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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 |
541c237c PE |
18 | * Bit 55 pte is soft-dirty (see Documentation/vm/soft-dirty.txt) |
19 | * Bits 56-60 zero | |
052fb0d6 | 20 | * Bit 61 page is file-page or shared-anon |
ef421be7 TT |
21 | * Bit 62 page swapped |
22 | * Bit 63 page present | |
23 | ||
24 | If the page is not present but in swap, then the PFN contains an | |
25 | encoding of the swap file number and the page's offset into the | |
26 | swap. Unmapped pages return a null PFN. This allows determining | |
27 | precisely which pages are mapped (or in swap) and comparing mapped | |
28 | pages between processes. | |
29 | ||
30 | Efficient users of this interface will use /proc/pid/maps to | |
31 | determine which areas of memory are actually mapped and llseek to | |
32 | skip over unmapped regions. | |
33 | ||
34 | * /proc/kpagecount. This file contains a 64-bit count of the number of | |
35 | times each page is mapped, indexed by PFN. | |
36 | ||
37 | * /proc/kpageflags. This file contains a 64-bit set of flags for each | |
38 | page, indexed by PFN. | |
39 | ||
c9ba78e2 | 40 | The flags are (from fs/proc/page.c, above kpageflags_read): |
ef421be7 TT |
41 | |
42 | 0. LOCKED | |
43 | 1. ERROR | |
44 | 2. REFERENCED | |
45 | 3. UPTODATE | |
46 | 4. DIRTY | |
47 | 5. LRU | |
48 | 6. ACTIVE | |
49 | 7. SLAB | |
50 | 8. WRITEBACK | |
51 | 9. RECLAIM | |
52 | 10. BUDDY | |
17e89501 WF |
53 | 11. MMAP |
54 | 12. ANON | |
55 | 13. SWAPCACHE | |
56 | 14. SWAPBACKED | |
57 | 15. COMPOUND_HEAD | |
58 | 16. COMPOUND_TAIL | |
59 | 16. HUGE | |
60 | 18. UNEVICTABLE | |
253fb02d | 61 | 19. HWPOISON |
17e89501 | 62 | 20. NOPAGE |
a1bbb5ec | 63 | 21. KSM |
807f0ccf | 64 | 22. THP |
17e89501 WF |
65 | |
66 | Short descriptions to the page flags: | |
67 | ||
68 | 0. LOCKED | |
69 | page is being locked for exclusive access, eg. by undergoing read/write IO | |
70 | ||
71 | 7. SLAB | |
72 | page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator | |
73 | When compound page is used, SLUB/SLQB will only set this flag on the head | |
74 | page; SLOB will not flag it at all. | |
75 | ||
76 | 10. BUDDY | |
77 | a free memory block managed by the buddy system allocator | |
78 | The buddy system organizes free memory in blocks of various orders. | |
79 | An order N block has 2^N physically contiguous pages, with the BUDDY flag | |
80 | set for and _only_ for the first page. | |
81 | ||
82 | 15. COMPOUND_HEAD | |
83 | 16. COMPOUND_TAIL | |
84 | A compound page with order N consists of 2^N physically contiguous pages. | |
85 | A compound page with order 2 takes the form of "HTTT", where H donates its | |
86 | head page and T donates its tail page(s). The major consumers of compound | |
87 | pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc. | |
88 | memory allocators and various device drivers. However in this interface, | |
89 | only huge/giga pages are made visible to end users. | |
90 | 17. HUGE | |
91 | this is an integral part of a HugeTLB page | |
92 | ||
253fb02d WF |
93 | 19. HWPOISON |
94 | hardware detected memory corruption on this page: don't touch the data! | |
95 | ||
17e89501 WF |
96 | 20. NOPAGE |
97 | no page frame exists at the requested address | |
98 | ||
a1bbb5ec WF |
99 | 21. KSM |
100 | identical memory pages dynamically shared between one or more processes | |
101 | ||
807f0ccf NH |
102 | 22. THP |
103 | contiguous pages which construct transparent hugepages | |
104 | ||
17e89501 WF |
105 | [IO related page flags] |
106 | 1. ERROR IO error occurred | |
107 | 3. UPTODATE page has up-to-date data | |
108 | ie. for file backed page: (in-memory data revision >= on-disk one) | |
109 | 4. DIRTY page has been written to, hence contains new data | |
110 | ie. for file backed page: (in-memory data revision > on-disk one) | |
111 | 8. WRITEBACK page is being synced to disk | |
112 | ||
113 | [LRU related page flags] | |
114 | 5. LRU page is in one of the LRU lists | |
115 | 6. ACTIVE page is in the active LRU list | |
116 | 18. UNEVICTABLE page is in the unevictable (non-)LRU list | |
117 | It is somehow pinned and not a candidate for LRU page reclaims, | |
118 | eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments | |
119 | 2. REFERENCED page has been referenced since last LRU list enqueue/requeue | |
120 | 9. RECLAIM page will be reclaimed soon after its pageout IO completed | |
121 | 11. MMAP a memory mapped page | |
122 | 12. ANON a memory mapped page that is not part of a file | |
123 | 13. SWAPCACHE page is mapped to swap space, ie. has an associated swap entry | |
124 | 14. SWAPBACKED page is backed by swap/RAM | |
125 | ||
126 | The page-types tool in this directory can be used to query the above flags. | |
ef421be7 TT |
127 | |
128 | Using pagemap to do something useful: | |
129 | ||
130 | The general procedure for using pagemap to find out about a process' memory | |
131 | usage goes like this: | |
132 | ||
133 | 1. Read /proc/pid/maps to determine which parts of the memory space are | |
134 | mapped to what. | |
135 | 2. Select the maps you are interested in -- all of them, or a particular | |
136 | library, or the stack or the heap, etc. | |
137 | 3. Open /proc/pid/pagemap and seek to the pages you would like to examine. | |
138 | 4. Read a u64 for each page from pagemap. | |
139 | 5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just | |
140 | read, seek to that entry in the file, and read the data you want. | |
141 | ||
142 | For example, to find the "unique set size" (USS), which is the amount of | |
143 | memory that a process is using that is not shared with any other process, | |
144 | you can go through every map in the process, find the PFNs, look those up | |
145 | in kpagecount, and tally up the number of pages that are only referenced | |
146 | once. | |
147 | ||
148 | Other notes: | |
149 | ||
150 | Reading from any of the files will return -EINVAL if you are not starting | |
f884ab15 | 151 | the read on an 8-byte boundary (e.g., if you sought an odd number of bytes |
ef421be7 | 152 | into the file), or if the size of the read is not a multiple of 8 bytes. |