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Documentation / vm / pagemap.txt




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Based on kernel version 3.9. Page generated on 2013-05-02 23:17 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	    * Bits 55-60 page shift (page size = 1<<page shift)
19	    * Bit  61    page is file-page or shared-anon
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/page.c, 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	    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	    19. HWPOISON
61	    20. NOPAGE
62	    21. KSM
63	    22. THP
64	
65	Short descriptions to the page flags:
66	
67	 0. LOCKED
68	    page is being locked for exclusive access, eg. by undergoing read/write IO
69	
70	 7. SLAB
71	    page is managed by the SLAB/SLOB/SLUB/SLQB kernel memory allocator
72	    When compound page is used, SLUB/SLQB will only set this flag on the head
73	    page; SLOB will not flag it at all.
74	
75	10. BUDDY
76	    a free memory block managed by the buddy system allocator
77	    The buddy system organizes free memory in blocks of various orders.
78	    An order N block has 2^N physically contiguous pages, with the BUDDY flag
79	    set for and _only_ for the first page.
80	
81	15. COMPOUND_HEAD
82	16. COMPOUND_TAIL
83	    A compound page with order N consists of 2^N physically contiguous pages.
84	    A compound page with order 2 takes the form of "HTTT", where H donates its
85	    head page and T donates its tail page(s).  The major consumers of compound
86	    pages are hugeTLB pages (Documentation/vm/hugetlbpage.txt), the SLUB etc.
87	    memory allocators and various device drivers. However in this interface,
88	    only huge/giga pages are made visible to end users.
89	17. HUGE
90	    this is an integral part of a HugeTLB page
91	
92	19. HWPOISON
93	    hardware detected memory corruption on this page: don't touch the data!
94	
95	20. NOPAGE
96	    no page frame exists at the requested address
97	
98	21. KSM
99	    identical memory pages dynamically shared between one or more processes
100	
101	22. THP
102	    contiguous pages which construct transparent hugepages
103	
104	    [IO related page flags]
105	 1. ERROR     IO error occurred
106	 3. UPTODATE  page has up-to-date data
107	              ie. for file backed page: (in-memory data revision >= on-disk one)
108	 4. DIRTY     page has been written to, hence contains new data
109	              ie. for file backed page: (in-memory data revision >  on-disk one)
110	 8. WRITEBACK page is being synced to disk
111	
112	    [LRU related page flags]
113	 5. LRU         page is in one of the LRU lists
114	 6. ACTIVE      page is in the active LRU list
115	18. UNEVICTABLE page is in the unevictable (non-)LRU list
116	                It is somehow pinned and not a candidate for LRU page reclaims,
117			eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments
118	 2. REFERENCED  page has been referenced since last LRU list enqueue/requeue
119	 9. RECLAIM     page will be reclaimed soon after its pageout IO completed
120	11. MMAP        a memory mapped page
121	12. ANON        a memory mapped page that is not part of a file
122	13. SWAPCACHE   page is mapped to swap space, ie. has an associated swap entry
123	14. SWAPBACKED  page is backed by swap/RAM
124	
125	The page-types tool in this directory can be used to query the above flags.
126	
127	Using pagemap to do something useful:
128	
129	The general procedure for using pagemap to find out about a process' memory
130	usage goes like this:
131	
132	 1. Read /proc/pid/maps to determine which parts of the memory space are
133	    mapped to what.
134	 2. Select the maps you are interested in -- all of them, or a particular
135	    library, or the stack or the heap, etc.
136	 3. Open /proc/pid/pagemap and seek to the pages you would like to examine.
137	 4. Read a u64 for each page from pagemap.
138	 5. Open /proc/kpagecount and/or /proc/kpageflags.  For each PFN you just
139	    read, seek to that entry in the file, and read the data you want.
140	
141	For example, to find the "unique set size" (USS), which is the amount of
142	memory that a process is using that is not shared with any other process,
143	you can go through every map in the process, find the PFNs, look those up
144	in kpagecount, and tally up the number of pages that are only referenced
145	once.
146	
147	Other notes:
148	
149	Reading from any of the files will return -EINVAL if you are not starting
150	the read on an 8-byte boundary (e.g., if you seeked an odd number of bytes
151	into the file), or if the size of the read is not a multiple of 8 bytes.
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