2 This document describes the Linux memory management "Unevictable LRU"
3 infrastructure and the use of this infrastructure to manage several types
4 of "unevictable" pages. The document attempts to provide the overall
5 rationale behind this mechanism and the rationale for some of the design
6 decisions that drove the implementation. The latter design rationale is
7 discussed in the context of an implementation description. Admittedly, one
8 can obtain the implementation details--the "what does it do?"--by reading the
9 code. One hopes that the descriptions below add value by provide the answer
10 to "why does it do that?".
12 Unevictable LRU Infrastructure:
14 The Unevictable LRU adds an additional LRU list to track unevictable pages
15 and to hide these pages from vmscan. This mechanism is based on a patch by
16 Larry Woodman of Red Hat to address several scalability problems with page
17 reclaim in Linux. The problems have been observed at customer sites on large
18 memory x86_64 systems. For example, a non-numal x86_64 platform with 128GB
19 of main memory will have over 32 million 4k pages in a single zone. When a
20 large fraction of these pages are not evictable for any reason [see below],
21 vmscan will spend a lot of time scanning the LRU lists looking for the small
22 fraction of pages that are evictable. This can result in a situation where
23 all cpus are spending 100% of their time in vmscan for hours or days on end,
24 with the system completely unresponsive.
26 The Unevictable LRU infrastructure addresses the following classes of
30 + page mapped into SHM_LOCKed shared memory regions
31 + page mapped into VM_LOCKED [mlock()ed] vmas
33 The infrastructure might be able to handle other conditions that make pages
34 unevictable, either by definition or by circumstance, in the future.
37 The Unevictable LRU List
39 The Unevictable LRU infrastructure consists of an additional, per-zone, LRU list
40 called the "unevictable" list and an associated page flag, PG_unevictable, to
41 indicate that the page is being managed on the unevictable list. The
42 PG_unevictable flag is analogous to, and mutually exclusive with, the PG_active
43 flag in that it indicates on which LRU list a page resides when PG_lru is set.
44 The unevictable LRU list is source configurable based on the UNEVICTABLE_LRU
47 The Unevictable LRU infrastructure maintains unevictable pages on an additional
48 LRU list for a few reasons:
50 1) We get to "treat unevictable pages just like we treat other pages in the
51 system, which means we get to use the same code to manipulate them, the
52 same code to isolate them (for migrate, etc.), the same code to keep track
53 of the statistics, etc..." [Rik van Riel]
55 2) We want to be able to migrate unevictable pages between nodes--for memory
56 defragmentation, workload management and memory hotplug. The linux kernel
57 can only migrate pages that it can successfully isolate from the lru lists.
58 If we were to maintain pages elsewise than on an lru-like list, where they
59 can be found by isolate_lru_page(), we would prevent their migration, unless
60 we reworked migration code to find the unevictable pages.
63 The unevictable LRU list does not differentiate between file backed and swap
64 backed [anon] pages. This differentiation is only important while the pages
65 are, in fact, evictable.
67 The unevictable LRU list benefits from the "arrayification" of the per-zone
68 LRU lists and statistics originally proposed and posted by Christoph Lameter.
70 The unevictable list does not use the lru pagevec mechanism. Rather,
71 unevictable pages are placed directly on the page's zone's unevictable
72 list under the zone lru_lock. The reason for this is to prevent stranding
73 of pages on the unevictable list when one task has the page isolated from the
74 lru and other tasks are changing the "evictability" state of the page.
77 Unevictable LRU and Memory Controller Interaction
79 The memory controller data structure automatically gets a per zone unevictable
80 lru list as a result of the "arrayification" of the per-zone LRU lists. The
81 memory controller tracks the movement of pages to and from the unevictable list.
82 When a memory control group comes under memory pressure, the controller will
83 not attempt to reclaim pages on the unevictable list. This has a couple of
84 effects. Because the pages are "hidden" from reclaim on the unevictable list,
85 the reclaim process can be more efficient, dealing only with pages that have
86 a chance of being reclaimed. On the other hand, if too many of the pages
87 charged to the control group are unevictable, the evictable portion of the
88 working set of the tasks in the control group may not fit into the available
89 memory. This can cause the control group to thrash or to oom-kill tasks.
