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.