1==============================
2Unevictable LRU Infrastructure
3==============================
4
5.. contents:: :local:
6
7
8Introduction
9============
10
11This document describes the Linux memory manager's "Unevictable LRU"
12infrastructure and the use of this to manage several types of "unevictable"
13folios.
14
15The document attempts to provide the overall rationale behind this mechanism
16and the rationale for some of the design decisions that drove the
17implementation.  The latter design rationale is discussed in the context of an
18implementation description.  Admittedly, one can obtain the implementation
19details - the "what does it do?" - by reading the code.  One hopes that the
20descriptions below add value by provide the answer to "why does it do that?".
21
22
23
24The Unevictable LRU
25===================
26
27The Unevictable LRU facility adds an additional LRU list to track unevictable
28folios and to hide these folios from vmscan.  This mechanism is based on a patch
29by Larry Woodman of Red Hat to address several scalability problems with folio
30reclaim in Linux.  The problems have been observed at customer sites on large
31memory x86_64 systems.
32
33To illustrate this with an example, a non-NUMA x86_64 platform with 128GB of
34main memory will have over 32 million 4k pages in a single node.  When a large
35fraction of these pages are not evictable for any reason [see below], vmscan
36will spend a lot of time scanning the LRU lists looking for the small fraction
37of pages that are evictable.  This can result in a situation where all CPUs are
38spending 100% of their time in vmscan for hours or days on end, with the system
39completely unresponsive.
40
41The unevictable list addresses the following classes of unevictable pages:
42
43 * Those owned by ramfs.
44
45 * Those owned by tmpfs with the noswap mount option.
46
47 * Those mapped into SHM_LOCK'd shared memory regions.
48
49 * Those mapped into VM_LOCKED [mlock()ed] VMAs.
50
51The infrastructure may also be able to handle other conditions that make pages
52unevictable, either by definition or by circumstance, in the future.
53
54
55The Unevictable LRU Folio List
56------------------------------
57
58The Unevictable LRU folio list is a lie.  It was never an LRU-ordered
59list, but a companion to the LRU-ordered anonymous and file, active and
60inactive folio lists; and now it is not even a folio list.  But following
61familiar convention, here in this document and in the source, we often
62imagine it as a fifth LRU folio list.
63
64The Unevictable LRU infrastructure consists of an additional, per-node, LRU list
65called the "unevictable" list and an associated folio flag, PG_unevictable, to
66indicate that the folio is being managed on the unevictable list.
67
68The PG_unevictable flag is analogous to, and mutually exclusive with, the
69PG_active flag in that it indicates on which LRU list a folio resides when
70PG_lru is set.
71
72The Unevictable LRU infrastructure maintains unevictable folios as if they were
73on an additional LRU list for a few reasons:
74
75 (1) We get to "treat unevictable folios just like we treat other folios in the
76     system - which means we get to use the same code to manipulate them, the
77     same code to isolate them (for migrate, etc.), the same code to keep track
78     of the statistics, etc..." [Rik van Riel]
79
80 (2) We want to be able to migrate unevictable folios between nodes for memory
81     defragmentation, workload management and memory hotplug.  The Linux kernel
82     can only migrate folios that it can successfully isolate from the LRU
83     lists (or "Movable" pages: outside of consideration here).  If we were to
84     maintain folios elsewhere than on an LRU-like list, where they can be
85     detected by folio_isolate_lru(), we would prevent their migration.
86
87The unevictable list does not differentiate between file-backed and
88anonymous, swap-backed folios.  This differentiation is only important
89while the folios are, in fact, evictable.
90
91The unevictable list benefits from the "arrayification" of the per-node LRU
92lists and statistics originally proposed and posted by Christoph Lameter.
93
94
95Memory Control Group Interaction
96--------------------------------
97
98The unevictable LRU facility interacts with the memory control group [aka
99memory controller; see Documentation/admin-guide/cgroup-v1/memory.rst] by
100extending the lru_list enum.
101
102The memory controller data structure automatically gets a per-node unevictable
103list as a result of the "arrayification" of the per-node LRU lists (one per
104lru_list enum element).  The memory controller tracks the movement of pages to
105and from the unevictable list.