92 Unevictable LRU: Detecting Unevictable Pages
94 The function page_evictable(page, vma) in vmscan.c determines whether a
95 page is evictable or not. For ramfs pages and pages in SHM_LOCKed regions,
96 page_evictable() tests a new address space flag, AS_UNEVICTABLE, in the page's
97 address space using a wrapper function. Wrapper functions are used to set,
98 clear and test the flag to reduce the requirement for #ifdef's throughout the
99 source code. AS_UNEVICTABLE is set on ramfs inode/mapping when it is created.
100 This flag remains for the life of the inode.
102 For shared memory regions, AS_UNEVICTABLE is set when an application
103 successfully SHM_LOCKs the region and is removed when the region is
104 SHM_UNLOCKed. Note that shmctl(SHM_LOCK, ...) does not populate the page
105 tables for the region as does, for example, mlock(). So, we make no special
106 effort to push any pages in the SHM_LOCKed region to the unevictable list.
107 Vmscan will do this when/if it encounters the pages during reclaim. On
108 SHM_UNLOCK, shmctl() scans the pages in the region and "rescues" them from the
109 unevictable list if no other condition keeps them unevictable. If a SHM_LOCKed
110 region is destroyed, the pages are also "rescued" from the unevictable list in
111 the process of freeing them.
113 page_evictable() detects mlock()ed pages by testing an additional page flag,
114 PG_mlocked via the PageMlocked() wrapper. If the page is NOT mlocked, and a
115 non-NULL vma is supplied, page_evictable() will check whether the vma is
116 VM_LOCKED via is_mlocked_vma(). is_mlocked_vma() will SetPageMlocked() and
117 update the appropriate statistics if the vma is VM_LOCKED. This method allows
118 efficient "culling" of pages in the fault path that are being faulted in to
122 Unevictable Pages and Vmscan [shrink_*_list()]
124 If unevictable pages are culled in the fault path, or moved to the unevictable
125 list at mlock() or mmap() time, vmscan will never encounter the pages until
126 they have become evictable again, for example, via munlock() and have been
127 "rescued" from the unevictable list. However, there may be situations where we
128 decide, for the sake of expediency, to leave a unevictable page on one of the
129 regular active/inactive LRU lists for vmscan to deal with. Vmscan checks for
130 such pages in all of the shrink_{active|inactive|page}_list() functions and
131 will "cull" such pages that it encounters--that is, it diverts those pages to
132 the unevictable list for the zone being scanned.
134 There may be situations where a page is mapped into a VM_LOCKED vma, but the
135 page is not marked as PageMlocked. Such pages will make it all the way to
136 shrink_page_list() where they will be detected when vmscan walks the reverse
137 map in try_to_unmap(). If try_to_unmap() returns SWAP_MLOCK, shrink_page_list()
138 will cull the page at that point.
140 To "cull" an unevictable page, vmscan simply puts the page back on the lru
141 list using putback_lru_page()--the inverse operation to isolate_lru_page()--
142 after dropping the page lock. Because the condition which makes the page
143 unevictable may change once the page is unlocked, putback_lru_page() will
144 recheck the unevictable state of a page that it places on the unevictable lru
145 list. If the page has become unevictable, putback_lru_page() removes it from
146 the list and retries, including the page_unevictable() test. Because such a
147 race is a rare event and movement of pages onto the unevictable list should be
148 rare, these extra evictabilty checks should not occur in the majority of calls
149 to putback_lru_page().
152 Mlocked Page: Prior Work
154 The "Unevictable Mlocked Pages" infrastructure is based on work originally
155 posted by Nick Piggin in an RFC patch entitled "mm: mlocked pages off LRU".
