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 Note that for anonymous pages, shrink_page_list() attempts to add the page to
141 the swap cache before it tries to unmap the page. To avoid this unnecessary
142 consumption of swap space, shrink_page_list() calls try_to_munlock() to check
143 whether any VM_LOCKED vmas map the page without attempting to unmap the page.
144 If try_to_munlock() returns SWAP_MLOCK, shrink_page_list() will cull the page
145 without consuming swap space. try_to_munlock() will be described below.
147 To "cull" an unevictable page, vmscan simply puts the page back on the lru
148 list using putback_lru_page()--the inverse operation to isolate_lru_page()--
149 after dropping the page lock. Because the condition which makes the page
150 unevictable may change once the page is unlocked, putback_lru_page() will
151 recheck the unevictable state of a page that it places on the unevictable lru
152 list. If the page has become unevictable, putback_lru_page() removes it from
153 the list and retries, including the page_unevictable() test. Because such a
154 race is a rare event and movement of pages onto the unevictable list should be
155 rare, these extra evictabilty checks should not occur in the majority of calls
156 to putback_lru_page().
159 Mlocked Page: Prior Work
161 The "Unevictable Mlocked Pages" infrastructure is based on work originally
162 posted by Nick Piggin in an RFC patch entitled "mm: mlocked pages off LRU".
163 Nick posted his patch as an alternative to a patch posted by Christoph
164 Lameter to achieve the same objective--hiding mlocked pages from vmscan.
165 In Nick's patch, he used one of the struct page lru list link fields as a count
166 of VM_LOCKED vmas that map the page. This use of the link field for a count
167 prevented the management of the pages on an LRU list. Thus, mlocked pages were
168 not migratable as isolate_lru_page() could not find them and the lru list link
169 field was not available to the migration subsystem. Nick resolved this by
170 putting mlocked pages back on the lru list before attempting to isolate them,
171 thus abandoning the count of VM_LOCKED vmas. When Nick's patch was integrated
172 with the Unevictable LRU work, the count was replaced by walking the reverse
173 map to determine whether any VM_LOCKED vmas mapped the page. More on this
177 Mlocked Pages: Basic Management
179 Mlocked pages--pages mapped into a VM_LOCKED vma--represent one class of
180 unevictable pages. When such a page has been "noticed" by the memory
181 management subsystem, the page is marked with the PG_mlocked [PageMlocked()]
182 flag. A PageMlocked() page will be placed on the unevictable LRU list when
183 it is added to the LRU. Pages can be "noticed" by memory management in
186 1) in the mlock()/mlockall() system call handlers.
187 2) in the mmap() system call handler when mmap()ing a region with the
188 MAP_LOCKED flag, or mmap()ing a region in a task that has called
189 mlockall() with the MCL_FUTURE flag. Both of these conditions result
190 in the VM_LOCKED flag being set for the vma.
191 3) in the fault path, if mlocked pages are "culled" in the fault path,
192 and when a VM_LOCKED stack segment is expanded.
193 4) as mentioned above, in vmscan:shrink_page_list() with attempting to
194 reclaim a page in a VM_LOCKED vma--via try_to_unmap() or try_to_munlock().
196 Mlocked pages become unlocked and rescued from the unevictable list when:
198 1) mapped in a range unlocked via the munlock()/munlockall() system calls.
199 2) munmapped() out of the last VM_LOCKED vma that maps the page, including
200 unmapping at task exit.
201 3) when the page is truncated from the last VM_LOCKED vma of an mmap()ed file.
202 4) before a page is COWed in a VM_LOCKED vma.
205 Mlocked Pages: mlock()/mlockall() System Call Handling
207 Both [do_]mlock() and [do_]mlockall() system call handlers call mlock_fixup()
208 for each vma in the range specified by the call. In the case of mlockall(),
209 this is the entire active address space of the task. Note that mlock_fixup()
210 is used for both mlock()ing and munlock()ing a range of memory. A call to
211 mlock() an already VM_LOCKED vma, or to munlock() a vma that is not VM_LOCKED
212 is treated as a no-op--mlock_fixup() simply returns.
214 If the vma passes some filtering described in "Mlocked Pages: Filtering Vmas"
215 below, mlock_fixup() will attempt to merge the vma with its neighbors or split
216 off a subset of the vma if the range does not cover the entire vma. Once the
217 vma has been merged or split or neither, mlock_fixup() will call
218 __mlock_vma_pages_range() to fault in the pages via get_user_pages() and
219 to mark the pages as mlocked via mlock_vma_page().
