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.. _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``.
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.rst)
* 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
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.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 - 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.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.
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.
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