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Based on kernel version 4.2. Page generated on 2015-09-09 12:15 EST.

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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.
17	 (*) mlock()'d pages.
19	     - History.
20	     - Basic management.
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.
28	     - try_to_unmap().
29	     - try_to_munlock() reverse map scan.
30	     - Page reclaim in shrink_*_list().
33	============
35	============
37	This document describes the Linux memory manager's "Unevictable LRU"
38	infrastructure and the use of this to manage several types of "unevictable"
39	pages.
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?".
49	===================
51	===================
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.
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
88	PG_lru is set.
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
104	     itself.
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,
109	in fact, evictable.
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.
122	--------------------------------
124	The unevictable LRU facility interacts with the memory control group [aka
125	memory controller; see Documentation/cgroups/memory.txt] by extending the
126	lru_list enum.
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
135	effects:
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.
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
166		unevictable.
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.
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
192	a reclamation scan.
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
198	freeing them.
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.
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
233	putback_lru_page().
236	=============
238	=============
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.
246	-------
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.
267	----------------
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
280	     MAP_LOCKED flag;
282	 (3) mmapping a region in a task that has called mlockall() with the MCL_FUTURE
283	     flag
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
292	already have it set.
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;
302	     or
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
343	more.
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.
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
392	ignored for munlock.
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
404	mlocking.
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.
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.
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.
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.
519	try_to_unmap()
520	--------------
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
530	map.
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 pages, as these types of pages have
535	different reverse map mechanisms.
537	 (*) try_to_unmap_anon()
539	     To unmap anonymous pages, each VMA in the list anchored in the anon_vma
540	     must be visited - at least until a VM_LOCKED VMA is encountered.  If the
541	     page is being unmapped for migration, VM_LOCKED VMAs do not stop the
542	     process because mlocked pages are migratable.  However, for reclaim, if
543	     the page is mapped into a VM_LOCKED VMA, the scan stops.
545	     try_to_unmap_anon() attempts to acquire in read mode the mmap semaphore of
546	     the mm_struct to which the VMA belongs.  If this is successful, it will
547	     mlock the page via mlock_vma_page() - we wouldn't have gotten to
548	     try_to_unmap_anon() if the page were already mlocked - and will return
549	     SWAP_MLOCK, indicating that the page is unevictable.
551	     If the mmap semaphore cannot be acquired, we are not sure whether the page
552	     is really unevictable or not.  In this case, try_to_unmap_anon() will
553	     return SWAP_AGAIN.
555	 (*) try_to_unmap_file() - linear mappings
557	     Unmapping of a mapped file page works the same as for anonymous mappings,
558	     except that the scan visits all VMAs that map the page's index/page offset
559	     in the page's mapping's reverse map priority search tree.  It also visits
560	     each VMA in the page's mapping's non-linear list, if the list is
561	     non-empty.
563	     As for anonymous pages, on encountering a VM_LOCKED VMA for a mapped file
564	     page, try_to_unmap_file() will attempt to acquire the associated
565	     mm_struct's mmap semaphore to mlock the page, returning SWAP_MLOCK if this
566	     is successful, and SWAP_AGAIN, if not.
568	 (*) try_to_unmap_file() - non-linear mappings
570	     If a page's mapping contains a non-empty non-linear mapping VMA list, then
571	     try_to_un{map|lock}() must also visit each VMA in that list to determine
572	     whether the page is mapped in a VM_LOCKED VMA.  Again, the scan must visit
573	     all VMAs in the non-linear list to ensure that the pages is not/should not
574	     be mlocked.
576	     If a VM_LOCKED VMA is found in the list, the scan could terminate.
577	     However, there is no easy way to determine whether the page is actually
578	     mapped in a given VMA - either for unmapping or testing whether the
579	     VM_LOCKED VMA actually pins the page.
581	     try_to_unmap_file() handles non-linear mappings by scanning a certain
582	     number of pages - a "cluster" - in each non-linear VMA associated with the
583	     page's mapping, for each file mapped page that vmscan tries to unmap.  If
584	     this happens to unmap the page we're trying to unmap, try_to_unmap() will
585	     notice this on return (page_mapcount(page) will be 0) and return
586	     SWAP_SUCCESS.  Otherwise, it will return SWAP_AGAIN, causing vmscan to
587	     recirculate this page.  We take advantage of the cluster scan in
588	     try_to_unmap_cluster() as follows:
590		For each non-linear VMA, try_to_unmap_cluster() attempts to acquire the
591		mmap semaphore of the associated mm_struct for read without blocking.
593		If this attempt is successful and the VMA is VM_LOCKED,
594		try_to_unmap_cluster() will retain the mmap semaphore for the scan;
595		otherwise it drops it here.
597		Then, for each page in the cluster, if we're holding the mmap semaphore
598		for a locked VMA, try_to_unmap_cluster() calls mlock_vma_page() to
599		mlock the page.  This call is a no-op if the page is already locked,
600		but will mlock any pages in the non-linear mapping that happen to be
601		unlocked.