221 Note that the vma being mlocked might be mapped with PROT_NONE. In this case,
222 get_user_pages() will be unable to fault in the pages. That's OK. If pages
223 do end up getting faulted into this VM_LOCKED vma, we'll handle them in the
224 fault path or in vmscan.
226 Also note that a page returned by get_user_pages() could be truncated or
227 migrated out from under us, while we're trying to mlock it. To detect
228 this, __mlock_vma_pages_range() tests the page_mapping after acquiring
229 the page lock. If the page is still associated with its mapping, we'll
230 go ahead and call mlock_vma_page(). If the mapping is gone, we just
231 unlock the page and move on. Worse case, this results in page mapped
232 in a VM_LOCKED vma remaining on a normal LRU list without being
233 PageMlocked(). Again, vmscan will detect and cull such pages.
235 mlock_vma_page(), called with the page locked [N.B., not "mlocked"], will
236 TestSetPageMlocked() for each page returned by get_user_pages(). We use
237 TestSetPageMlocked() because the page might already be mlocked by another
238 task/vma and we don't want to do extra work. We especially do not want to
239 count an mlocked page more than once in the statistics. If the page was
240 already mlocked, mlock_vma_page() is done.
242 If the page was NOT already mlocked, mlock_vma_page() attempts to isolate the
243 page from the LRU, as it is likely on the appropriate active or inactive list
244 at that time. If the isolate_lru_page() succeeds, mlock_vma_page() will
245 putback the page--putback_lru_page()--which will notice that the page is now
246 mlocked and divert the page to the zone's unevictable LRU list. If
247 mlock_vma_page() is unable to isolate the page from the LRU, vmscan will handle
248 it later if/when it attempts to reclaim the page.
251 Mlocked Pages: Filtering Special Vmas
253 mlock_fixup() filters several classes of "special" vmas:
255 1) vmas with VM_IO|VM_PFNMAP set are skipped entirely. The pages behind
256 these mappings are inherently pinned, so we don't need to mark them as
257 mlocked. In any case, most of the pages have no struct page in which to
258 so mark the page. Because of this, get_user_pages() will fail for these
259 vmas, so there is no sense in attempting to visit them.
261 2) vmas mapping hugetlbfs page are already effectively pinned into memory.
262 We don't need nor want to mlock() these pages. However, to preserve the
263 prior behavior of mlock()--before the unevictable/mlock changes--mlock_fixup()
264 will call make_pages_present() in the hugetlbfs vma range to allocate the
265 huge pages and populate the ptes.
267 3) vmas with VM_DONTEXPAND|VM_RESERVED are generally user space mappings of
268 kernel pages, such as the vdso page, relay channel pages, etc. These pages
269 are inherently unevictable and are not managed on the LRU lists.
270 mlock_fixup() treats these vmas the same as hugetlbfs vmas. It calls
271 make_pages_present() to populate the ptes.
273 Note that for all of these special vmas, mlock_fixup() does not set the
274 VM_LOCKED flag. Therefore, we won't have to deal with them later during
275 munlock() or munmap()--for example, at task exit. Neither does mlock_fixup()
276 account these vmas against the task's "locked_vm".
278 Mlocked Pages: Downgrading the Mmap Semaphore.
280 mlock_fixup() must be called with the mmap semaphore held for write, because
281 it may have to merge or split vmas. However, mlocking a large region of
282 memory can take a long time--especially if vmscan must reclaim pages to
283 satisfy the regions requirements. Faulting in a large region with the mmap
284 semaphore held for write can hold off other faults on the address space, in
285 the case of a multi-threaded task. It can also hold off scans of the task's
286 address space via /proc. While testing under heavy load, it was observed that
287 the ps(1) command could be held off for many minutes while a large segment was
290 To address this issue, and to make the system more responsive during mlock()ing
291 of large segments, mlock_fixup() downgrades the mmap semaphore to read mode
292 during the call to __mlock_vma_pages_range(). This works fine. However, the
293 callers of mlock_fixup() expect the semaphore to be returned in write mode.
294 So, mlock_fixup() "upgrades" the semphore to write mode. Linux does not
295 support an atomic upgrade_sem() call, so mlock_fixup() must drop the semaphore
296 and reacquire it in write mode. In a multi-threaded task, it is possible for
297 the task memory map to change while the semaphore is dropped. Therefore,
298 mlock_fixup() looks up the vma at the range start address after reacquiring
299 the semaphore in write mode and verifies that it still covers the original
300 range. If not, mlock_fixup() returns an error [-EAGAIN]. All callers of
301 mlock_fixup() have been changed to deal with this new error condition.