156 Nick posted his patch as an alternative to a patch posted by Christoph
157 Lameter to achieve the same objective--hiding mlocked pages from vmscan.
158 In Nick's patch, he used one of the struct page lru list link fields as a count
159 of VM_LOCKED vmas that map the page. This use of the link field for a count
160 prevented the management of the pages on an LRU list. Thus, mlocked pages were
161 not migratable as isolate_lru_page() could not find them and the lru list link
162 field was not available to the migration subsystem. Nick resolved this by
163 putting mlocked pages back on the lru list before attempting to isolate them,
164 thus abandoning the count of VM_LOCKED vmas. When Nick's patch was integrated
165 with the Unevictable LRU work, the count was replaced by walking the reverse
166 map to determine whether any VM_LOCKED vmas mapped the page. More on this
170 Mlocked Pages: Basic Management
172 Mlocked pages--pages mapped into a VM_LOCKED vma--represent one class of
173 unevictable pages. When such a page has been "noticed" by the memory
174 management subsystem, the page is marked with the PG_mlocked [PageMlocked()]
175 flag. A PageMlocked() page will be placed on the unevictable LRU list when
176 it is added to the LRU. Pages can be "noticed" by memory management in
179 1) in the mlock()/mlockall() system call handlers.
180 2) in the mmap() system call handler when mmap()ing a region with the
181 MAP_LOCKED flag, or mmap()ing a region in a task that has called
182 mlockall() with the MCL_FUTURE flag. Both of these conditions result
183 in the VM_LOCKED flag being set for the vma.
184 3) in the fault path, if mlocked pages are "culled" in the fault path,
185 and when a VM_LOCKED stack segment is expanded.
186 4) as mentioned above, in vmscan:shrink_page_list() when attempting to
187 reclaim a page in a VM_LOCKED vma via try_to_unmap().
189 Mlocked pages become unlocked and rescued from the unevictable list when:
191 1) mapped in a range unlocked via the munlock()/munlockall() system calls.
192 2) munmapped() out of the last VM_LOCKED vma that maps the page, including
193 unmapping at task exit.
194 3) when the page is truncated from the last VM_LOCKED vma of an mmap()ed file.
195 4) before a page is COWed in a VM_LOCKED vma.
198 Mlocked Pages: mlock()/mlockall() System Call Handling
200 Both [do_]mlock() and [do_]mlockall() system call handlers call mlock_fixup()
201 for each vma in the range specified by the call. In the case of mlockall(),
202 this is the entire active address space of the task. Note that mlock_fixup()
203 is used for both mlock()ing and munlock()ing a range of memory. A call to
204 mlock() an already VM_LOCKED vma, or to munlock() a vma that is not VM_LOCKED
205 is treated as a no-op--mlock_fixup() simply returns.
207 If the vma passes some filtering described in "Mlocked Pages: Filtering Vmas"
208 below, mlock_fixup() will attempt to merge the vma with its neighbors or split
209 off a subset of the vma if the range does not cover the entire vma. Once the
210 vma has been merged or split or neither, mlock_fixup() will call
211 __mlock_vma_pages_range() to fault in the pages via get_user_pages() and
212 to mark the pages as mlocked via mlock_vma_page().
214 Note that the vma being mlocked might be mapped with PROT_NONE. In this case,
215 get_user_pages() will be unable to fault in the pages. That's OK. If pages
216 do end up getting faulted into this VM_LOCKED vma, we'll handle them in the
217 fault path or in vmscan.
219 Also note that a page returned by get_user_pages() could be truncated or
220 migrated out from under us, while we're trying to mlock it. To detect
221 this, __mlock_vma_pages_range() tests the page_mapping after acquiring
222 the page lock. If the page is still associated with its mapping, we'll
223 go ahead and call mlock_vma_page(). If the mapping is gone, we just
224 unlock the page and move on. Worse case, this results in page mapped
225 in a VM_LOCKED vma remaining on a normal LRU list without being
226 PageMlocked(). Again, vmscan will detect and cull such pages.
228 mlock_vma_page(), called with the page locked [N.B., not "mlocked"], will
229 TestSetPageMlocked() for each page returned by get_user_pages(). We use
230 TestSetPageMlocked() because the page might already be mlocked by another
231 task/vma and we don't want to do extra work. We especially do not want to
232 count an mlocked page more than once in the statistics. If the page was
233 already mlocked, mlock_vma_page() is done.
