 40
 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
 64Short descriptions to the page flags:
 65
 66 0. LOCKED
 67    page is being locked for exclusive access, eg. by undergoing read/write IO
 68
 69 7. SLAB
 70    page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator
 71    When compound page is used, SLUB/SLQB will only set this flag on the head
 72    page; SLOB will not flag it at all.
 73
 7410. BUDDY
 75    a free memory block managed by the buddy system allocator
 76    The buddy system organizes free memory in blocks of various orders.
 77    An order N block has 2^N physically contiguous pages, with the BUDDY flag
 78    set for and _only_ for the first page.
 79
 8015. COMPOUND_HEAD
 8116. COMPOUND_TAIL
 82    A compound page with order N consists of 2^N physically contiguous pages.
 83    A compound page with order 2 takes the form of "HTTT", where H donates its
 84    head page and T donates its tail page(s).  The major consumers of compound
 85    pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc.
 86    memory allocators and various device drivers. However in this interface,
 87    only huge/giga pages are made visible to end users.
 8817. HUGE
 89    this is an integral part of a HugeTLB page
 90
 9119. HWPOISON
 92    hardware detected memory corruption on this page: don't touch the data!
 93
 9420. NOPAGE
 95    no page frame exists at the requested address
 96
 9721. KSM
 98    identical memory pages dynamically shared between one or more processes
 99
100    [IO related page flags]
101 1. ERROR     IO error occurred
102 3. UPTODATE  page has up-to-date data
103              ie. for file backed page: (in-memory data revision >= on-disk one)
104 4. DIRTY     page has been written to, hence contains new data
105              ie. for file backed page: (in-memory data revision >  on-disk one)
106 8. WRITEBACK page is being synced to disk
107
108    [LRU related page flags]
109 5. LRU         page is in one of the LRU lists
110 6. ACTIVE      page is in the active LRU list
11118. UNEVICTABLE page is in the unevictable (non-)LRU list
112                It is somehow pinned and not a candidate for LRU page reclaims,
113		eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments
114 2. REFERENCED  page has been referenced since last LRU list enqueue/requeue
115 9. RECLAIM     page will be reclaimed soon after its pageout IO completed
11611. MMAP        a memory mapped page
11712. ANON        a memory mapped page that is not part of a file
11813. SWAPCACHE   page is mapped to swap space, ie. has an associated swap entry
11914. SWAPBACKED  page is backed by swap/RAM
120
121The page-types tool in this directory can be used to query the above flags.
122
123Using pagemap to do something useful:
124
125The general procedure for using pagemap to find out about a process' memory
126usage goes like this:
127
128 1. Read /proc/pid/maps to determine which parts of the memory space are
129    mapped to what.
130 2. Select the maps you are interested in -- all of them, or a particular
131    library, or the stack or the heap, etc.
132 3. Open /proc/pid/pagemap and seek to the pages you would like to examine.
133 4. Read a u64 for each page from pagemap.
134 5. Open /proc/kpagecount and/or /proc/kpageflags.  For each PFN you just
135    read, seek to that entry in the file, and read the data you want.
136
137For example, to find the "unique set size" (USS), which is the amount of
138memory that a process is using that is not shared with any other process,
139you can go through every map in the process, find the PFNs, look those up
140in kpagecount, and tally up the number of pages that are only referenced
141once.
142
143Other notes:
144
145Reading from any of the files will return -EINVAL if you are not starting
146the read on an 8-byte boundary (e.g., if you seeked an odd number of bytes
147into the file), or if the size of the read is not a multiple of 8 bytes.