Documentation/admin-guide/mm/pagemap.rst

=============================
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/SLUB kernel memory allocator.
   When compound page is used, either will only set this flag on the head
   page.
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.

Pagemap Scan IOCTL
==================

The ``PAGEMAP_SCAN`` IOCTL on the pagemap file can be used to get or optionally
clear the info about page table entries. The following operations are supported
in this IOCTL:

- Scan the address range and get the memory ranges matching the provided criteria.
  This is performed when the output buffer is specified.
- Write-protect the pages. The ``PM_SCAN_WP_MATCHING`` is used to write-protect
  the pages of interest. The ``PM_SCAN_CHECK_WPASYNC`` aborts the operation if
  non-Async Write Protected pages are found. The ``PM_SCAN_WP_MATCHING`` can be
  used with or without ``PM_SCAN_CHECK_WPASYNC``.
- Both of those operations can be combined into one atomic operation where we can
  get and write protect the pages as well.

Following flags about pages are currently supported:

- ``PAGE_IS_WPALLOWED`` - Page has async-write-protection enabled
- ``PAGE_IS_WRITTEN`` - Page has been written to from the time it was write protected
- ``PAGE_IS_FILE`` - Page is file backed
- ``PAGE_IS_PRESENT`` - Page is present in the memory
- ``PAGE_IS_SWAPPED`` - Page is in swapped
- ``PAGE_IS_PFNZERO`` - Page has zero PFN
- ``PAGE_IS_HUGE`` - Page is THP or Hugetlb backed
- ``PAGE_IS_SOFT_DIRTY`` - Page is soft-dirty

The ``struct pm_scan_arg`` is used as the argument of the IOCTL.

 1. The size of the ``struct pm_scan_arg`` must be specified in the ``size``
    field. This field will be helpful in recognizing the structure if extensions
    are done later.
 2. The flags can be specified in the ``flags`` field. The ``PM_SCAN_WP_MATCHING``
    and ``PM_SCAN_CHECK_WPASYNC`` are the only added flags at this time. The get
    operation is optionally performed depending upon if the output buffer is
    provided or not.
 3. The range is specified through ``start`` and ``end``.
 4. The walk can abort before visiting the complete range such as the user buffer
    can get full etc. The walk ending address is specified in``end_walk``.
 5. The output buffer of ``struct page_region`` array and size is specified in
    ``vec`` and ``vec_len``.
 6. The optional maximum requested pages are specified in the ``max_pages``.
 7. The masks are specified in ``category_mask``, ``category_anyof_mask``,
    ``category_inverted`` and ``return_mask``.

Find pages which have been written and WP them as well::

   struct pm_scan_arg arg = {
   .size = sizeof(arg),
   .flags = PM_SCAN_CHECK_WPASYNC | PM_SCAN_CHECK_WPASYNC,
   ..
   .category_mask = PAGE_IS_WRITTEN,
   .return_mask = PAGE_IS_WRITTEN,
   };

Find pages which have been written, are file backed, not swapped and either
present or huge::

   struct pm_scan_arg arg = {
   .size = sizeof(arg),
   .flags = 0,
   ..
   .category_mask = PAGE_IS_WRITTEN | PAGE_IS_SWAPPED,
   .category_inverted = PAGE_IS_SWAPPED,
   .category_anyof_mask = PAGE_IS_PRESENT | PAGE_IS_HUGE,
   .return_mask = PAGE_IS_WRITTEN | PAGE_IS_SWAPPED |
                  PAGE_IS_PRESENT | PAGE_IS_HUGE,
   };

The ``PAGE_IS_WRITTEN`` flag can be considered as a better-performing alternative
of soft-dirty flag. It doesn't get affected by VMA merging of the kernel and hence
the user can find the true soft-dirty pages in case of normal pages. (There may
still be extra dirty pages reported for THP or Hugetlb pages.)

"PAGE_IS_WRITTEN" category is used with uffd write protect-enabled ranges to
implement memory dirty tracking in userspace:

 1. The userfaultfd file descriptor is created with ``userfaultfd`` syscall.
 2. The ``UFFD_FEATURE_WP_UNPOPULATED`` and ``UFFD_FEATURE_WP_ASYNC`` features
    are set by ``UFFDIO_API`` IOCTL.
 3. The memory range is registered with ``UFFDIO_REGISTER_MODE_WP`` mode
    through ``UFFDIO_REGISTER`` IOCTL.
 4. Then any part of the registered memory or the whole memory region must
    be write protected using ``PAGEMAP_SCAN`` IOCTL with flag ``PM_SCAN_WP_MATCHING``
    or the ``UFFDIO_WRITEPROTECT`` IOCTL can be used. Both of these perform the
    same operation. The former is better in terms of performance.
 5. Now the ``PAGEMAP_SCAN`` IOCTL can be used to either just find pages which
    have been written to since they were last marked and/or optionally write protect
    the pages as well.