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2 UNEVICTABLE LRU INFRASTRUCTURE
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9 (*) The Unevictable LRU
11 - The unevictable page list.
12 - Memory control group interaction.
13 - Marking address spaces unevictable.
14 - Detecting Unevictable Pages.
15 - vmscan's handling of unevictable pages.
21 - mlock()/mlockall() system call handling.
22 - Filtering special vmas.
23 - munlock()/munlockall() system call handling.
24 - Migrating mlocked pages.
25 - Compacting mlocked pages.
26 - mmap(MAP_LOCKED) system call handling.
27 - munmap()/exit()/exec() system call handling.
29 - try_to_munlock() reverse map scan.
30 - Page reclaim in shrink_*_list().
37 This document describes the Linux memory manager's "Unevictable LRU"
38 infrastructure and the use of this to manage several types of "unevictable"
41 The document attempts to provide the overall rationale behind this mechanism
42 and the rationale for some of the design decisions that drove the
43 implementation. The latter design rationale is discussed in the context of an
44 implementation description. Admittedly, one can obtain the implementation
45 details - the "what does it do?" - by reading the code. One hopes that the
46 descriptions below add value by provide the answer to "why does it do that?".
53 The Unevictable LRU facility adds an additional LRU list to track unevictable
54 pages and to hide these pages from vmscan. This mechanism is based on a patch
55 by Larry Woodman of Red Hat to address several scalability problems with page
56 reclaim in Linux. The problems have been observed at customer sites on large
57 memory x86_64 systems.
59 To illustrate this with an example, a non-NUMA x86_64 platform with 128GB of
60 main memory will have over 32 million 4k pages in a single zone. When a large
61 fraction of these pages are not evictable for any reason [see below], vmscan
62 will spend a lot of time scanning the LRU lists looking for the small fraction
63 of pages that are evictable. This can result in a situation where all CPUs are
64 spending 100% of their time in vmscan for hours or days on end, with the system
65 completely unresponsive.
67 The unevictable list addresses the following classes of unevictable pages:
69 (*) Those owned by ramfs.
71 (*) Those mapped into SHM_LOCK'd shared memory regions.
73 (*) Those mapped into VM_LOCKED [mlock()ed] VMAs.
75 The infrastructure may also be able to handle other conditions that make pages
76 unevictable, either by definition or by circumstance, in the future.
79 THE UNEVICTABLE PAGE LIST
80 -------------------------
82 The Unevictable LRU infrastructure consists of an additional, per-zone, LRU list
83 called the "unevictable" list and an associated page flag, PG_unevictable, to
84 indicate that the page is being managed on the unevictable list.
86 The PG_unevictable flag is analogous to, and mutually exclusive with, the
87 PG_active flag in that it indicates on which LRU list a page resides when
90 The Unevictable LRU infrastructure maintains unevictable pages on an additional
91 LRU list for a few reasons:
93 (1) We get to "treat unevictable pages just like we treat other pages in the
94 system - which means we get to use the same code to manipulate them, the
95 same code to isolate them (for migrate, etc.), the same code to keep track
96 of the statistics, etc..." [Rik van Riel]
98 (2) We want to be able to migrate unevictable pages between nodes for memory
99 defragmentation, workload management and memory hotplug. The linux kernel
100 can only migrate pages that it can successfully isolate from the LRU
101 lists. If we were to maintain pages elsewhere than on an LRU-like list,
102 where they can be found by isolate_lru_page(), we would prevent their
103 migration, unless we reworked migration code to find the unevictable pages
107 The unevictable list does not differentiate between file-backed and anonymous,
108 swap-backed pages. This differentiation is only important while the pages are,
111 The unevictable list benefits from the "arrayification" of the per-zone LRU
112 lists and statistics originally proposed and posted by Christoph Lameter.
114 The unevictable list does not use the LRU pagevec mechanism. Rather,
115 unevictable pages are placed directly on the page's zone's unevictable list
116 under the zone lru_lock. This allows us to prevent the stranding of pages on
117 the unevictable list when one task has the page isolated from the LRU and other
118 tasks are changing the "evictability" state of the page.