106
107When a memory control group comes under memory pressure, the controller will
108not attempt to reclaim pages on the unevictable list.  This has a couple of
109effects:
110
111 (1) Because the pages are "hidden" from reclaim on the unevictable list, the
112     reclaim process can be more efficient, dealing only with pages that have a
113     chance of being reclaimed.
114
115 (2) On the other hand, if too many of the pages charged to the control group
116     are unevictable, the evictable portion of the working set of the tasks in
117     the control group may not fit into the available memory.  This can cause
118     the control group to thrash or to OOM-kill tasks.
119
120
121.. _mark_addr_space_unevict:
122
123Marking Address Spaces Unevictable
124----------------------------------
125
126For facilities such as ramfs none of the pages attached to the address space
127may be evicted.  To prevent eviction of any such pages, the AS_UNEVICTABLE
128address space flag is provided, and this can be manipulated by a filesystem
129using a number of wrapper functions:
130
131 * ``void mapping_set_unevictable(struct address_space *mapping);``
132
133	Mark the address space as being completely unevictable.
134
135 * ``void mapping_clear_unevictable(struct address_space *mapping);``
136
137	Mark the address space as being evictable.
138
139 * ``int mapping_unevictable(struct address_space *mapping);``
140
141	Query the address space, and return true if it is completely
142	unevictable.
143
144These are currently used in three places in the kernel:
145
146 (1) By ramfs to mark the address spaces of its inodes when they are created,
147     and this mark remains for the life of the inode.
148
149 (2) By SYSV SHM to mark SHM_LOCK'd address spaces until SHM_UNLOCK is called.
150     Note that SHM_LOCK is not required to page in the locked pages if they're
151     swapped out; the application must touch the pages manually if it wants to
152     ensure they're in memory.
153
154 (3) By the i915 driver to mark pinned address space until it's unpinned. The
155     amount of unevictable memory marked by i915 driver is roughly the bounded
156     object size in debugfs/dri/0/i915_gem_objects.
157
158
159Detecting Unevictable Pages
160---------------------------
161
162The function folio_evictable() in mm/internal.h determines whether a folio is
163evictable or not using the query function outlined above [see section
164:ref:`Marking address spaces unevictable <mark_addr_space_unevict>`]
165to check the AS_UNEVICTABLE flag.
166
167For address spaces that are so marked after being populated (as SHM regions
168might be), the lock action (e.g. SHM_LOCK) can be lazy, and need not populate
169the page tables for the region as does, for example, mlock(), nor need it make
170any special effort to push any pages in the SHM_LOCK'd area to the unevictable
171list.  Instead, vmscan will do this if and when it encounters the folios during
172a reclamation scan.
173
174On an unlock action (such as SHM_UNLOCK), the unlocker (e.g. shmctl()) must scan
175the pages in the region and "rescue" them from the unevictable list if no other
176condition is keeping them unevictable.  If an unevictable region is destroyed,
177the pages are also "rescued" from the unevictable list in the process of
178freeing them.
179
180folio_evictable() also checks for mlocked folios by calling
181folio_test_mlocked(), which is set when a folio is faulted into a
182VM_LOCKED VMA, or found in a VMA being VM_LOCKED.
183
184
185Vmscan's Handling of Unevictable Folios
186---------------------------------------
187
188If unevictable folios are culled in the fault path, or moved to the unevictable
189list at mlock() or mmap() time, vmscan will not encounter the folios until they
190have become evictable again (via munlock() for example) and have been "rescued"
191from the unevictable list.  However, there may be situations where we decide,
192for the sake of expediency, to leave an unevictable folio on one of the regular
193active/inactive LRU lists for vmscan to deal with.  vmscan checks for such
194folios in all of the shrink_{active|inactive|page}_list() functions and will
195"cull" such folios that it encounters: that is, it diverts those folios to the
196unevictable list for the memory cgroup and node being scanned.
