Documentation/admin-guide/mm/pagemap.rst - linux - Git at Google

 =============================
 Examining Process Page Tables
 =============================

 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/admin-guide/mm/soft-dirty.rst)
     * Bit  56    page exclusively mapped (since 4.2)
     * Bit  57    pte is uffd-wp write-protected (since 5.13) (see
       Documentation/admin-guide/mm/userfaultfd.rst)
     * Bits 58-60 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.

 The page-types tool in the tools/mm directory can be used to query the
 number of times a page is mapped.

  * ``/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. OFFLINE
     24. ZERO_PAGE
     25. IDLE
     26. PGTABLE

  * ``/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
    The page is being locked for exclusive access, e.g. by undergoing read/write
    IO.
 7 - SLAB
    The 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
     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/admin-guide/mm/hugetlbpage.rst),
     the SLUB etc.  memory allocators and various device drivers.
     However in this interface, only huge/giga pages are made visible
     to end users.
 16 - COMPOUND_TAIL
     A compound page tail (see description above).
 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 - OFFLINE
     The page is logically offline.
 24 - ZERO_PAGE
     Zero page for pfn_zero or huge_zero page.
 25 - IDLE
     The page has not been accessed since it was marked idle (see
     Documentation/admin-guide/mm/idle_page_tracking.rst).
     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.
 26 - PGTABLE
     The page is in use as a page table.

 IO related page flags
 ---------------------

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

 LRU related page flags
 ----------------------

 5 - LRU
    The page is in one of the LRU lists.
 6 - ACTIVE
    The page is in the active LRU list.
 18 - UNEVICTABLE
    The page is in the unevictable (non-)LRU list It is somehow pinned and
    not a candidate for LRU page reclaims, e.g. ramfs pages,
    shmctl(SHM_LOCK) and mlock() memory segments.
 2 - REFERENCED
    The page has been referenced since last LRU list enqueue/requeue.
 9 - RECLAIM
    The 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
    The page is mapped to swap space, i.e. has an associated swap entry.
 14 - SWAPBACKED
    The page is backed by swap/RAM.

 The page-types tool in the tools/mm 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.

 Exceptions for Shared Memory
 ============================

 Page table entries for shared pages are cleared when the pages are zapped or
 swapped out. This makes swapped out pages indistinguishable from never-allocated
 ones.

 In kernel space, the swap location can still be retrieved from the page cache.
 However, values stored only on the normal PTE get lost irretrievably when the
 page is swapped out (i.e. SOFT_DIRTY).

 In user space, whether the page is present, swapped or none can be deduced with
 the help of lseek and/or mincore system calls.

 lseek() can differentiate between accessed pages (present or swapped out) and
 holes (none/non-allocated) by specifying the SEEK_DATA flag on the file where
 the pages are backed. For anonymous shared pages, the file can be found in
 ``/proc/pid/map_files/``.

 mincore() can differentiate between pages in memory (present, including swap
 cache) and out of memory (swapped out or none/non-allocated).

 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.
	=============================
	Examining Process Page Tables
	=============================

	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/admin-guide/mm/soft-dirty.rst)
	* Bit 56 page exclusively mapped (since 4.2)
	* Bit 57 pte is uffd-wp write-protected (since 5.13) (see
	Documentation/admin-guide/mm/userfaultfd.rst)
	* Bits 58-60 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.

	The page-types tool in the tools/mm directory can be used to query the
	number of times a page is mapped.

	* ``/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. OFFLINE
	24. ZERO_PAGE
	25. IDLE
	26. PGTABLE

	* ``/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
	The page is being locked for exclusive access, e.g. by undergoing read/write
	IO.
	7 - SLAB
	The 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
	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/admin-guide/mm/hugetlbpage.rst),
	the SLUB etc. memory allocators and various device drivers.
	However in this interface, only huge/giga pages are made visible
	to end users.
	16 - COMPOUND_TAIL
	A compound page tail (see description above).
	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 - OFFLINE
	The page is logically offline.
	24 - ZERO_PAGE
	Zero page for pfn_zero or huge_zero page.
	25 - IDLE
	The page has not been accessed since it was marked idle (see
	Documentation/admin-guide/mm/idle_page_tracking.rst).
	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.
	26 - PGTABLE
	The page is in use as a page table.

	IO related page flags
	---------------------

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

	LRU related page flags
	----------------------

	5 - LRU
	The page is in one of the LRU lists.
	6 - ACTIVE
	The page is in the active LRU list.
	18 - UNEVICTABLE
	The page is in the unevictable (non-)LRU list It is somehow pinned and
	not a candidate for LRU page reclaims, e.g. ramfs pages,
	shmctl(SHM_LOCK) and mlock() memory segments.
	2 - REFERENCED
	The page has been referenced since last LRU list enqueue/requeue.
	9 - RECLAIM
	The 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
	The page is mapped to swap space, i.e. has an associated swap entry.
	14 - SWAPBACKED
	The page is backed by swap/RAM.

	The page-types tool in the tools/mm 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.

	Exceptions for Shared Memory
	============================

	Page table entries for shared pages are cleared when the pages are zapped or
	swapped out. This makes swapped out pages indistinguishable from never-allocated
	ones.

	In kernel space, the swap location can still be retrieved from the page cache.
	However, values stored only on the normal PTE get lost irretrievably when the
	page is swapped out (i.e. SOFT_DIRTY).

	In user space, whether the page is present, swapped or none can be deduced with
	the help of lseek and/or mincore system calls.

	lseek() can differentiate between accessed pages (present or swapped out) and
	holes (none/non-allocated) by specifying the SEEK_DATA flag on the file where
	the pages are backed. For anonymous shared pages, the file can be found in
	``/proc/pid/map_files/``.

	mincore() can differentiate between pages in memory (present, including swap
	cache) and out of memory (swapped out or none/non-allocated).

	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.