Table of Contents

Proc - pagemap file

The /proc/pid/pagemap gives the PFN (page frame number), which can be used to find the pageflags using /proc/kpageflags and number of times a page is mapped using /proc/kpagecount.

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 three 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):

BitsDescription
0-54Page frame number (PFN) if present.
0-4Swap type if swapped.
5-54Swap offset if swapped.
55-60Page shift (page size = 1«page shift).
61Reserved for future use.
62Page swapped.
63Page present.

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):

No. Name Description
0 LOCKED Page is being locked for exclusive access, e.g. by undergoing read/write IO.
1 ERROR IO error occurred.
2 REFERENCED Page has been referenced since last LRU list enqueue/requeue.
3 UPTODATE Page has up-to-date data. i.e. for file backed page: (in-memory data revision >= on-disk one).
4 DIRTY Page has been written to, hence contains new data. i.e. for file backed page: (in-memory data revision > on-disk one).
5 LRU Page is in one of the LRU lists.
6 ACTIVE Page is in the active LRU list.
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.
8 WRITEBACK Page is being synced to disk.
9 RECLAIM Page will be reclaimed soon after its pageout IO completed.
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.
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, i.e. has an associated swap entry.
14 SWAPBACKED Page is backed by swap/RAM.
15 COMPOUND_HEAD A compound page with order N consists of 2N 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. See Huge Table 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 with order N consists of 2N 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. See Huge Table Pages, the SLUB etc. Memory allocators and various device drivers. However in this interface, only huge/giga pages are made visible to end users.
17 HUGE This is an integral part of a Huge Page Table page.
18 UNEVICTABLE 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.
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.

The page-types tool in this 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.

NOTE: 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 seeked an odd number of bytes into the file), or if the size of the read is not a multiple of 8 bytes.