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 Soft-Dirty PTEs)
    • Bit 56 page exclusively mapped (since 4.2)
    • Bit 57 pte is uffd-wp write-protected (since 5.13) (see Userfaultfd)
    • 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.

The page-types tool in the tools/vm 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):

    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
    24. OFFLINE
    25. ZERO_PAGE
    26. IDLE
    27. 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
page is being locked for exclusive access, e.g. 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 (HugeTLB Pages), 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
page is logically offline
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 Idle Page Tracking). 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
page is in use as a page table

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.