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ef421be7 TT |
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. |