pagemap, from the userspace perspective
---------------------------------------

pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
userspace programs to examine the page tables and related information by
reading files in /proc.

There are four components to pagemap:

 * /proc/pid/pagemap.  This file lets a userspace process find out which
   physical frame each virtual page is mapped to.  It contains one 64-bit
   value for each virtual page, containing the following data (from
   fs/proc/task_mmu.c, above pagemap_read):

    * Bits 0-54  page frame number (PFN) if present
    * Bits 0-4   swap type if swapped
    * Bits 5-54  swap offset if swapped
    * Bit  55    pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
    * Bit  56    page exclusively mapped (since 4.2)
    * Bits 57-60 zero
    * Bit  61    page is file-page or shared-anon (since 3.5)
    * Bit  62    page swapped
    * Bit  63    page present

   Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs.
   In 4.0 and 4.1 opens by unprivileged fail with -EPERM.  Starting from
   4.2 the PFN field is zeroed if the user does not have CAP_SYS_ADMIN.
   Reason: information about PFNs helps in exploiting Rowhammer vulnerability.

   If the page is not present but in swap, then the PFN contains an
   encoding of the swap file number and the page's offset into the
   swap. Unmapped pages return a null PFN. This allows determining
   precisely which pages are mapped (or in swap) and comparing mapped
   pages between processes.

   Efficient users of this interface will use /proc/pid/maps to
   determine which areas of memory are actually mapped and llseek to
   skip over unmapped regions.

 * /proc/kpagecount.  This file contains a 64-bit count of the number of
   times each page is mapped, indexed by PFN.

 * /proc/kpageflags.  This file contains a 64-bit set of flags for each
   page, indexed by PFN.

   The flags are (from fs/proc/page.c, above kpageflags_read):

     0. LOCKED
     1. ERROR
     2. REFERENCED
     3. UPTODATE
     4. DIRTY
     5. LRU
     6. ACTIVE
     7. SLAB
     8. WRITEBACK
     9. RECLAIM
    10. BUDDY
    11. MMAP
    12. ANON
    13. SWAPCACHE
    14. SWAPBACKED
    15. COMPOUND_HEAD
    16. COMPOUND_TAIL
    17. HUGE
    18. UNEVICTABLE
    19. HWPOISON
    20. NOPAGE
    21. KSM
    22. THP
    23. BALLOON
    24. ZERO_PAGE
    25. IDLE

 * /proc/kpagecgroup.  This file contains a 64-bit inode number of the
   memory cgroup each page is charged to, indexed by PFN. Only available when
   CONFIG_MEMCG is set.

Short descriptions to the page flags:

 0. LOCKED
    page is being locked for exclusive access, eg. by undergoing read/write IO

 7. SLAB
    page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator
    When compound page is used, SLUB/SLQB will only set this flag on the head
    page; SLOB will not flag it at all.

10. BUDDY
    a free memory block managed by the buddy system allocator
    The buddy system organizes free memory in blocks of various orders.
    An order N block has 2^N physically contiguous pages, with the BUDDY flag
    set for and _only_ for the first page.

15. COMPOUND_HEAD
16. COMPOUND_TAIL
    A compound page with order N consists of 2^N physically contiguous pages.
    A compound page with order 2 takes the form of "HTTT", where H donates its
    head page and T donates its tail page(s).  The major consumers of compound
    pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc.
    memory allocators and various device drivers. However in this interface,
    only huge/giga pages are made visible to end users.
17. HUGE
    this is an integral part of a HugeTLB page

19. HWPOISON
    hardware detected memory corruption on this page: don't touch the data!

20. NOPAGE
    no page frame exists at the requested address

21. KSM
    identical memory pages dynamically shared between one or more processes

22. THP
    contiguous pages which construct transparent hugepages

23. BALLOON
    balloon compaction page

24. ZERO_PAGE
    zero page for pfn_zero or huge_zero page

25. IDLE
    page has not been accessed since it was marked idle (see
    Documentation/vm/idle_page_tracking.txt). Note that this flag may be
    stale in case the page was accessed via a PTE. To make sure the flag
    is up-to-date one has to read /sys/kernel/mm/page_idle/bitmap first.

    [IO related page flags]
 1. ERROR     IO error occurred
 3. UPTODATE  page has up-to-date data
              ie. for file backed page: (in-memory data revision >= on-disk one)
 4. DIRTY     page has been written to, hence contains new data
              ie. for file backed page: (in-memory data revision >  on-disk one)
 8. WRITEBACK page is being synced to disk

    [LRU related page flags]
 5. LRU         page is in one of the LRU lists
 6. ACTIVE      page is in the active LRU list
18. UNEVICTABLE page is in the unevictable (non-)LRU list
                It is somehow pinned and not a candidate for LRU page reclaims,
		eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments
 2. REFERENCED  page has been referenced since last LRU list enqueue/requeue
 9. RECLAIM     page will be reclaimed soon after its pageout IO completed
11. MMAP        a memory mapped page
12. ANON        a memory mapped page that is not part of a file
13. SWAPCACHE   page is mapped to swap space, ie. has an associated swap entry
14. SWAPBACKED  page is backed by swap/RAM

The page-types tool in the tools/vm directory can be used to query the
above flags.

Using pagemap to do something useful:

The general procedure for using pagemap to find out about a process' memory
usage goes like this:

 1. Read /proc/pid/maps to determine which parts of the memory space are
    mapped to what.
 2. Select the maps you are interested in -- all of them, or a particular
    library, or the stack or the heap, etc.
 3. Open /proc/pid/pagemap and seek to the pages you would like to examine.
 4. Read a u64 for each page from pagemap.
 5. Open /proc/kpagecount and/or /proc/kpageflags.  For each PFN you just
    read, seek to that entry in the file, and read the data you want.

For example, to find the "unique set size" (USS), which is the amount of
memory that a process is using that is not shared with any other process,
you can go through every map in the process, find the PFNs, look those up
in kpagecount, and tally up the number of pages that are only referenced
once.

Other notes:

Reading from any of the files will return -EINVAL if you are not starting
the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
into the file), or if the size of the read is not a multiple of 8 bytes.

Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
always 12 at most architectures). Since Linux 3.11 their meaning changes
after first clear of soft-dirty bits. Since Linux 4.2 they are used for
flags unconditionally.