303 Note: when munlocking a region, all of the pages should already be resident--
304 unless we have racing threads mlocking() and munlocking() regions. So,
305 unlocking should not have to wait for page allocations nor faults of any kind.
306 Therefore mlock_fixup() does not downgrade the semaphore for munlock().
309 Mlocked Pages: munlock()/munlockall() System Call Handling
311 The munlock() and munlockall() system calls are handled by the same functions--
312 do_mlock[all]()--as the mlock() and mlockall() system calls with the unlock
313 vs lock operation indicated by an argument. So, these system calls are also
314 handled by mlock_fixup(). Again, if called for an already munlock()ed vma,
315 mlock_fixup() simply returns. Because of the vma filtering discussed above,
316 VM_LOCKED will not be set in any "special" vmas. So, these vmas will be
319 If the vma is VM_LOCKED, mlock_fixup() again attempts to merge or split off
320 the specified range. The range is then munlocked via the function
321 __mlock_vma_pages_range()--the same function used to mlock a vma range--
322 passing a flag to indicate that munlock() is being performed.
324 Because the vma access protections could have been changed to PROT_NONE after
325 faulting in and mlocking some pages, get_user_pages() was unreliable for visiting
326 these pages for munlocking. Because we don't want to leave pages mlocked(),
327 get_user_pages() was enhanced to accept a flag to ignore the permissions when
328 fetching the pages--all of which should be resident as a result of previous
331 For munlock(), __mlock_vma_pages_range() unlocks individual pages by calling
332 munlock_vma_page(). munlock_vma_page() unconditionally clears the PG_mlocked
333 flag using TestClearPageMlocked(). As with mlock_vma_page(), munlock_vma_page()
334 use the Test*PageMlocked() function to handle the case where the page might
335 have already been unlocked by another task. If the page was mlocked,
336 munlock_vma_page() updates that zone statistics for the number of mlocked
337 pages. Note, however, that at this point we haven't checked whether the page
338 is mapped by other VM_LOCKED vmas.
340 We can't call try_to_munlock(), the function that walks the reverse map to check
341 for other VM_LOCKED vmas, without first isolating the page from the LRU.
342 try_to_munlock() is a variant of try_to_unmap() and thus requires that the page
343 not be on an lru list. [More on these below.] However, the call to
344 isolate_lru_page() could fail, in which case we couldn't try_to_munlock().
345 So, we go ahead and clear PG_mlocked up front, as this might be the only chance
346 we have. If we can successfully isolate the page, we go ahead and
347 try_to_munlock(), which will restore the PG_mlocked flag and update the zone
348 page statistics if it finds another vma holding the page mlocked. If we fail
349 to isolate the page, we'll have left a potentially mlocked page on the LRU.
350 This is fine, because we'll catch it later when/if vmscan tries to reclaim the
351 page. This should be relatively rare.
353 Mlocked Pages: Migrating Them...
355 A page that is being migrated has been isolated from the lru lists and is
356 held locked across unmapping of the page, updating the page's mapping
357 [address_space] entry and copying the contents and state, until the
358 page table entry has been replaced with an entry that refers to the new
359 page. Linux supports migration of mlocked pages and other unevictable
360 pages. This involves simply moving the PageMlocked and PageUnevictable states
361 from the old page to the new page.
363 Note that page migration can race with mlocking or munlocking of the same
364 page. This has been discussed from the mlock/munlock perspective in the
365 respective sections above. Both processes [migration, m[un]locking], hold
366 the page locked. This provides the first level of synchronization. Page
367 migration zeros out the page_mapping of the old page before unlocking it,
368 so m[un]lock can skip these pages by testing the page mapping under page
371 When completing page migration, we place the new and old pages back onto the
372 lru after dropping the page lock. The "unneeded" page--old page on success,
373 new page on failure--will be freed when the reference count held by the
374 migration process is released. To ensure that we don't strand pages on the
375 unevictable list because of a race between munlock and migration, page
376 migration uses the putback_lru_page() function to add migrated pages back to
380 Mlocked Pages: mmap(MAP_LOCKED) System Call Handling
382 In addition the the mlock()/mlockall() system calls, an application can request
383 that a region of memory be mlocked using the MAP_LOCKED flag with the mmap()
384 call. Furthermore, any mmap() call or brk() call that expands the heap by a
385 task that has previously called mlockall() with the MCL_FUTURE flag will result
386 in the newly mapped memory being mlocked. Before the unevictable/mlock changes,
387 the kernel simply called make_pages_present() to allocate pages and populate
390 To mlock a range of memory under the unevictable/mlock infrastructure, the
391 mmap() handler and task address space expansion functions call
392 mlock_vma_pages_range() specifying the vma and the address range to mlock.