121 MEMORY CONTROL GROUP INTERACTION
122 --------------------------------
124 The unevictable LRU facility interacts with the memory control group [aka
125 memory controller; see Documentation/cgroups/memory.txt] by extending the
128 The memory controller data structure automatically gets a per-zone unevictable
129 list as a result of the "arrayification" of the per-zone LRU lists (one per
130 lru_list enum element). The memory controller tracks the movement of pages to
131 and from the unevictable list.
133 When a memory control group comes under memory pressure, the controller will
134 not attempt to reclaim pages on the unevictable list. This has a couple of
137 (1) Because the pages are "hidden" from reclaim on the unevictable list, the
138 reclaim process can be more efficient, dealing only with pages that have a
139 chance of being reclaimed.
141 (2) On the other hand, if too many of the pages charged to the control group
142 are unevictable, the evictable portion of the working set of the tasks in
143 the control group may not fit into the available memory. This can cause
144 the control group to thrash or to OOM-kill tasks.
147 MARKING ADDRESS SPACES UNEVICTABLE
148 ----------------------------------
150 For facilities such as ramfs none of the pages attached to the address space
151 may be evicted. To prevent eviction of any such pages, the AS_UNEVICTABLE
152 address space flag is provided, and this can be manipulated by a filesystem
153 using a number of wrapper functions:
155 (*) void mapping_set_unevictable(struct address_space *mapping);
157 Mark the address space as being completely unevictable.
159 (*) void mapping_clear_unevictable(struct address_space *mapping);
161 Mark the address space as being evictable.
163 (*) int mapping_unevictable(struct address_space *mapping);
165 Query the address space, and return true if it is completely
168 These are currently used in two places in the kernel:
170 (1) By ramfs to mark the address spaces of its inodes when they are created,
171 and this mark remains for the life of the inode.
173 (2) By SYSV SHM to mark SHM_LOCK'd address spaces until SHM_UNLOCK is called.
175 Note that SHM_LOCK is not required to page in the locked pages if they're
176 swapped out; the application must touch the pages manually if it wants to
177 ensure they're in memory.
180 DETECTING UNEVICTABLE PAGES
181 ---------------------------
183 The function page_evictable() in vmscan.c determines whether a page is
184 evictable or not using the query function outlined above [see section "Marking
185 address spaces unevictable"] to check the AS_UNEVICTABLE flag.
187 For address spaces that are so marked after being populated (as SHM regions
188 might be), the lock action (eg: SHM_LOCK) can be lazy, and need not populate
189 the page tables for the region as does, for example, mlock(), nor need it make
190 any special effort to push any pages in the SHM_LOCK'd area to the unevictable
191 list. Instead, vmscan will do this if and when it encounters the pages during
194 On an unlock action (such as SHM_UNLOCK), the unlocker (eg: shmctl()) must scan
195 the pages in the region and "rescue" them from the unevictable list if no other
196 condition is keeping them unevictable. If an unevictable region is destroyed,
197 the pages are also "rescued" from the unevictable list in the process of
200 page_evictable() also checks for mlocked pages by testing an additional page
201 flag, PG_mlocked (as wrapped by PageMlocked()), which is set when a page is
202 faulted into a VM_LOCKED vma, or found in a vma being VM_LOCKED.
205 VMSCAN'S HANDLING OF UNEVICTABLE PAGES
206 --------------------------------------
208 If unevictable pages are culled in the fault path, or moved to the unevictable
209 list at mlock() or mmap() time, vmscan will not encounter the pages until they
210 have become evictable again (via munlock() for example) and have been "rescued"
211 from the unevictable list. However, there may be situations where we decide,
212 for the sake of expediency, to leave a unevictable page on one of the regular
213 active/inactive LRU lists for vmscan to deal with. vmscan checks for such
214 pages in all of the shrink_{active|inactive|page}_list() functions and will
215 "cull" such pages that it encounters: that is, it diverts those pages to the
216 unevictable list for the zone being scanned.
