Commit 25c3bf8a authored by Mike Rapoport's avatar Mike Rapoport Committed by Jonathan Corbet
Browse files

docs/vm: pagemap.txt: convert to ReST format 



Signed-off-by: default avatarMike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 137b4552

Documentation/vm/pagemap.txt

+89 −75
Original line number Diff line number Diff line 
pagemap, from the userspace perspective

---------------------------------------

.. _pagemap:



======================================

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.

reading files in ``/proc``.



There are four components to pagemap:



 * /proc/pid/pagemap.  This file lets a userspace process find out which

 * ``/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):
@@ -37,13 +40,13 @@ There are four components to pagemap: 
   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

 * ``/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

 * ``/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):

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



    0. LOCKED

    1. ERROR
@@ -72,98 +75,108 @@ There are four components to pagemap: 
    24. ZERO_PAGE

    25. IDLE



 * /proc/kpagecgroup.  This file contains a 64-bit inode number of the

 * ``/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

0 - LOCKED

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



 7. SLAB

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

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

15 - COMPOUND_HEAD

    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

16 - COMPOUND_TAIL

    A compound page tail (see description above).

17 - HUGE

    this is an integral part of a HugeTLB page



19. HWPOISON

19 - HWPOISON

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



20. NOPAGE

20 - NOPAGE

    no page frame exists at the requested address



21. KSM

21 - KSM

    identical memory pages dynamically shared between one or more processes



22. THP

22 - THP

    contiguous pages which construct transparent hugepages



23. BALLOON

23 - BALLOON

    balloon compaction page



24. ZERO_PAGE

24 - ZERO_PAGE

    zero page for pfn_zero or huge_zero page



25. IDLE

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.

    is up-to-date one has to read ``/sys/kernel/mm/page_idle/bitmap`` first.



IO related page flags

---------------------



    [IO related page flags]

 1. ERROR     IO error occurred

 3. UPTODATE  page has up-to-date data

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

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

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:

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

 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.

 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.

 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,
@@ -171,7 +184,8 @@ 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:

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