235 If the page was NOT already mlocked, mlock_vma_page() attempts to isolate the
236 page from the LRU, as it is likely on the appropriate active or inactive list
237 at that time. If the isolate_lru_page() succeeds, mlock_vma_page() will
238 putback the page--putback_lru_page()--which will notice that the page is now
239 mlocked and divert the page to the zone's unevictable LRU list. If
240 mlock_vma_page() is unable to isolate the page from the LRU, vmscan will handle
241 it later if/when it attempts to reclaim the page.
244 Mlocked Pages: Filtering Special Vmas
246 mlock_fixup() filters several classes of "special" vmas:
248 1) vmas with VM_IO|VM_PFNMAP set are skipped entirely. The pages behind
249 these mappings are inherently pinned, so we don't need to mark them as
250 mlocked. In any case, most of the pages have no struct page in which to
251 so mark the page. Because of this, get_user_pages() will fail for these
252 vmas, so there is no sense in attempting to visit them.
254 2) vmas mapping hugetlbfs page are already effectively pinned into memory.
255 We don't need nor want to mlock() these pages. However, to preserve the
256 prior behavior of mlock()--before the unevictable/mlock changes--
257 mlock_fixup() will call make_pages_present() in the hugetlbfs vma range
258 to allocate the huge pages and populate the ptes.
260 3) vmas with VM_DONTEXPAND|VM_RESERVED are generally user space mappings of
261 kernel pages, such as the vdso page, relay channel pages, etc. These pages
262 are inherently unevictable and are not managed on the LRU lists.
263 mlock_fixup() treats these vmas the same as hugetlbfs vmas. It calls
264 make_pages_present() to populate the ptes.
266 Note that for all of these special vmas, mlock_fixup() does not set the
267 VM_LOCKED flag. Therefore, we won't have to deal with them later during
268 munlock() or munmap()--for example, at task exit. Neither does mlock_fixup()
269 account these vmas against the task's "locked_vm".
271 Mlocked Pages: Downgrading the Mmap Semaphore.
273 mlock_fixup() must be called with the mmap semaphore held for write, because
274 it may have to merge or split vmas. However, mlocking a large region of
275 memory can take a long time--especially if vmscan must reclaim pages to
276 satisfy the regions requirements. Faulting in a large region with the mmap
277 semaphore held for write can hold off other faults on the address space, in
278 the case of a multi-threaded task. It can also hold off scans of the task's
279 address space via /proc. While testing under heavy load, it was observed that
280 the ps(1) command could be held off for many minutes while a large segment was
283 To address this issue, and to make the system more responsive during mlock()ing
284 of large segments, mlock_fixup() downgrades the mmap semaphore to read mode
285 during the call to __mlock_vma_pages_range(). This works fine. However, the
286 callers of mlock_fixup() expect the semaphore to be returned in write mode.
287 So, mlock_fixup() "upgrades" the semphore to write mode. Linux does not
288 support an atomic upgrade_sem() call, so mlock_fixup() must drop the semaphore
289 and reacquire it in write mode. In a multi-threaded task, it is possible for
290 the task memory map to change while the semaphore is dropped. Therefore,
291 mlock_fixup() looks up the vma at the range start address after reacquiring
292 the semaphore in write mode and verifies that it still covers the original
293 range. If not, mlock_fixup() returns an error [-EAGAIN]. All callers of
294 mlock_fixup() have been changed to deal with this new error condition.
296 Note: when munlocking a region, all of the pages should already be resident--
297 unless we have racing threads mlocking() and munlocking() regions. So,
298 unlocking should not have to wait for page allocations nor faults of any kind.