218 There may be situations where a page is mapped into a VM_LOCKED VMA, but the
219 page is not marked as PG_mlocked. Such pages will make it all the way to
220 shrink_page_list() where they will be detected when vmscan walks the reverse
221 map in try_to_unmap(). If try_to_unmap() returns SWAP_MLOCK,
222 shrink_page_list() will cull the page at that point.
224 To "cull" an unevictable page, vmscan simply puts the page back on the LRU list
225 using putback_lru_page() - the inverse operation to isolate_lru_page() - after
226 dropping the page lock. Because the condition which makes the page unevictable
227 may change once the page is unlocked, putback_lru_page() will recheck the
228 unevictable state of a page that it places on the unevictable list. If the
229 page has become unevictable, putback_lru_page() removes it from the list and
230 retries, including the page_unevictable() test. Because such a race is a rare
231 event and movement of pages onto the unevictable list should be rare, these
232 extra evictabilty checks should not occur in the majority of calls to
240 The unevictable page list is also useful for mlock(), in addition to ramfs and
241 SYSV SHM. Note that mlock() is only available in CONFIG_MMU=y situations; in
242 NOMMU situations, all mappings are effectively mlocked.
248 The "Unevictable mlocked Pages" infrastructure is based on work originally
249 posted by Nick Piggin in an RFC patch entitled "mm: mlocked pages off LRU".
250 Nick posted his patch as an alternative to a patch posted by Christoph Lameter
251 to achieve the same objective: hiding mlocked pages from vmscan.
253 In Nick's patch, he used one of the struct page LRU list link fields as a count
254 of VM_LOCKED VMAs that map the page. This use of the link field for a count
255 prevented the management of the pages on an LRU list, and thus mlocked pages
256 were not migratable as isolate_lru_page() could not find them, and the LRU list
257 link field was not available to the migration subsystem.
259 Nick resolved this by putting mlocked pages back on the lru list before
260 attempting to isolate them, thus abandoning the count of VM_LOCKED VMAs. When
261 Nick's patch was integrated with the Unevictable LRU work, the count was
262 replaced by walking the reverse map to determine whether any VM_LOCKED VMAs
263 mapped the page. More on this below.
269 mlocked pages - pages mapped into a VM_LOCKED VMA - are a class of unevictable
270 pages. When such a page has been "noticed" by the memory management subsystem,
271 the page is marked with the PG_mlocked flag. This can be manipulated using the
272 PageMlocked() functions.
274 A PG_mlocked page will be placed on the unevictable list when it is added to
275 the LRU. Such pages can be "noticed" by memory management in several places:
277 (1) in the mlock()/mlockall() system call handlers;
279 (2) in the mmap() system call handler when mmapping a region with the
282 (3) mmapping a region in a task that has called mlockall() with the MCL_FUTURE
285 (4) in the fault path, if mlocked pages are "culled" in the fault path,
286 and when a VM_LOCKED stack segment is expanded; or
288 (5) as mentioned above, in vmscan:shrink_page_list() when attempting to
289 reclaim a page in a VM_LOCKED VMA via try_to_unmap()
291 all of which result in the VM_LOCKED flag being set for the VMA if it doesn't
294 mlocked pages become unlocked and rescued from the unevictable list when:
296 (1) mapped in a range unlocked via the munlock()/munlockall() system calls;
298 (2) munmap()'d out of the last VM_LOCKED VMA that maps the page, including
299 unmapping at task exit;
301 (3) when the page is truncated from the last VM_LOCKED VMA of an mmapped file;
304 (4) before a page is COW'd in a VM_LOCKED VMA.
