1 pagemap, from the userspace perspective
2 ---------------------------------------
4 pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
5 userspace programs to examine the page tables and related information by
6 reading files in /proc.
8 There are three components to pagemap:
10 * /proc/pid/pagemap. This file lets a userspace process find out which
11 physical frame each virtual page is mapped to. It contains one 64-bit
12 value for each virtual page, containing the following data (from
13 fs/proc/task_mmu.c, above pagemap_read):
15 * Bits 0-55 page frame number (PFN) if present
16 * Bits 0-4 swap type if swapped
17 * Bits 5-55 swap offset if swapped
18 * Bits 55-60 page shift (page size = 1<<page shift)
19 * Bit 61 reserved for future use
23 If the page is not present but in swap, then the PFN contains an
24 encoding of the swap file number and the page's offset into the
25 swap. Unmapped pages return a null PFN. This allows determining
26 precisely which pages are mapped (or in swap) and comparing mapped
27 pages between processes.
29 Efficient users of this interface will use /proc/pid/maps to
30 determine which areas of memory are actually mapped and llseek to
31 skip over unmapped regions.
33 * /proc/kpagecount. This file contains a 64-bit count of the number of
34 times each page is mapped, indexed by PFN.
36 * /proc/kpageflags. This file contains a 64-bit set of flags for each
39 The flags are (from fs/proc/proc_misc, above kpageflags_read):
53 Using pagemap to do something useful:
55 The general procedure for using pagemap to find out about a process' memory
58 1. Read /proc/pid/maps to determine which parts of the memory space are
60 2. Select the maps you are interested in -- all of them, or a particular
61 library, or the stack or the heap, etc.
62 3. Open /proc/pid/pagemap and seek to the pages you would like to examine.
63 4. Read a u64 for each page from pagemap.
64 5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just
65 read, seek to that entry in the file, and read the data you want.
67 For example, to find the "unique set size" (USS), which is the amount of
68 memory that a process is using that is not shared with any other process,
69 you can go through every map in the process, find the PFNs, look those up
70 in kpagecount, and tally up the number of pages that are only referenced
75 Reading from any of the files will return -EINVAL if you are not starting
76 the read on an 8-byte boundary (e.g., if you seeked an odd number of bytes
77 into the file), or if the size of the read is not a multiple of 8 bytes.