299 Therefore mlock_fixup() does not downgrade the semaphore for munlock().
302 Mlocked Pages: munlock()/munlockall() System Call Handling
304 The munlock() and munlockall() system calls are handled by the same functions--
305 do_mlock[all]()--as the mlock() and mlockall() system calls with the unlock
306 vs lock operation indicated by an argument. So, these system calls are also
307 handled by mlock_fixup(). Again, if called for an already munlock()ed vma,
308 mlock_fixup() simply returns. Because of the vma filtering discussed above,
309 VM_LOCKED will not be set in any "special" vmas. So, these vmas will be
312 If the vma is VM_LOCKED, mlock_fixup() again attempts to merge or split off
313 the specified range. The range is then munlocked via the function
314 __mlock_vma_pages_range()--the same function used to mlock a vma range--
315 passing a flag to indicate that munlock() is being performed.
317 Because the vma access protections could have been changed to PROT_NONE after
318 faulting in and mlocking pages, get_user_pages() was unreliable for visiting
319 these pages for munlocking. Because we don't want to leave pages mlocked(),
320 get_user_pages() was enhanced to accept a flag to ignore the permissions when
321 fetching the pages--all of which should be resident as a result of previous
324 For munlock(), __mlock_vma_pages_range() unlocks individual pages by calling
325 munlock_vma_page(). munlock_vma_page() unconditionally clears the PG_mlocked
326 flag using TestClearPageMlocked(). As with mlock_vma_page(), munlock_vma_page()
327 use the Test*PageMlocked() function to handle the case where the page might
328 have already been unlocked by another task. If the page was mlocked,
329 munlock_vma_page() updates that zone statistics for the number of mlocked
330 pages. Note, however, that at this point we haven't checked whether the page
331 is mapped by other VM_LOCKED vmas.
333 We can't call try_to_munlock(), the function that walks the reverse map to check
334 for other VM_LOCKED vmas, without first isolating the page from the LRU.
335 try_to_munlock() is a variant of try_to_unmap() and thus requires that the page
336 not be on an lru list. [More on these below.] However, the call to
337 isolate_lru_page() could fail, in which case we couldn't try_to_munlock().
338 So, we go ahead and clear PG_mlocked up front, as this might be the only chance
339 we have. If we can successfully isolate the page, we go ahead and
340 try_to_munlock(), which will restore the PG_mlocked flag and update the zone
341 page statistics if it finds another vma holding the page mlocked. If we fail
342 to isolate the page, we'll have left a potentially mlocked page on the LRU.
343 This is fine, because we'll catch it later when/if vmscan tries to reclaim the
344 page. This should be relatively rare.
346 Mlocked Pages: Migrating Them...
348 A page that is being migrated has been isolated from the lru lists and is
349 held locked across unmapping of the page, updating the page's mapping
350 [address_space] entry and copying the contents and state, until the
351 page table entry has been replaced with an entry that refers to the new
352 page. Linux supports migration of mlocked pages and other unevictable
353 pages. This involves simply moving the PageMlocked and PageUnevictable states
354 from the old page to the new page.
356 Note that page migration can race with mlocking or munlocking of the same
357 page. This has been discussed from the mlock/munlock perspective in the
358 respective sections above. Both processes [migration, m[un]locking], hold
359 the page locked. This provides the first level of synchronization. Page
360 migration zeros out the page_mapping of the old page before unlocking it,
361 so m[un]lock can skip these pages by testing the page mapping under page
364 When completing page migration, we place the new and old pages back onto the
365 lru after dropping the page lock. The "unneeded" page--old page on success,
366 new page on failure--will be freed when the reference count held by the
367 migration process is released. To ensure that we don't strand pages on the
368 unevictable list because of a race between munlock and migration, page
369 migration uses the putback_lru_page() function to add migrated pages back to
373 Mlocked Pages: mmap(MAP_LOCKED) System Call Handling
375 In addition the the mlock()/mlockall() system calls, an application can request
376 that a region of memory be mlocked using the MAP_LOCKED flag with the mmap()
377 call. Furthermore, any mmap() call or brk() call that expands the heap by a
378 task that has previously called mlockall() with the MCL_FUTURE flag will result
379 in the newly mapped memory being mlocked. Before the unevictable/mlock changes,
380 the kernel simply called make_pages_present() to allocate pages and populate
383 To mlock a range of memory under the unevictable/mlock infrastructure, the
384 mmap() handler and task address space expansion functions call
385 mlock_vma_pages_range() specifying the vma and the address range to mlock.