307 mlock()/mlockall() SYSTEM CALL HANDLING
308 ---------------------------------------
310 Both [do_]mlock() and [do_]mlockall() system call handlers call mlock_fixup()
311 for each VMA in the range specified by the call. In the case of mlockall(),
312 this is the entire active address space of the task. Note that mlock_fixup()
313 is used for both mlocking and munlocking a range of memory. A call to mlock()
314 an already VM_LOCKED VMA, or to munlock() a VMA that is not VM_LOCKED is
315 treated as a no-op, and mlock_fixup() simply returns.
317 If the VMA passes some filtering as described in "Filtering Special Vmas"
318 below, mlock_fixup() will attempt to merge the VMA with its neighbors or split
319 off a subset of the VMA if the range does not cover the entire VMA. Once the
320 VMA has been merged or split or neither, mlock_fixup() will call
321 populate_vma_page_range() to fault in the pages via get_user_pages() and to
322 mark the pages as mlocked via mlock_vma_page().
324 Note that the VMA being mlocked might be mapped with PROT_NONE. In this case,
325 get_user_pages() will be unable to fault in the pages. That's okay. If pages
326 do end up getting faulted into this VM_LOCKED VMA, we'll handle them in the
327 fault path or in vmscan.
329 Also note that a page returned by get_user_pages() could be truncated or
330 migrated out from under us, while we're trying to mlock it. To detect this,
331 populate_vma_page_range() checks page_mapping() after acquiring the page lock.
332 If the page is still associated with its mapping, we'll go ahead and call
333 mlock_vma_page(). If the mapping is gone, we just unlock the page and move on.
334 In the worst case, this will result in a page mapped in a VM_LOCKED VMA
335 remaining on a normal LRU list without being PageMlocked(). Again, vmscan will
336 detect and cull such pages.
338 mlock_vma_page() will call TestSetPageMlocked() for each page returned by
339 get_user_pages(). We use TestSetPageMlocked() because the page might already
340 be mlocked by another task/VMA and we don't want to do extra work. We
341 especially do not want to count an mlocked page more than once in the
342 statistics. If the page was already mlocked, mlock_vma_page() need do nothing
345 If the page was NOT already mlocked, mlock_vma_page() attempts to isolate the
346 page from the LRU, as it is likely on the appropriate active or inactive list
347 at that time. If the isolate_lru_page() succeeds, mlock_vma_page() will put
348 back the page - by calling putback_lru_page() - which will notice that the page
349 is now mlocked and divert the page to the zone's unevictable list. If
350 mlock_vma_page() is unable to isolate the page from the LRU, vmscan will handle
351 it later if and when it attempts to reclaim the page.
354 FILTERING SPECIAL VMAS
355 ----------------------
357 mlock_fixup() filters several classes of "special" VMAs:
359 1) VMAs with VM_IO or VM_PFNMAP set are skipped entirely. The pages behind
360 these mappings are inherently pinned, so we don't need to mark them as
361 mlocked. In any case, most of the pages have no struct page in which to so
362 mark the page. Because of this, get_user_pages() will fail for these VMAs,
363 so there is no sense in attempting to visit them.
365 2) VMAs mapping hugetlbfs page are already effectively pinned into memory. We
366 neither need nor want to mlock() these pages. However, to preserve the
367 prior behavior of mlock() - before the unevictable/mlock changes -
368 mlock_fixup() will call make_pages_present() in the hugetlbfs VMA range to
369 allocate the huge pages and populate the ptes.
371 3) VMAs with VM_DONTEXPAND are generally userspace mappings of kernel pages,
372 such as the VDSO page, relay channel pages, etc. These pages
373 are inherently unevictable and are not managed on the LRU lists.
374 mlock_fixup() treats these VMAs the same as hugetlbfs VMAs. It calls
375 make_pages_present() to populate the ptes.