197
198There may be situations where a folio is mapped into a VM_LOCKED VMA,
199but the folio does not have the mlocked flag set.  Such folios will make
200it all the way to shrink_active_list() or shrink_page_list() where they
201will be detected when vmscan walks the reverse map in folio_referenced()
202or try_to_unmap().  The folio is culled to the unevictable list when it
203is released by the shrinker.
204
205To "cull" an unevictable folio, vmscan simply puts the folio back on
206the LRU list using folio_putback_lru() - the inverse operation to
207folio_isolate_lru() - after dropping the folio lock.  Because the
208condition which makes the folio unevictable may change once the folio
209is unlocked, __pagevec_lru_add_fn() will recheck the unevictable state
210of a folio before placing it on the unevictable list.
211
212
213MLOCKED Pages
214=============
215
216The unevictable folio list is also useful for mlock(), in addition to ramfs and
217SYSV SHM.  Note that mlock() is only available in CONFIG_MMU=y situations; in
218NOMMU situations, all mappings are effectively mlocked.
219
220
221History
222-------
223
224The "Unevictable mlocked Pages" infrastructure is based on work originally
225posted by Nick Piggin in an RFC patch entitled "mm: mlocked pages off LRU".
226Nick posted his patch as an alternative to a patch posted by Christoph Lameter
227to achieve the same objective: hiding mlocked pages from vmscan.
228
229In Nick's patch, he used one of the struct page LRU list link fields as a count
230of VM_LOCKED VMAs that map the page (Rik van Riel had the same idea three years
231earlier).  But this use of the link field for a count prevented the management
232of the pages on an LRU list, and thus mlocked pages were not migratable as
233isolate_lru_page() could not detect them, and the LRU list link field was not
234available to the migration subsystem.
235
236Nick resolved this by putting mlocked pages back on the LRU list before
237attempting to isolate them, thus abandoning the count of VM_LOCKED VMAs.  When
238Nick's patch was integrated with the Unevictable LRU work, the count was
239replaced by walking the reverse map when munlocking, to determine whether any
240other VM_LOCKED VMAs still mapped the page.
241
242However, walking the reverse map for each page when munlocking was ugly and
243inefficient, and could lead to catastrophic contention on a file's rmap lock,
244when many processes which had it mlocked were trying to exit.  In 5.18, the
245idea of keeping mlock_count in Unevictable LRU list link field was revived and
246put to work, without preventing the migration of mlocked pages.  This is why
247the "Unevictable LRU list" cannot be a linked list of pages now; but there was
248no use for that linked list anyway - though its size is maintained for meminfo.
249
250
251Basic Management
252----------------
253
254mlocked pages - pages mapped into a VM_LOCKED VMA - are a class of unevictable
255pages.  When such a page has been "noticed" by the memory management subsystem,
256the page is marked with the PG_mlocked flag.  This can be manipulated using the
257PageMlocked() functions.
258
259A PG_mlocked page will be placed on the unevictable list when it is added to
260the LRU.  Such pages can be "noticed" by memory management in several places:
261
262 (1) in the mlock()/mlock2()/mlockall() system call handlers;
263
264 (2) in the mmap() system call handler when mmapping a region with the
265     MAP_LOCKED flag;
266
267 (3) mmapping a region in a task that has called mlockall() with the MCL_FUTURE
268     flag;
269
270 (4) in the fault path and when a VM_LOCKED stack segment is expanded; or
271
272 (5) as mentioned above, in vmscan:shrink_page_list() when attempting to
273     reclaim a page in a VM_LOCKED VMA by folio_referenced() or try_to_unmap().
274
275mlocked pages become unlocked and rescued from the unevictable list when:
276
277 (1) mapped in a range unlocked via the munlock()/munlockall() system calls;
278
279 (2) munmap()'d out of the last VM_LOCKED VMA that maps the page, including
280     unmapping at task exit;
281
282 (3) when the page is truncated from the last VM_LOCKED VMA of an mmapped file;
283     or
284
285 (4) before a page is COW'd in a VM_LOCKED VMA.