603		If one of the pages so mlocked is the page passed in to try_to_unmap(),
604		try_to_unmap_cluster() will return SWAP_MLOCK, rather than the default
605		SWAP_AGAIN.  This will allow vmscan to cull the page, rather than
606		recirculating it on the inactive list.
608		Again, if try_to_unmap_cluster() cannot acquire the VMA's mmap sem, it
609		returns SWAP_AGAIN, indicating that the page is mapped by a VM_LOCKED
610		VMA, but couldn't be mlocked.
613	try_to_munlock() REVERSE MAP SCAN
614	---------------------------------
616	 [!] TODO/FIXME: a better name might be page_mlocked() - analogous to the
617	     page_referenced() reverse map walker.
619	When munlock_vma_page() [see section "munlock()/munlockall() System Call
620	Handling" above] tries to munlock a page, it needs to determine whether or not
621	the page is mapped by any VM_LOCKED VMA without actually attempting to unmap
622	all PTEs from the page.  For this purpose, the unevictable/mlock infrastructure
623	introduced a variant of try_to_unmap() called try_to_munlock().
625	try_to_munlock() calls the same functions as try_to_unmap() for anonymous and
626	mapped file pages with an additional argument specifying unlock versus unmap
627	processing.  Again, these functions walk the respective reverse maps looking
628	for VM_LOCKED VMAs.  When such a VMA is found for anonymous pages and file
629	pages mapped in linear VMAs, as in the try_to_unmap() case, the functions
630	attempt to acquire the associated mmap semaphore, mlock the page via
631	mlock_vma_page() and return SWAP_MLOCK.  This effectively undoes the
632	pre-clearing of the page's PG_mlocked done by munlock_vma_page.
634	If try_to_unmap() is unable to acquire a VM_LOCKED VMA's associated mmap
635	semaphore, it will return SWAP_AGAIN.  This will allow shrink_page_list() to
636	recycle the page on the inactive list and hope that it has better luck with the
637	page next time.
639	For file pages mapped into non-linear VMAs, the try_to_munlock() logic works
640	slightly differently.  On encountering a VM_LOCKED non-linear VMA that might
641	map the page, try_to_munlock() returns SWAP_AGAIN without actually mlocking the
642	page.  munlock_vma_page() will just leave the page unlocked and let vmscan deal
643	with it - the usual fallback position.
645	Note that try_to_munlock()'s reverse map walk must visit every VMA in a page's
646	reverse map to determine that a page is NOT mapped into any VM_LOCKED VMA.
647	However, the scan can terminate when it encounters a VM_LOCKED VMA and can
648	successfully acquire the VMA's mmap semaphore for read and mlock the page.
649	Although try_to_munlock() might be called a great many times when munlocking a
650	large region or tearing down a large address space that has been mlocked via
651	mlockall(), overall this is a fairly rare event.
654	PAGE RECLAIM IN shrink_*_list()
655	-------------------------------
657	shrink_active_list() culls any obviously unevictable pages - i.e.
658	!page_evictable(page) - diverting these to the unevictable list.
659	However, shrink_active_list() only sees unevictable pages that made it onto the
660	active/inactive lru lists.  Note that these pages do not have PageUnevictable
661	set - otherwise they would be on the unevictable list and shrink_active_list
662	would never see them.
664	Some examples of these unevictable pages on the LRU lists are:
666	 (1) ramfs pages that have been placed on the LRU lists when first allocated.
668	 (2) SHM_LOCK'd shared memory pages.  shmctl(SHM_LOCK) does not attempt to
669	     allocate or fault in the pages in the shared memory region.  This happens
670	     when an application accesses the page the first time after SHM_LOCK'ing
671	     the segment.
673	 (3) mlocked pages that could not be isolated from the LRU and moved to the
674	     unevictable list in mlock_vma_page().
676	 (4) Pages mapped into multiple VM_LOCKED VMAs, but try_to_munlock() couldn't
677	     acquire the VMA's mmap semaphore to test the flags and set PageMlocked.
678	     munlock_vma_page() was forced to let the page back on to the normal LRU
679	     list for vmscan to handle.
681	shrink_inactive_list() also diverts any unevictable pages that it finds on the
682	inactive lists to the appropriate zone's unevictable list.
684	shrink_inactive_list() should only see SHM_LOCK'd pages that became SHM_LOCK'd
685	after shrink_active_list() had moved them to the inactive list, or pages mapped
686	into VM_LOCKED VMAs that munlock_vma_page() couldn't isolate from the LRU to
687	recheck via try_to_munlock().  shrink_inactive_list() won't notice the latter,
688	but will pass on to shrink_page_list().
690	shrink_page_list() again culls obviously unevictable pages that it could
691	encounter for similar reason to shrink_inactive_list().  Pages mapped into
692	VM_LOCKED VMAs but without PG_mlocked set will make it all the way to
693	try_to_unmap().  shrink_page_list() will divert them to the unevictable list
694	when try_to_unmap() returns SWAP_MLOCK, as discussed above.
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