386 mlock_vma_pages_range() filters vmas like mlock_fixup(), as described above in
387 "Mlocked Pages: Filtering Vmas". It will clear the VM_LOCKED flag, which will
388 have already been set by the caller, in filtered vmas. Thus these vma's need
389 not be visited for munlock when the region is unmapped.
391 For "normal" vmas, mlock_vma_pages_range() calls __mlock_vma_pages_range() to
392 fault/allocate the pages and mlock them. Again, like mlock_fixup(),
393 mlock_vma_pages_range() downgrades the mmap semaphore to read mode before
394 attempting to fault/allocate and mlock the pages; and "upgrades" the semaphore
395 back to write mode before returning.
397 The callers of mlock_vma_pages_range() will have already added the memory
398 range to be mlocked to the task's "locked_vm". To account for filtered vmas,
399 mlock_vma_pages_range() returns the number of pages NOT mlocked. All of the
400 callers then subtract a non-negative return value from the task's locked_vm.
401 A negative return value represent an error--for example, from get_user_pages()
402 attempting to fault in a vma with PROT_NONE access. In this case, we leave
403 the memory range accounted as locked_vm, as the protections could be changed
404 later and pages allocated into that region.
407 Mlocked Pages: munmap()/exit()/exec() System Call Handling
409 When unmapping an mlocked region of memory, whether by an explicit call to
410 munmap() or via an internal unmap from exit() or exec() processing, we must
411 munlock the pages if we're removing the last VM_LOCKED vma that maps the pages.
412 Before the unevictable/mlock changes, mlocking did not mark the pages in any
413 way, so unmapping them required no processing.
415 To munlock a range of memory under the unevictable/mlock infrastructure, the
416 munmap() hander and task address space tear down function call
417 munlock_vma_pages_all(). The name reflects the observation that one always
418 specifies the entire vma range when munlock()ing during unmap of a region.
419 Because of the vma filtering when mlocking() regions, only "normal" vmas that
420 actually contain mlocked pages will be passed to munlock_vma_pages_all().
422 munlock_vma_pages_all() clears the VM_LOCKED vma flag and, like mlock_fixup()
423 for the munlock case, calls __munlock_vma_pages_range() to walk the page table
424 for the vma's memory range and munlock_vma_page() each resident page mapped by
425 the vma. This effectively munlocks the page, only if this is the last
426 VM_LOCKED vma that maps the page.
429 Mlocked Page: try_to_unmap()
431 [Note: the code changes represented by this section are really quite small
432 compared to the text to describe what happening and why, and to discuss the
435 Pages can, of course, be mapped into multiple vmas. Some of these vmas may
436 have VM_LOCKED flag set. It is possible for a page mapped into one or more
437 VM_LOCKED vmas not to have the PG_mlocked flag set and therefore reside on one
438 of the active or inactive LRU lists. This could happen if, for example, a
439 task in the process of munlock()ing the page could not isolate the page from
440 the LRU. As a result, vmscan/shrink_page_list() might encounter such a page
441 as described in "Unevictable Pages and Vmscan [shrink_*_list()]". To
442 handle this situation, try_to_unmap() has been enhanced to check for VM_LOCKED
443 vmas while it is walking a page's reverse map.