393 mlock_vma_pages_range() filters vmas like mlock_fixup(), as described above in
394 "Mlocked Pages: Filtering Vmas". It will clear the VM_LOCKED flag, which will
395 have already been set by the caller, in filtered vmas. Thus these vma's need
396 not be visited for munlock when the region is unmapped.
398 For "normal" vmas, mlock_vma_pages_range() calls __mlock_vma_pages_range() to
399 fault/allocate the pages and mlock them. Again, like mlock_fixup(),
400 mlock_vma_pages_range() downgrades the mmap semaphore to read mode before
401 attempting to fault/allocate and mlock the pages; and "upgrades" the semaphore
402 back to write mode before returning.
404 The callers of mlock_vma_pages_range() will have already added the memory
405 range to be mlocked to the task's "locked_vm". To account for filtered vmas,
406 mlock_vma_pages_range() returns the number of pages NOT mlocked. All of the
407 callers then subtract a non-negative return value from the task's locked_vm.
408 A negative return value represent an error--for example, from get_user_pages()
409 attempting to fault in a vma with PROT_NONE access. In this case, we leave
410 the memory range accounted as locked_vm, as the protections could be changed
411 later and pages allocated into that region.
414 Mlocked Pages: munmap()/exit()/exec() System Call Handling
416 When unmapping an mlocked region of memory, whether by an explicit call to
417 munmap() or via an internal unmap from exit() or exec() processing, we must
418 munlock the pages if we're removing the last VM_LOCKED vma that maps the pages.
419 Before the unevictable/mlock changes, mlocking did not mark the pages in any way,
420 so unmapping them required no processing.
422 To munlock a range of memory under the unevictable/mlock infrastructure, the
423 munmap() hander and task address space tear down function call
424 munlock_vma_pages_all(). The name reflects the observation that one always
425 specifies the entire vma range when munlock()ing during unmap of a region.
426 Because of the vma filtering when mlocking() regions, only "normal" vmas that
427 actually contain mlocked pages will be passed to munlock_vma_pages_all().
429 munlock_vma_pages_all() clears the VM_LOCKED vma flag and, like mlock_fixup()
430 for the munlock case, calls __munlock_vma_pages_range() to walk the page table
431 for the vma's memory range and munlock_vma_page() each resident page mapped by
432 the vma. This effectively munlocks the page, only if this is the last
433 VM_LOCKED vma that maps the page.
436 Mlocked Page: try_to_unmap()
438 [Note: the code changes represented by this section are really quite small
439 compared to the text to describe what happening and why, and to discuss the
442 Pages can, of course, be mapped into multiple vmas. Some of these vmas may
443 have VM_LOCKED flag set. It is possible for a page mapped into one or more
444 VM_LOCKED vmas not to have the PG_mlocked flag set and therefore reside on one
445 of the active or inactive LRU lists. This could happen if, for example, a
446 task in the process of munlock()ing the page could not isolate the page from
447 the LRU. As a result, vmscan/shrink_page_list() might encounter such a page
448 as described in "Unevictable Pages and Vmscan [shrink_*_list()]". To
449 handle this situation, try_to_unmap() has been enhanced to check for VM_LOCKED
450 vmas while it is walking a page's reverse map.
452 try_to_unmap() is always called, by either vmscan for reclaim or for page
453 migration, with the argument page locked and isolated from the LRU. BUG_ON()
454 assertions enforce this requirement. Separate functions handle anonymous and
455 mapped file pages, as these types of pages have different reverse map
460 To unmap anonymous pages, each vma in the list anchored in the anon_vma must be
461 visited--at least until a VM_LOCKED vma is encountered. If the page is being
462 unmapped for migration, VM_LOCKED vmas do not stop the process because mlocked
463 pages are migratable. However, for reclaim, if the page is mapped into a
464 VM_LOCKED vma, the scan stops. try_to_unmap() attempts to acquire the mmap
465 semphore of the mm_struct to which the vma belongs in read mode. If this is
466 successful, try_to_unmap() will mlock the page via mlock_vma_page()--we
467 wouldn't have gotten to try_to_unmap() if the page were already mlocked--and
468 will return SWAP_MLOCK, indicating that the page is unevictable. If the
469 mmap semaphore cannot be acquired, we are not sure whether the page is really
470 unevictable or not. In this case, try_to_unmap() will return SWAP_AGAIN.
