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author | Ingo Molnar <mingo@elte.hu> | 2008-06-16 05:24:43 -0400 |
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committer | Ingo Molnar <mingo@elte.hu> | 2008-06-16 05:24:43 -0400 |
commit | 8bbd54d69e9c66adbf544e21d8dcfb15fb9198f7 (patch) | |
tree | 95f30814fc759c2cb523dbea95bc531c7f8f3231 /Documentation/vm/pagemap.txt | |
parent | 8c2238eaaf0f774ca0f8d9daad7a616429bbb7f1 (diff) | |
parent | 066519068ad2fbe98c7f45552b1f592903a9c8c8 (diff) |
Merge branch 'linus' into core/softlockup
Diffstat (limited to 'Documentation/vm/pagemap.txt')
-rw-r--r-- | Documentation/vm/pagemap.txt | 77 |
1 files changed, 77 insertions, 0 deletions
diff --git a/Documentation/vm/pagemap.txt b/Documentation/vm/pagemap.txt new file mode 100644 index 000000000000..ce72c0fe6177 --- /dev/null +++ b/Documentation/vm/pagemap.txt | |||
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1 | pagemap, from the userspace perspective | ||
2 | --------------------------------------- | ||
3 | |||
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. | ||
7 | |||
8 | There are three components to pagemap: | ||
9 | |||
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): | ||
14 | |||
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 | ||
20 | * Bit 62 page swapped | ||
21 | * Bit 63 page present | ||
22 | |||
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. | ||
28 | |||
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. | ||
32 | |||
33 | * /proc/kpagecount. This file contains a 64-bit count of the number of | ||
34 | times each page is mapped, indexed by PFN. | ||
35 | |||
36 | * /proc/kpageflags. This file contains a 64-bit set of flags for each | ||
37 | page, indexed by PFN. | ||
38 | |||
39 | The flags are (from fs/proc/proc_misc, above kpageflags_read): | ||
40 | |||
41 | 0. LOCKED | ||
42 | 1. ERROR | ||
43 | 2. REFERENCED | ||
44 | 3. UPTODATE | ||
45 | 4. DIRTY | ||
46 | 5. LRU | ||
47 | 6. ACTIVE | ||
48 | 7. SLAB | ||
49 | 8. WRITEBACK | ||
50 | 9. RECLAIM | ||
51 | 10. BUDDY | ||
52 | |||
53 | Using pagemap to do something useful: | ||
54 | |||
55 | The general procedure for using pagemap to find out about a process' memory | ||
56 | usage goes like this: | ||
57 | |||
58 | 1. Read /proc/pid/maps to determine which parts of the memory space are | ||
59 | mapped to what. | ||
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. | ||
66 | |||
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 | ||
71 | once. | ||
72 | |||
73 | Other notes: | ||
74 | |||
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. | ||