286
287
288mlock()/mlock2()/mlockall() System Call Handling
289------------------------------------------------
290
291mlock(), mlock2() and mlockall() system call handlers proceed to mlock_fixup()
292for each VMA in the range specified by the call.  In the case of mlockall(),
293this is the entire active address space of the task.  Note that mlock_fixup()
294is used for both mlocking and munlocking a range of memory.  A call to mlock()
295an already VM_LOCKED VMA, or to munlock() a VMA that is not VM_LOCKED, is
296treated as a no-op and mlock_fixup() simply returns.
297
298If the VMA passes some filtering as described in "Filtering Special VMAs"
299below, mlock_fixup() will attempt to merge the VMA with its neighbors or split
300off a subset of the VMA if the range does not cover the entire VMA.  Any pages
301already present in the VMA are then marked as mlocked by mlock_folio() via
302mlock_pte_range() via walk_page_range() via mlock_vma_pages_range().
303
304Before returning from the system call, do_mlock() or mlockall() will call
305__mm_populate() to fault in the remaining pages via get_user_pages() and to
306mark those pages as mlocked as they are faulted.
307
308Note that the VMA being mlocked might be mapped with PROT_NONE.  In this case,
309get_user_pages() will be unable to fault in the pages.  That's okay.  If pages
310do end up getting faulted into this VM_LOCKED VMA, they will be handled in the
311fault path - which is also how mlock2()'s MLOCK_ONFAULT areas are handled.
312
313For each PTE (or PMD) being faulted into a VMA, the page add rmap function
314calls mlock_vma_folio(), which calls mlock_folio() when the VMA is VM_LOCKED
315(unless it is a PTE mapping of a part of a transparent huge page).  Or when
316it is a newly allocated anonymous page, folio_add_lru_vma() calls
317mlock_new_folio() instead: similar to mlock_folio(), but can make better
318judgments, since this page is held exclusively and known not to be on LRU yet.
319
320mlock_folio() sets PG_mlocked immediately, then places the page on the CPU's
321mlock folio batch, to batch up the rest of the work to be done under lru_lock by
322__mlock_folio().  __mlock_folio() sets PG_unevictable, initializes mlock_count
323and moves the page to unevictable state ("the unevictable LRU", but with
324mlock_count in place of LRU threading).  Or if the page was already PG_lru
325and PG_unevictable and PG_mlocked, it simply increments the mlock_count.
326
327But in practice that may not work ideally: the page may not yet be on an LRU, or
328it may have been temporarily isolated from LRU.  In such cases the mlock_count
329field cannot be touched, but will be set to 0 later when __munlock_folio()
330returns the page to "LRU".  Races prohibit mlock_count from being set to 1 then:
331rather than risk stranding a page indefinitely as unevictable, always err with
332mlock_count on the low side, so that when munlocked the page will be rescued to
333an evictable LRU, then perhaps be mlocked again later if vmscan finds it in a
334VM_LOCKED VMA.
335
336
337Filtering Special VMAs
338----------------------
339
340mlock_fixup() filters several classes of "special" VMAs:
341
3421) VMAs with VM_IO or VM_PFNMAP set are skipped entirely.  The pages behind
343   these mappings are inherently pinned, so we don't need to mark them as
344   mlocked.  In any case, most of the pages have no struct page in which to so
345   mark the page.  Because of this, get_user_pages() will fail for these VMAs,
346   so there is no sense in attempting to visit them.
347
3482) VMAs mapping hugetlbfs page are already effectively pinned into memory.  We
349   neither need nor want to mlock() these pages.  But __mm_populate() includes
350   hugetlbfs ranges, allocating the huge pages and populating the PTEs.
351
3523) VMAs with VM_DONTEXPAND are generally userspace mappings of kernel pages,
353   such as the VDSO page, relay channel pages, etc.  These pages are inherently
354   unevictable and are not managed on the LRU lists.  __mm_populate() includes
355   these ranges, populating the PTEs if not already populated.
356
3574) VMAs with VM_MIXEDMAP set are not marked VM_LOCKED, but __mm_populate()
358   includes these ranges, populating the PTEs if not already populated.
359
360Note that for all of these special VMAs, mlock_fixup() does not set the
361VM_LOCKED flag.  Therefore, we won't have to deal with them later during
362munlock(), munmap() or task exit.  Neither does mlock_fixup() account these
363VMAs against the task's "locked_vm".
