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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 | ||
c9ba78e2 | 15 | * Bits 0-54 page frame number (PFN) if present |
ef421be7 | 16 | * Bits 0-4 swap type if swapped |
c9ba78e2 | 17 | * Bits 5-54 swap offset if swapped |
ef421be7 TT |
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 | ||
c9ba78e2 | 39 | The flags are (from fs/proc/page.c, above kpageflags_read): |
ef421be7 TT |
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 | |
17e89501 WF |
52 | 11. MMAP |
53 | 12. ANON | |
54 | 13. SWAPCACHE | |
55 | 14. SWAPBACKED | |
56 | 15. COMPOUND_HEAD | |
57 | 16. COMPOUND_TAIL | |
58 | 16. HUGE | |
59 | 18. UNEVICTABLE | |
60 | 20. NOPAGE | |
61 | ||
62 | Short descriptions to the page flags: | |
63 | ||
64 | 0. LOCKED | |
65 | page is being locked for exclusive access, eg. by undergoing read/write IO | |
66 | ||
67 | 7. SLAB | |
68 | page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator | |
69 | When compound page is used, SLUB/SLQB will only set this flag on the head | |
70 | page; SLOB will not flag it at all. | |
71 | ||
72 | 10. BUDDY | |
73 | a free memory block managed by the buddy system allocator | |
74 | The buddy system organizes free memory in blocks of various orders. | |
75 | An order N block has 2^N physically contiguous pages, with the BUDDY flag | |
76 | set for and _only_ for the first page. | |
77 | ||
78 | 15. COMPOUND_HEAD | |
79 | 16. COMPOUND_TAIL | |
80 | A compound page with order N consists of 2^N physically contiguous pages. | |
81 | A compound page with order 2 takes the form of "HTTT", where H donates its | |
82 | head page and T donates its tail page(s). The major consumers of compound | |
83 | pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc. | |
84 | memory allocators and various device drivers. However in this interface, | |
85 | only huge/giga pages are made visible to end users. | |
86 | 17. HUGE | |
87 | this is an integral part of a HugeTLB page | |
88 | ||
89 | 20. NOPAGE | |
90 | no page frame exists at the requested address | |
91 | ||
92 | [IO related page flags] | |
93 | 1. ERROR IO error occurred | |
94 | 3. UPTODATE page has up-to-date data | |
95 | ie. for file backed page: (in-memory data revision >= on-disk one) | |
96 | 4. DIRTY page has been written to, hence contains new data | |
97 | ie. for file backed page: (in-memory data revision > on-disk one) | |
98 | 8. WRITEBACK page is being synced to disk | |
99 | ||
100 | [LRU related page flags] | |
101 | 5. LRU page is in one of the LRU lists | |
102 | 6. ACTIVE page is in the active LRU list | |
103 | 18. UNEVICTABLE page is in the unevictable (non-)LRU list | |
104 | It is somehow pinned and not a candidate for LRU page reclaims, | |
105 | eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments | |
106 | 2. REFERENCED page has been referenced since last LRU list enqueue/requeue | |
107 | 9. RECLAIM page will be reclaimed soon after its pageout IO completed | |
108 | 11. MMAP a memory mapped page | |
109 | 12. ANON a memory mapped page that is not part of a file | |
110 | 13. SWAPCACHE page is mapped to swap space, ie. has an associated swap entry | |
111 | 14. SWAPBACKED page is backed by swap/RAM | |
112 | ||
113 | The page-types tool in this directory can be used to query the above flags. | |
ef421be7 TT |
114 | |
115 | Using pagemap to do something useful: | |
116 | ||
117 | The general procedure for using pagemap to find out about a process' memory | |
118 | usage goes like this: | |
119 | ||
120 | 1. Read /proc/pid/maps to determine which parts of the memory space are | |
121 | mapped to what. | |
122 | 2. Select the maps you are interested in -- all of them, or a particular | |
123 | library, or the stack or the heap, etc. | |
124 | 3. Open /proc/pid/pagemap and seek to the pages you would like to examine. | |
125 | 4. Read a u64 for each page from pagemap. | |
126 | 5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just | |
127 | read, seek to that entry in the file, and read the data you want. | |
128 | ||
129 | For example, to find the "unique set size" (USS), which is the amount of | |
130 | memory that a process is using that is not shared with any other process, | |
131 | you can go through every map in the process, find the PFNs, look those up | |
132 | in kpagecount, and tally up the number of pages that are only referenced | |
133 | once. | |
134 | ||
135 | Other notes: | |
136 | ||
137 | Reading from any of the files will return -EINVAL if you are not starting | |
138 | the read on an 8-byte boundary (e.g., if you seeked an odd number of bytes | |
139 | into the file), or if the size of the read is not a multiple of 8 bytes. |