Examining Process Page Tables — The Linux Kernel documentation

# 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)

>   > - Bit 58 pte is a guard region (since 6.15) (see madvise (2) man page)

>   > - Bits 59-60 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.

>   Traditionally, bit 56 indicates that a page is mapped exactly once and bit 56 is clear when a page is mapped multiple times, even when mapped in the same process multiple times. In some kernel configurations, the semantics for pages part of a larger allocation (e.g., THP) can differ: bit 56 is set if all pages part of the corresponding large allocation are certainly mapped in the same process, even if the page is mapped multiple times in that process. Bit 56 is clear when any page page of the larger allocation is maybe mapped in a different process. In some cases, a large allocation might be treated as “maybe mapped by multiple processes” even though this is no longer the case.

>   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. Some kernel configurations do not track the precise number of times a page part of a larger allocation (e.g., THP) is mapped. In these configurations, the average number of mappings per page in this larger allocation is returned instead. However, if any page of the large allocation is mapped, the returned value will be at least 1.

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 all pages. Before 4.6 only the first page of the block had the flag set.

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 THP of any size and mapped by any granularity.

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 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

  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.

## 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 PMD-mapped THP or Hugetlb backed

- `PAGE_IS_SOFT_DIRTY` - Page is soft-dirty

- `PAGE_IS_GUARD` - Page is a part of a guard region

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.