364
365
366munlock()/munlockall() System Call Handling
367-------------------------------------------
368
369The munlock() and munlockall() system calls are handled by the same
370mlock_fixup() function as mlock(), mlock2() and mlockall() system calls are.
371If called to munlock an already munlocked VMA, mlock_fixup() simply returns.
372Because of the VMA filtering discussed above, VM_LOCKED will not be set in
373any "special" VMAs.  So, those VMAs will be ignored for munlock.
374
375If the VMA is VM_LOCKED, mlock_fixup() again attempts to merge or split off the
376specified range.  All pages in the VMA are then munlocked by munlock_folio() via
377mlock_pte_range() via walk_page_range() via mlock_vma_pages_range() - the same
378function used when mlocking a VMA range, with new flags for the VMA indicating
379that it is munlock() being performed.
380
381munlock_folio() uses the mlock pagevec to batch up work to be done
382under lru_lock by  __munlock_folio().  __munlock_folio() decrements the
383folio's mlock_count, and when that reaches 0 it clears the mlocked flag
384and clears the unevictable flag, moving the folio from unevictable state
385to the inactive LRU.
386
387But in practice that may not work ideally: the folio may not yet have reached
388"the unevictable LRU", or it may have been temporarily isolated from it.  In
389those cases its mlock_count field is unusable and must be assumed to be 0: so
390that the folio will be rescued to an evictable LRU, then perhaps be mlocked
391again later if vmscan finds it in a VM_LOCKED VMA.
392
393
394Migrating MLOCKED Pages
395-----------------------
396
397A page that is being migrated has been isolated from the LRU lists and is held
398locked across unmapping of the page, updating the page's address space entry
399and copying the contents and state, until the page table entry has been
400replaced with an entry that refers to the new page.  Linux supports migration
401of mlocked pages and other unevictable pages.  PG_mlocked is cleared from the
402the old page when it is unmapped from the last VM_LOCKED VMA, and set when the
403new page is mapped in place of migration entry in a VM_LOCKED VMA.  If the page
404was unevictable because mlocked, PG_unevictable follows PG_mlocked; but if the
405page was unevictable for other reasons, PG_unevictable is copied explicitly.
406
407Note that page migration can race with mlocking or munlocking of the same page.
408There is mostly no problem since page migration requires unmapping all PTEs of
409the old page (including munlock where VM_LOCKED), then mapping in the new page
410(including mlock where VM_LOCKED).  The page table locks provide sufficient
411synchronization.
412
413However, since mlock_vma_pages_range() starts by setting VM_LOCKED on a VMA,
414before mlocking any pages already present, if one of those pages were migrated
415before mlock_pte_range() reached it, it would get counted twice in mlock_count.
416To prevent that, mlock_vma_pages_range() temporarily marks the VMA as VM_IO,
417so that mlock_vma_folio() will skip it.
418
419To complete page migration, we place the old and new pages back onto the LRU
420afterwards.  The "unneeded" page - old page on success, new page on failure -
421is freed when the reference count held by the migration process is released.
422
423
424Compacting MLOCKED Pages
425------------------------
426
427The memory map can be scanned for compactable regions and the default behavior
428is to let unevictable pages be moved.  /proc/sys/vm/compact_unevictable_allowed
429controls this behavior (see Documentation/admin-guide/sysctl/vm.rst).  The work
430of compaction is mostly handled by the page migration code and the same work
431flow as described in Migrating MLOCKED Pages will apply.
432
433
434MLOCKING Transparent Huge Pages
435-------------------------------
436
437A transparent huge page is represented by a single entry on an LRU list.
438Therefore, we can only make unevictable an entire compound page, not
439individual subpages.
440
441If a user tries to mlock() part of a huge page, and no user mlock()s the
442whole of the huge page, we want the rest of the page to be reclaimable.
443
444We cannot just split the page on partial mlock() as split_huge_page() can
445fail and a new intermittent failure mode for the syscall is undesirable.