377 Note that for all of these special VMAs, mlock_fixup() does not set the
378 VM_LOCKED flag. Therefore, we won't have to deal with them later during
379 munlock(), munmap() or task exit. Neither does mlock_fixup() account these
380 VMAs against the task's "locked_vm".
383 munlock()/munlockall() SYSTEM CALL HANDLING
384 -------------------------------------------
386 The munlock() and munlockall() system calls are handled by the same functions -
387 do_mlock[all]() - as the mlock() and mlockall() system calls with the unlock vs
388 lock operation indicated by an argument. So, these system calls are also
389 handled by mlock_fixup(). Again, if called for an already munlocked VMA,
390 mlock_fixup() simply returns. Because of the VMA filtering discussed above,
391 VM_LOCKED will not be set in any "special" VMAs. So, these VMAs will be
394 If the VMA is VM_LOCKED, mlock_fixup() again attempts to merge or split off the
395 specified range. The range is then munlocked via the function
396 populate_vma_page_range() - the same function used to mlock a VMA range -
397 passing a flag to indicate that munlock() is being performed.
399 Because the VMA access protections could have been changed to PROT_NONE after
400 faulting in and mlocking pages, get_user_pages() was unreliable for visiting
401 these pages for munlocking. Because we don't want to leave pages mlocked,
402 get_user_pages() was enhanced to accept a flag to ignore the permissions when
403 fetching the pages - all of which should be resident as a result of previous
406 For munlock(), populate_vma_page_range() unlocks individual pages by calling
407 munlock_vma_page(). munlock_vma_page() unconditionally clears the PG_mlocked
408 flag using TestClearPageMlocked(). As with mlock_vma_page(),
409 munlock_vma_page() use the Test*PageMlocked() function to handle the case where
410 the page might have already been unlocked by another task. If the page was
411 mlocked, munlock_vma_page() updates that zone statistics for the number of
412 mlocked pages. Note, however, that at this point we haven't checked whether
413 the page is mapped by other VM_LOCKED VMAs.
415 We can't call try_to_munlock(), the function that walks the reverse map to
416 check for other VM_LOCKED VMAs, without first isolating the page from the LRU.
417 try_to_munlock() is a variant of try_to_unmap() and thus requires that the page
418 not be on an LRU list [more on these below]. However, the call to
419 isolate_lru_page() could fail, in which case we couldn't try_to_munlock(). So,
420 we go ahead and clear PG_mlocked up front, as this might be the only chance we
421 have. If we can successfully isolate the page, we go ahead and
422 try_to_munlock(), which will restore the PG_mlocked flag and update the zone
423 page statistics if it finds another VMA holding the page mlocked. If we fail
424 to isolate the page, we'll have left a potentially mlocked page on the LRU.
425 This is fine, because we'll catch it later if and if vmscan tries to reclaim
426 the page. This should be relatively rare.
429 MIGRATING MLOCKED PAGES
430 -----------------------
432 A page that is being migrated has been isolated from the LRU lists and is held
433 locked across unmapping of the page, updating the page's address space entry
434 and copying the contents and state, until the page table entry has been
435 replaced with an entry that refers to the new page. Linux supports migration
436 of mlocked pages and other unevictable pages. This involves simply moving the
437 PG_mlocked and PG_unevictable states from the old page to the new page.
439 Note that page migration can race with mlocking or munlocking of the same page.
440 This has been discussed from the mlock/munlock perspective in the respective
441 sections above. Both processes (migration and m[un]locking) hold the page
442 locked. This provides the first level of synchronization. Page migration
443 zeros out the page_mapping of the old page before unlocking it, so m[un]lock
444 can skip these pages by testing the page mapping under page lock.
446 To complete page migration, we place the new and old pages back onto the LRU
447 after dropping the page lock. The "unneeded" page - old page on success, new
448 page on failure - will be freed when the reference count held by the migration
449 process is released. To ensure that we don't strand pages on the unevictable
450 list because of a race between munlock and migration, page migration uses the
451 putback_lru_page() function to add migrated pages back to the LRU.