445 try_to_unmap() is always called, by either vmscan for reclaim or for page
446 migration, with the argument page locked and isolated from the LRU. BUG_ON()
447 assertions enforce this requirement. Separate functions handle anonymous and
448 mapped file pages, as these types of pages have different reverse map
453 To unmap anonymous pages, each vma in the list anchored in the anon_vma must be
454 visited--at least until a VM_LOCKED vma is encountered. If the page is being
455 unmapped for migration, VM_LOCKED vmas do not stop the process because mlocked
456 pages are migratable. However, for reclaim, if the page is mapped into a
457 VM_LOCKED vma, the scan stops. try_to_unmap() attempts to acquire the mmap
458 semphore of the mm_struct to which the vma belongs in read mode. If this is
459 successful, try_to_unmap() will mlock the page via mlock_vma_page()--we
460 wouldn't have gotten to try_to_unmap() if the page were already mlocked--and
461 will return SWAP_MLOCK, indicating that the page is unevictable. If the
462 mmap semaphore cannot be acquired, we are not sure whether the page is really
463 unevictable or not. In this case, try_to_unmap() will return SWAP_AGAIN.
465 try_to_unmap_file() -- linear mappings
467 Unmapping of a mapped file page works the same, except that the scan visits
468 all vmas that maps the page's index/page offset in the page's mapping's
469 reverse map priority search tree. It must also visit each vma in the page's
470 mapping's non-linear list, if the list is non-empty. As for anonymous pages,
471 on encountering a VM_LOCKED vma for a mapped file page, try_to_unmap() will
472 attempt to acquire the associated mm_struct's mmap semaphore to mlock the page,
473 returning SWAP_MLOCK if this is successful, and SWAP_AGAIN, if not.
475 try_to_unmap_file() -- non-linear mappings
477 If a page's mapping contains a non-empty non-linear mapping vma list, then
478 try_to_un{map|lock}() must also visit each vma in that list to determine
479 whether the page is mapped in a VM_LOCKED vma. Again, the scan must visit
480 all vmas in the non-linear list to ensure that the pages is not/should not be
481 mlocked. If a VM_LOCKED vma is found in the list, the scan could terminate.
482 However, there is no easy way to determine whether the page is actually mapped
483 in a given vma--either for unmapping or testing whether the VM_LOCKED vma
484 actually pins the page.
486 So, try_to_unmap_file() handles non-linear mappings by scanning a certain
487 number of pages--a "cluster"--in each non-linear vma associated with the page's
488 mapping, for each file mapped page that vmscan tries to unmap. If this happens
489 to unmap the page we're trying to unmap, try_to_unmap() will notice this on
490 return--(page_mapcount(page) == 0)--and return SWAP_SUCCESS. Otherwise, it
491 will return SWAP_AGAIN, causing vmscan to recirculate this page. We take
492 advantage of the cluster scan in try_to_unmap_cluster() as follows:
494 For each non-linear vma, try_to_unmap_cluster() attempts to acquire the mmap
495 semaphore of the associated mm_struct for read without blocking. If this
496 attempt is successful and the vma is VM_LOCKED, try_to_unmap_cluster() will
497 retain the mmap semaphore for the scan; otherwise it drops it here. Then,
498 for each page in the cluster, if we're holding the mmap semaphore for a locked
499 vma, try_to_unmap_cluster() calls mlock_vma_page() to mlock the page. This
500 call is a no-op if the page is already locked, but will mlock any pages in
501 the non-linear mapping that happen to be unlocked. If one of the pages so
502 mlocked is the page passed in to try_to_unmap(), try_to_unmap_cluster() will
503 return SWAP_MLOCK, rather than the default SWAP_AGAIN. This will allow vmscan
504 to cull the page, rather than recirculating it on the inactive list. Again,
505 if try_to_unmap_cluster() cannot acquire the vma's mmap sem, it returns
506 SWAP_AGAIN, indicating that the page is mapped by a VM_LOCKED vma, but
510 Mlocked pages: try_to_munlock() Reverse Map Scan
512 TODO/FIXME: a better name might be page_mlocked()--analogous to the
513 page_referenced() reverse map walker.
