1 =============================
2 Examining Process Page Tables
3 =============================
4
5 pagemap is a new (as of 2.6.25) set of interfa
6 userspace programs to examine the page tables
7 reading files in ``/proc``.
8
9 There are four components to pagemap:
10
11 * ``/proc/pid/pagemap``. This file lets a us
12 physical frame each virtual page is mapped
13 value for each virtual page, containing the
14 ``fs/proc/task_mmu.c``, above pagemap_read)
15
16 * Bits 0-54 page frame number (PFN) if pr
17 * Bits 0-4 swap type if swapped
18 * Bits 5-54 swap offset if swapped
19 * Bit 55 pte is soft-dirty (see
20 Documentation/admin-guide/mm/soft-dirty.
21 * Bit 56 page exclusively mapped (sinc
22 * Bit 57 pte is uffd-wp write-protecte
23 Documentation/admin-guide/mm/userfaultfd
24 * Bits 58-60 zero
25 * Bit 61 page is file-page or shared-a
26 * Bit 62 page swapped
27 * Bit 63 page present
28
29 Since Linux 4.0 only users with the CAP_SYS
30 In 4.0 and 4.1 opens by unprivileged fail w
31 4.2 the PFN field is zeroed if the user doe
32 Reason: information about PFNs helps in exp
33
34 If the page is not present but in swap, the
35 encoding of the swap file number and the pa
36 swap. Unmapped pages return a null PFN. Thi
37 precisely which pages are mapped (or in swa
38 pages between processes.
39
40 Efficient users of this interface will use
41 determine which areas of memory are actuall
42 skip over unmapped regions.
43
44 * ``/proc/kpagecount``. This file contains a
45 times each page is mapped, indexed by PFN.
46
47 The page-types tool in the tools/mm directory
48 number of times a page is mapped.
49
50 * ``/proc/kpageflags``. This file contains a
51 page, indexed by PFN.
52
53 The flags are (from ``fs/proc/page.c``, abo
54
55 0. LOCKED
56 1. ERROR
57 2. REFERENCED
58 3. UPTODATE
59 4. DIRTY
60 5. LRU
61 6. ACTIVE
62 7. SLAB
63 8. WRITEBACK
64 9. RECLAIM
65 10. BUDDY
66 11. MMAP
67 12. ANON
68 13. SWAPCACHE
69 14. SWAPBACKED
70 15. COMPOUND_HEAD
71 16. COMPOUND_TAIL
72 17. HUGE
73 18. UNEVICTABLE
74 19. HWPOISON
75 20. NOPAGE
76 21. KSM
77 22. THP
78 23. OFFLINE
79 24. ZERO_PAGE
80 25. IDLE
81 26. PGTABLE
82
83 * ``/proc/kpagecgroup``. This file contains
84 memory cgroup each page is charged to, inde
85 CONFIG_MEMCG is set.
86
87 Short descriptions to the page flags
88 ====================================
89
90 0 - LOCKED
91 The page is being locked for exclusive acce
92 IO.
93 7 - SLAB
94 The page is managed by the SLAB/SLUB kernel
95 When compound page is used, either will onl
96 page.
97 10 - BUDDY
98 A free memory block managed by the buddy s
99 The buddy system organizes free memory in
100 An order N block has 2^N physically contig
101 set for and _only_ for the first page.
102 15 - COMPOUND_HEAD
103 A compound page with order N consists of 2
104 A compound page with order 2 takes the for
105 head page and T donates its tail page(s).
106 pages are hugeTLB pages (Documentation/adm
107 the SLUB etc. memory allocators and vario
108 However in this interface, only huge/giga
109 to end users.
110 16 - COMPOUND_TAIL
111 A compound page tail (see description abov
112 17 - HUGE
113 This is an integral part of a HugeTLB page
114 19 - HWPOISON
115 Hardware detected memory corruption on thi
116 20 - NOPAGE
117 No page frame exists at the requested addr
118 21 - KSM
119 Identical memory pages dynamically shared
120 22 - THP
121 Contiguous pages which construct THP of an
122 23 - OFFLINE
123 The page is logically offline.
124 24 - ZERO_PAGE
125 Zero page for pfn_zero or huge_zero page.
126 25 - IDLE
127 The page has not been accessed since it wa
128 Documentation/admin-guide/mm/idle_page_tra
129 Note that this flag may be stale in case t
130 a PTE. To make sure the flag is up-to-date
131 ``/sys/kernel/mm/page_idle/bitmap`` first.
132 26 - PGTABLE
133 The page is in use as a page table.
134
135 IO related page flags
136 ---------------------
137
138 1 - ERROR
139 IO error occurred.
140 3 - UPTODATE
141 The page has up-to-date data.
142 ie. for file backed page: (in-memory data r
143 4 - DIRTY
144 The page has been written to, hence contain
145 i.e. for file backed page: (in-memory data
146 8 - WRITEBACK
147 The page is being synced to disk.
148
149 LRU related page flags
150 ----------------------
151
152 5 - LRU
153 The page is in one of the LRU lists.
154 6 - ACTIVE
155 The page is in the active LRU list.
156 18 - UNEVICTABLE
157 The page is in the unevictable (non-)LRU li
158 not a candidate for LRU page reclaims, e.g.