454 COMPACTING MLOCKED PAGES
455 ------------------------
457 The unevictable LRU can be scanned for compactable regions and the default
458 behavior is to do so. /proc/sys/vm/compact_unevictable_allowed controls
459 this behavior (see Documentation/sysctl/vm.txt). Once scanning of the
460 unevictable LRU is enabled, the work of compaction is mostly handled by
461 the page migration code and the same work flow as described in MIGRATING
462 MLOCKED PAGES will apply.
465 mmap(MAP_LOCKED) SYSTEM CALL HANDLING
466 -------------------------------------
468 In addition the mlock()/mlockall() system calls, an application can request
469 that a region of memory be mlocked supplying the MAP_LOCKED flag to the mmap()
470 call. There is one important and subtle difference here, though. mmap() + mlock()
471 will fail if the range cannot be faulted in (e.g. because mm_populate fails)
472 and returns with ENOMEM while mmap(MAP_LOCKED) will not fail. The mmaped
473 area will still have properties of the locked area - aka. pages will not get
474 swapped out - but major page faults to fault memory in might still happen.
476 Furthermore, any mmap() call or brk() call that expands the heap by a
477 task that has previously called mlockall() with the MCL_FUTURE flag will result
478 in the newly mapped memory being mlocked. Before the unevictable/mlock
479 changes, the kernel simply called make_pages_present() to allocate pages and
480 populate the page table.
482 To mlock a range of memory under the unevictable/mlock infrastructure, the
483 mmap() handler and task address space expansion functions call
484 populate_vma_page_range() specifying the vma and the address range to mlock.
486 The callers of populate_vma_page_range() will have already added the memory range
487 to be mlocked to the task's "locked_vm". To account for filtered VMAs,
488 populate_vma_page_range() returns the number of pages NOT mlocked. All of the
489 callers then subtract a non-negative return value from the task's locked_vm. A
490 negative return value represent an error - for example, from get_user_pages()
491 attempting to fault in a VMA with PROT_NONE access. In this case, we leave the
492 memory range accounted as locked_vm, as the protections could be changed later
493 and pages allocated into that region.
496 munmap()/exit()/exec() SYSTEM CALL HANDLING
497 -------------------------------------------
499 When unmapping an mlocked region of memory, whether by an explicit call to
500 munmap() or via an internal unmap from exit() or exec() processing, we must
501 munlock the pages if we're removing the last VM_LOCKED VMA that maps the pages.
502 Before the unevictable/mlock changes, mlocking did not mark the pages in any
503 way, so unmapping them required no processing.
505 To munlock a range of memory under the unevictable/mlock infrastructure, the
506 munmap() handler and task address space call tear down function
507 munlock_vma_pages_all(). The name reflects the observation that one always
508 specifies the entire VMA range when munlock()ing during unmap of a region.
509 Because of the VMA filtering when mlocking() regions, only "normal" VMAs that
510 actually contain mlocked pages will be passed to munlock_vma_pages_all().
512 munlock_vma_pages_all() clears the VM_LOCKED VMA flag and, like mlock_fixup()
513 for the munlock case, calls __munlock_vma_pages_range() to walk the page table
514 for the VMA's memory range and munlock_vma_page() each resident page mapped by
515 the VMA. This effectively munlocks the page, only if this is the last
516 VM_LOCKED VMA that maps the page.
522 Pages can, of course, be mapped into multiple VMAs. Some of these VMAs may
523 have VM_LOCKED flag set. It is possible for a page mapped into one or more
524 VM_LOCKED VMAs not to have the PG_mlocked flag set and therefore reside on one
525 of the active or inactive LRU lists. This could happen if, for example, a task
526 in the process of munlocking the page could not isolate the page from the LRU.
527 As a result, vmscan/shrink_page_list() might encounter such a page as described
528 in section "vmscan's handling of unevictable pages". To handle this situation,
529 try_to_unmap() checks for VM_LOCKED VMAs while it is walking a page's reverse
532 try_to_unmap() is always called, by either vmscan for reclaim or for page
533 migration, with the argument page locked and isolated from the LRU. Separate
534 functions handle anonymous and mapped file and KSM pages, as these types of
535 pages have different reverse map lookup mechanisms, with different locking.
536 In each case, whether rmap_walk_anon() or rmap_walk_file() or rmap_walk_ksm(),
537 it will call try_to_unmap_one() for every VMA which might contain the page.