515 When munlock_vma_page()--see "Mlocked Pages: munlock()/munlockall()
516 System Call Handling" above--tries to munlock a page, it needs to
517 determine whether or not the page is mapped by any VM_LOCKED vma, without
518 actually attempting to unmap all ptes from the page. For this purpose, the
519 unevictable/mlock infrastructure introduced a variant of try_to_unmap() called
522 try_to_munlock() calls the same functions as try_to_unmap() for anonymous and
523 mapped file pages with an additional argument specifing unlock versus unmap
524 processing. Again, these functions walk the respective reverse maps looking
525 for VM_LOCKED vmas. When such a vma is found for anonymous pages and file
526 pages mapped in linear VMAs, as in the try_to_unmap() case, the functions
527 attempt to acquire the associated mmap semphore, mlock the page via
528 mlock_vma_page() and return SWAP_MLOCK. This effectively undoes the
529 pre-clearing of the page's PG_mlocked done by munlock_vma_page.
531 If try_to_unmap() is unable to acquire a VM_LOCKED vma's associated mmap
532 semaphore, it will return SWAP_AGAIN. This will allow shrink_page_list()
533 to recycle the page on the inactive list and hope that it has better luck
534 with the page next time.
536 For file pages mapped into non-linear vmas, the try_to_munlock() logic works
537 slightly differently. On encountering a VM_LOCKED non-linear vma that might
538 map the page, try_to_munlock() returns SWAP_AGAIN without actually mlocking
539 the page. munlock_vma_page() will just leave the page unlocked and let
540 vmscan deal with it--the usual fallback position.
542 Note that try_to_munlock()'s reverse map walk must visit every vma in a pages'
543 reverse map to determine that a page is NOT mapped into any VM_LOCKED vma.
544 However, the scan can terminate when it encounters a VM_LOCKED vma and can
545 successfully acquire the vma's mmap semphore for read and mlock the page.
546 Although try_to_munlock() can be called many [very many!] times when
547 munlock()ing a large region or tearing down a large address space that has been
548 mlocked via mlockall(), overall this is a fairly rare event.
550 Mlocked Page: Page Reclaim in shrink_*_list()
552 shrink_active_list() culls any obviously unevictable pages--i.e.,
553 !page_evictable(page, NULL)--diverting these to the unevictable lru
554 list. However, shrink_active_list() only sees unevictable pages that
555 made it onto the active/inactive lru lists. Note that these pages do not
556 have PageUnevictable set--otherwise, they would be on the unevictable list and
557 shrink_active_list would never see them.
559 Some examples of these unevictable pages on the LRU lists are:
561 1) ramfs pages that have been placed on the lru lists when first allocated.
563 2) SHM_LOCKed shared memory pages. shmctl(SHM_LOCK) does not attempt to
564 allocate or fault in the pages in the shared memory region. This happens
565 when an application accesses the page the first time after SHM_LOCKing
568 3) Mlocked pages that could not be isolated from the lru and moved to the
569 unevictable list in mlock_vma_page().
571 3) Pages mapped into multiple VM_LOCKED vmas, but try_to_munlock() couldn't
572 acquire the vma's mmap semaphore to test the flags and set PageMlocked.
573 munlock_vma_page() was forced to let the page back on to the normal
574 LRU list for vmscan to handle.
576 shrink_inactive_list() also culls any unevictable pages that it finds on
577 the inactive lists, again diverting them to the appropriate zone's unevictable
578 lru list. shrink_inactive_list() should only see SHM_LOCKed pages that became
579 SHM_LOCKed after shrink_active_list() had moved them to the inactive list, or
580 pages mapped into VM_LOCKED vmas that munlock_vma_page() couldn't isolate from
581 the lru to recheck via try_to_munlock(). shrink_inactive_list() won't notice
582 the latter, but will pass on to shrink_page_list().
584 shrink_page_list() again culls obviously unevictable pages that it could
585 encounter for similar reason to shrink_inactive_list(). Pages mapped into
586 VM_LOCKED vmas but without PG_mlocked set will make it all the way to
587 try_to_unmap(). shrink_page_list() will divert them to the unevictable list
588 when try_to_unmap() returns SWAP_MLOCK, as discussed above.
projects / linux-2.6 / blob