159 shmctl(SHM_LOCK) and mlock() memory segment
160 2 - REFERENCED
161 The page has been referenced since last LRU
162 9 - RECLAIM
163 The page will be reclaimed soon after its p
164 11 - MMAP
165 A memory mapped page.
166 12 - ANON
167 A memory mapped page that is not part of a
168 13 - SWAPCACHE
169 The page is mapped to swap space, i.e. has
170 14 - SWAPBACKED
171 The page is backed by swap/RAM.
172
173 The page-types tool in the tools/mm directory
174 above flags.
175
176 Exceptions for Shared Memory
177 ============================
178
179 Page table entries for shared pages are cleare
180 swapped out. This makes swapped out pages indi
181 ones.
182
183 In kernel space, the swap location can still b
184 However, values stored only on the normal PTE
185 page is swapped out (i.e. SOFT_DIRTY).
186
187 In user space, whether the page is present, sw
188 the help of lseek and/or mincore system calls.
189
190 lseek() can differentiate between accessed pag
191 holes (none/non-allocated) by specifying the S
192 the pages are backed. For anonymous shared pag
193 ``/proc/pid/map_files/``.
194
195 mincore() can differentiate between pages in m
196 cache) and out of memory (swapped out or none/
197
198 Other notes
199 ===========
200
201 Reading from any of the files will return -EIN
202 the read on an 8-byte boundary (e.g., if you s
203 into the file), or if the size of the read is
204
205 Before Linux 3.11 pagemap bits 55-60 were used
206 always 12 at most architectures). Since Linux
207 after first clear of soft-dirty bits. Since Li
208 flags unconditionally.
209
210 Pagemap Scan IOCTL
211 ==================
212
213 The ``PAGEMAP_SCAN`` IOCTL on the pagemap file
214 clear the info about page table entries. The f
215 in this IOCTL:
216
217 - Scan the address range and get the memory ra
218 This is performed when the output buffer is
219 - Write-protect the pages. The ``PM_SCAN_WP_MA
220 the pages of interest. The ``PM_SCAN_CHECK_W
221 non-Async Write Protected pages are found. T
222 used with or without ``PM_SCAN_CHECK_WPASYNC
223 - Both of those operations can be combined int
224 get and write protect the pages as well.
225
226 Following flags about pages are currently supp
227
228 - ``PAGE_IS_WPALLOWED`` - Page has async-write
229 - ``PAGE_IS_WRITTEN`` - Page has been written
230 - ``PAGE_IS_FILE`` - Page is file backed
231 - ``PAGE_IS_PRESENT`` - Page is present in the
232 - ``PAGE_IS_SWAPPED`` - Page is in swapped
233 - ``PAGE_IS_PFNZERO`` - Page has zero PFN
234 - ``PAGE_IS_HUGE`` - Page is PMD-mapped THP or
235 - ``PAGE_IS_SOFT_DIRTY`` - Page is soft-dirty
236
237 The ``struct pm_scan_arg`` is used as the argu
238
239 1. The size of the ``struct pm_scan_arg`` mus
240 field. This field will be helpful in recog
241 are done later.
242 2. The flags can be specified in the ``flags`
243 and ``PM_SCAN_CHECK_WPASYNC`` are the only
244 operation is optionally performed dependin
245 provided or not.
246 3. The range is specified through ``start`` a
247 4. The walk can abort before visiting the com
248 can get full etc. The walk ending address
249 5. The output buffer of ``struct page_region`
250 ``vec`` and ``vec_len``.
251 6. The optional maximum requested pages are s
252 7. The masks are specified in ``category_mask
253 ``category_inverted`` and ``return_mask``.
254
255 Find pages which have been written and WP them
256
257 struct pm_scan_arg arg = {
258 .size = sizeof(arg),
259 .flags = PM_SCAN_CHECK_WPASYNC | PM_SCAN_CH
260 ..
261 .category_mask = PAGE_IS_WRITTEN,
262 .return_mask = PAGE_IS_WRITTEN,
263 };
264
265 Find pages which have been written, are file b
266 present or huge::
267
268 struct pm_scan_arg arg = {
269 .size = sizeof(arg),
270 .flags = 0,
271 ..
272 .category_mask = PAGE_IS_WRITTEN | PAGE_IS_
273 .category_inverted = PAGE_IS_SWAPPED,
274 .category_anyof_mask = PAGE_IS_PRESENT | PA
275 .return_mask = PAGE_IS_WRITTEN | PAGE_IS_SW
276 PAGE_IS_PRESENT | PAGE_IS_HU
277 };
278
279 The ``PAGE_IS_WRITTEN`` flag can be considered
280 of soft-dirty flag. It doesn't get affected by
281 the user can find the true soft-dirty pages in
282 still be extra dirty pages reported for THP or
283
284 "PAGE_IS_WRITTEN" category is used with uffd w
285 implement memory dirty tracking in userspace:
286
287 1. The userfaultfd file descriptor is created
288 2. The ``UFFD_FEATURE_WP_UNPOPULATED`` and ``
289 are set by ``UFFDIO_API`` IOCTL.
290 3. The memory range is registered with ``UFFD
291 through ``UFFDIO_REGISTER`` IOCTL.
292 4. Then any part of the registered memory or
293 be write protected using ``PAGEMAP_SCAN``
294 or the ``UFFDIO_WRITEPROTECT`` IOCTL can b
295 same operation. The former is better in te
296 5. Now the ``PAGEMAP_SCAN`` IOCTL can be used
297 have been written to since they were last
298 the pages as well.
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