472 try_to_unmap_file() -- linear mappings
474 Unmapping of a mapped file page works the same, except that the scan visits
475 all vmas that maps the page's index/page offset in the page's mapping's
476 reverse map priority search tree. It must also visit each vma in the page's
477 mapping's non-linear list, if the list is non-empty. As for anonymous pages,
478 on encountering a VM_LOCKED vma for a mapped file page, try_to_unmap() will
479 attempt to acquire the associated mm_struct's mmap semaphore to mlock the page,
480 returning SWAP_MLOCK if this is successful, and SWAP_AGAIN, if not.
482 try_to_unmap_file() -- non-linear mappings
484 If a page's mapping contains a non-empty non-linear mapping vma list, then
485 try_to_un{map|lock}() must also visit each vma in that list to determine
486 whether the page is mapped in a VM_LOCKED vma. Again, the scan must visit
487 all vmas in the non-linear list to ensure that the pages is not/should not be
488 mlocked. If a VM_LOCKED vma is found in the list, the scan could terminate.
489 However, there is no easy way to determine whether the page is actually mapped
490 in a given vma--either for unmapping or testing whether the VM_LOCKED vma
491 actually pins the page.
493 So, try_to_unmap_file() handles non-linear mappings by scanning a certain
494 number of pages--a "cluster"--in each non-linear vma associated with the page's
495 mapping, for each file mapped page that vmscan tries to unmap. If this happens
496 to unmap the page we're trying to unmap, try_to_unmap() will notice this on
497 return--(page_mapcount(page) == 0)--and return SWAP_SUCCESS. Otherwise, it
498 will return SWAP_AGAIN, causing vmscan to recirculate this page. We take
499 advantage of the cluster scan in try_to_unmap_cluster() as follows:
501 For each non-linear vma, try_to_unmap_cluster() attempts to acquire the mmap
502 semaphore of the associated mm_struct for read without blocking. If this
503 attempt is successful and the vma is VM_LOCKED, try_to_unmap_cluster() will
504 retain the mmap semaphore for the scan; otherwise it drops it here. Then,
505 for each page in the cluster, if we're holding the mmap semaphore for a locked
506 vma, try_to_unmap_cluster() calls mlock_vma_page() to mlock the page. This
507 call is a no-op if the page is already locked, but will mlock any pages in
508 the non-linear mapping that happen to be unlocked. If one of the pages so
509 mlocked is the page passed in to try_to_unmap(), try_to_unmap_cluster() will
510 return SWAP_MLOCK, rather than the default SWAP_AGAIN. This will allow vmscan
511 to cull the page, rather than recirculating it on the inactive list. Again,
512 if try_to_unmap_cluster() cannot acquire the vma's mmap sem, it returns
513 SWAP_AGAIN, indicating that the page is mapped by a VM_LOCKED vma, but
517 Mlocked pages: try_to_munlock() Reverse Map Scan
519 TODO/FIXME: a better name might be page_mlocked()--analogous to the
520 page_referenced() reverse map walker--especially if we continue to call this
521 from shrink_page_list(). See related TODO/FIXME below.
523 When munlock_vma_page()--see "Mlocked Pages: munlock()/munlockall() System
524 Call Handling" above--tries to munlock a page, or when shrink_page_list()
525 encounters an anonymous page that is not yet in the swap cache, they need to
526 determine whether or not the page is mapped by any VM_LOCKED vma, without
527 actually attempting to unmap all ptes from the page. For this purpose, the
528 unevictable/mlock infrastructure introduced a variant of try_to_unmap() called
531 try_to_munlock() calls the same functions as try_to_unmap() for anonymous and
532 mapped file pages with an additional argument specifing unlock versus unmap
533 processing. Again, these functions walk the respective reverse maps looking
534 for VM_LOCKED vmas. When such a vma is found for anonymous pages and file
535 pages mapped in linear VMAs, as in the try_to_unmap() case, the functions
536 attempt to acquire the associated mmap semphore, mlock the page via
537 mlock_vma_page() and return SWAP_MLOCK. This effectively undoes the
538 pre-clearing of the page's PG_mlocked done by munlock_vma_page() and informs
539 shrink_page_list() that the anonymous page should be culled rather than added
540 to the swap cache in preparation for a try_to_unmap() that will almost
543 If try_to_unmap() is unable to acquire a VM_LOCKED vma's associated mmap
544 semaphore, it will return SWAP_AGAIN. This will allow shrink_page_list()
545 to recycle the page on the inactive list and hope that it has better luck
546 with the page next time.
