Kernel-3.10.0-957.el7_pagemap

published: updated: Kernel_3.10.0-957.el7_内核文档

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 three 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)
    • Bits 56-60 zero
    • Bit 61 page is file-page or shared-anon
    • Bit 62 page swapped
    • Bit 63 page present

    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):

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

Short descriptions to the page flags:

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

  2. 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.

  3. 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.

  4. COMPOUND_HEAD

  5. 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.

  6. HUGE
    this is an integral part of a HugeTLB page

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

  8. NOPAGE
    no page frame exists at the requested address

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

  10. THP
    contiguous pages which construct transparent hugepages

    [IO related page flags]

  11. ERROR IO error occurred

  12. UPTODATE page has up-to-date data

          ie. for file backed page: (in-memory data revision >= on-disk one)

  13. DIRTY page has been written to, hence contains new data

          ie. for file backed page: (in-memory data revision >  on-disk one)

  14. WRITEBACK page is being synced to disk

    [LRU related page flags]

  15. LRU page is in one of the LRU lists

  16. ACTIVE page is in the active LRU list

  17. 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

  18. REFERENCED page has been referenced since last LRU list enqueue/requeue

  19. RECLAIM page will be reclaimed soon after its pageout IO completed

  20. MMAP a memory mapped page

  21. ANON a memory mapped page that is not part of a file

  22. SWAPCACHE page is mapped to swap space, ie. has an associated swap entry

  23. SWAPBACKED page is backed by swap/RAM

The page-types tool in this 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 seeked an odd number of bytes
into the file), or if the size of the read is not a multiple of 8 bytes.

oosTech by Sam Lee