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Documentation / vm / pagemap.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

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 0-54  page frame number (PFN) if present
16	    * Bits 0-4   swap type if swapped
17	    * Bits 5-54  swap offset if swapped
18	    * Bit  55    pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
19	    * Bits 56-60 zero
20	    * Bit  61    page is file-page or shared-anon
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	
40	   The flags are (from fs/proc/page.c, above kpageflags_read):
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
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
61	    19. HWPOISON
62	    20. NOPAGE
63	    21. KSM
64	    22. THP
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	
93	19. HWPOISON
94	    hardware detected memory corruption on this page: don't touch the data!
95	
96	20. NOPAGE
97	    no page frame exists at the requested address
98	
99	21. KSM
100	    identical memory pages dynamically shared between one or more processes
101	
102	22. THP
103	    contiguous pages which construct transparent hugepages
104	
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.
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
151	the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
152	into the file), or if the size of the read is not a multiple of 8 bytes.
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