548 For file pages mapped into non-linear vmas, the try_to_munlock() logic works
549 slightly differently. On encountering a VM_LOCKED non-linear vma that might
550 map the page, try_to_munlock() returns SWAP_AGAIN without actually mlocking
551 the page. munlock_vma_page() will just leave the page unlocked and let
552 vmscan deal with it--the usual fallback position.
554 Note that try_to_munlock()'s reverse map walk must visit every vma in a pages'
555 reverse map to determine that a page is NOT mapped into any VM_LOCKED vma.
556 However, the scan can terminate when it encounters a VM_LOCKED vma and can
557 successfully acquire the vma's mmap semphore for read and mlock the page.
558 Although try_to_munlock() can be called many [very many!] times when
559 munlock()ing a large region or tearing down a large address space that has been
560 mlocked via mlockall(), overall this is a fairly rare event. In addition,
561 although shrink_page_list() calls try_to_munlock() for every anonymous page that
562 it handles that is not yet in the swap cache, on average anonymous pages will
563 have very short reverse map lists.
565 Mlocked Page: Page Reclaim in shrink_*_list()
567 shrink_active_list() culls any obviously unevictable pages--i.e.,
568 !page_evictable(page, NULL)--diverting these to the unevictable lru
569 list. However, shrink_active_list() only sees unevictable pages that
570 made it onto the active/inactive lru lists. Note that these pages do not
571 have PageUnevictable set--otherwise, they would be on the unevictable list and
572 shrink_active_list would never see them.
574 Some examples of these unevictable pages on the LRU lists are:
576 1) ramfs pages that have been placed on the lru lists when first allocated.
578 2) SHM_LOCKed shared memory pages. shmctl(SHM_LOCK) does not attempt to
579 allocate or fault in the pages in the shared memory region. This happens
580 when an application accesses the page the first time after SHM_LOCKing
583 3) Mlocked pages that could not be isolated from the lru and moved to the
584 unevictable list in mlock_vma_page().
586 3) Pages mapped into multiple VM_LOCKED vmas, but try_to_munlock() couldn't
587 acquire the vma's mmap semaphore to test the flags and set PageMlocked.
588 munlock_vma_page() was forced to let the page back on to the normal
589 LRU list for vmscan to handle.
591 shrink_inactive_list() also culls any unevictable pages that it finds
592 on the inactive lists, again diverting them to the appropriate zone's unevictable
593 lru list. shrink_inactive_list() should only see SHM_LOCKed pages that became
594 SHM_LOCKed after shrink_active_list() had moved them to the inactive list, or
595 pages mapped into VM_LOCKED vmas that munlock_vma_page() couldn't isolate from
596 the lru to recheck via try_to_munlock(). shrink_inactive_list() won't notice
597 the latter, but will pass on to shrink_page_list().
599 shrink_page_list() again culls obviously unevictable pages that it could
600 encounter for similar reason to shrink_inactive_list(). As already discussed,
601 shrink_page_list() proactively looks for anonymous pages that should have
602 PG_mlocked set but don't--these would not be detected by page_evictable()--to
603 avoid adding them to the swap cache unnecessarily. File pages mapped into
604 VM_LOCKED vmas but without PG_mlocked set will make it all the way to
605 try_to_unmap(). shrink_page_list() will divert them to the unevictable list when
606 try_to_unmap() returns SWAP_MLOCK, as discussed above.
608 TODO/FIXME: If we can enhance the swap cache to reliably remove entries
609 with page_count(page) > 2, as long as all ptes are mapped to the page and
610 not the swap entry, we can probably remove the call to try_to_munlock() in
611 shrink_page_list() and just remove the page from the swap cache when
612 try_to_unmap() returns SWAP_MLOCK. Currently, remove_exclusive_swap_page()
613 doesn't seem to allow that.
projects / linux-2.6-omap-h63xx.git / blob