539 When trying to reclaim, if try_to_unmap_one() finds the page in a VM_LOCKED
540 VMA, it will then mlock the page via mlock_vma_page() instead of unmapping it,
541 and return SWAP_MLOCK to indicate that the page is unevictable: and the scan
544 mlock_vma_page() is called while holding the page table's lock (in addition
545 to the page lock, and the rmap lock): to serialize against concurrent mlock or
546 munlock or munmap system calls, mm teardown (munlock_vma_pages_all), reclaim,
547 holepunching, and truncation of file pages and their anonymous COWed pages.
550 try_to_munlock() REVERSE MAP SCAN
551 ---------------------------------
553 [!] TODO/FIXME: a better name might be page_mlocked() - analogous to the
554 page_referenced() reverse map walker.
556 When munlock_vma_page() [see section "munlock()/munlockall() System Call
557 Handling" above] tries to munlock a page, it needs to determine whether or not
558 the page is mapped by any VM_LOCKED VMA without actually attempting to unmap
559 all PTEs from the page. For this purpose, the unevictable/mlock infrastructure
560 introduced a variant of try_to_unmap() called try_to_munlock().
562 try_to_munlock() calls the same functions as try_to_unmap() for anonymous and
563 mapped file and KSM pages with a flag argument specifying unlock versus unmap
564 processing. Again, these functions walk the respective reverse maps looking
565 for VM_LOCKED VMAs. When such a VMA is found, as in the try_to_unmap() case,
566 the functions mlock the page via mlock_vma_page() and return SWAP_MLOCK. This
567 undoes the pre-clearing of the page's PG_mlocked done by munlock_vma_page.
569 Note that try_to_munlock()'s reverse map walk must visit every VMA in a page's
570 reverse map to determine that a page is NOT mapped into any VM_LOCKED VMA.
571 However, the scan can terminate when it encounters a VM_LOCKED VMA.
572 Although try_to_munlock() might be called a great many times when munlocking a
573 large region or tearing down a large address space that has been mlocked via
574 mlockall(), overall this is a fairly rare event.
577 PAGE RECLAIM IN shrink_*_list()
578 -------------------------------
580 shrink_active_list() culls any obviously unevictable pages - i.e.
581 !page_evictable(page) - diverting these to the unevictable list.
582 However, shrink_active_list() only sees unevictable pages that made it onto the
583 active/inactive lru lists. Note that these pages do not have PageUnevictable
584 set - otherwise they would be on the unevictable list and shrink_active_list
585 would never see them.
587 Some examples of these unevictable pages on the LRU lists are:
589 (1) ramfs pages that have been placed on the LRU lists when first allocated.
591 (2) SHM_LOCK'd shared memory pages. shmctl(SHM_LOCK) does not attempt to
592 allocate or fault in the pages in the shared memory region. This happens
593 when an application accesses the page the first time after SHM_LOCK'ing
596 (3) mlocked pages that could not be isolated from the LRU and moved to the
597 unevictable list in mlock_vma_page().
599 shrink_inactive_list() also diverts any unevictable pages that it finds on the
600 inactive lists to the appropriate zone's unevictable list.
602 shrink_inactive_list() should only see SHM_LOCK'd pages that became SHM_LOCK'd
603 after shrink_active_list() had moved them to the inactive list, or pages mapped
604 into VM_LOCKED VMAs that munlock_vma_page() couldn't isolate from the LRU to
605 recheck via try_to_munlock(). shrink_inactive_list() won't notice the latter,
606 but will pass on to shrink_page_list().
608 shrink_page_list() again culls obviously unevictable pages that it could
609 encounter for similar reason to shrink_inactive_list(). Pages mapped into
610 VM_LOCKED VMAs but without PG_mlocked set will make it all the way to
611 try_to_unmap(). shrink_page_list() will divert them to the unevictable list
612 when try_to_unmap() returns SWAP_MLOCK, as discussed above.