446
447We handle this by keeping PTE-mlocked huge pages on evictable LRU lists:
448the PMD on the border of a VM_LOCKED VMA will be split into a PTE table.
449
450This way the huge page is accessible for vmscan.  Under memory pressure the
451page will be split, subpages which belong to VM_LOCKED VMAs will be moved
452to the unevictable LRU and the rest can be reclaimed.
453
454/proc/meminfo's Unevictable and Mlocked amounts do not include those parts
455of a transparent huge page which are mapped only by PTEs in VM_LOCKED VMAs.
456
457
458mmap(MAP_LOCKED) System Call Handling
459-------------------------------------
460
461In addition to the mlock(), mlock2() and mlockall() system calls, an application
462can request that a region of memory be mlocked by supplying the MAP_LOCKED flag
463to the mmap() call.  There is one important and subtle difference here, though.
464mmap() + mlock() will fail if the range cannot be faulted in (e.g. because
465mm_populate fails) and returns with ENOMEM while mmap(MAP_LOCKED) will not fail.
466The mmapped area will still have properties of the locked area - pages will not
467get swapped out - but major page faults to fault memory in might still happen.
468
469Furthermore, any mmap() call or brk() call that expands the heap by a task
470that has previously called mlockall() with the MCL_FUTURE flag will result
471in the newly mapped memory being mlocked.  Before the unevictable/mlock
472changes, the kernel simply called make_pages_present() to allocate pages
473and populate the page table.
474
475To mlock a range of memory under the unevictable/mlock infrastructure,
476the mmap() handler and task address space expansion functions call
477populate_vma_page_range() specifying the vma and the address range to mlock.
478
479
480munmap()/exit()/exec() System Call Handling
481-------------------------------------------
482
483When unmapping an mlocked region of memory, whether by an explicit call to
484munmap() or via an internal unmap from exit() or exec() processing, we must
485munlock the pages if we're removing the last VM_LOCKED VMA that maps the pages.
486Before the unevictable/mlock changes, mlocking did not mark the pages in any
487way, so unmapping them required no processing.
488
489For each PTE (or PMD) being unmapped from a VMA, page_remove_rmap() calls
490munlock_vma_folio(), which calls munlock_folio() when the VMA is VM_LOCKED
491(unless it was a PTE mapping of a part of a transparent huge page).
492
493munlock_folio() uses the mlock pagevec to batch up work to be done
494under lru_lock by  __munlock_folio().  __munlock_folio() decrements the
495folio's mlock_count, and when that reaches 0 it clears the mlocked flag
496and clears the unevictable flag, moving the folio from unevictable state
497to the inactive LRU.
498
499But in practice that may not work ideally: the folio may not yet have reached
500"the unevictable LRU", or it may have been temporarily isolated from it.  In
501those cases its mlock_count field is unusable and must be assumed to be 0: so
502that the folio will be rescued to an evictable LRU, then perhaps be mlocked
503again later if vmscan finds it in a VM_LOCKED VMA.
504
505
506Truncating MLOCKED Pages
507------------------------
508
509File truncation or hole punching forcibly unmaps the deleted pages from
510userspace; truncation even unmaps and deletes any private anonymous pages
511which had been Copied-On-Write from the file pages now being truncated.
512
513Mlocked pages can be munlocked and deleted in this way: like with munmap(),
514for each PTE (or PMD) being unmapped from a VMA, page_remove_rmap() calls
515munlock_vma_folio(), which calls munlock_folio() when the VMA is VM_LOCKED
516(unless it was a PTE mapping of a part of a transparent huge page).
517
518However, if there is a racing munlock(), since mlock_vma_pages_range() starts
519munlocking by clearing VM_LOCKED from a VMA, before munlocking all the pages
520present, if one of those pages were unmapped by truncation or hole punch before
521mlock_pte_range() reached it, it would not be recognized as mlocked by this VMA,
522and would not be counted out of mlock_count.  In this rare case, a page may
523still appear as PG_mlocked after it has been fully unmapped: and it is left to
524release_pages() (or __page_cache_release()) to clear it and update statistics
525before freeing (this event is counted in /proc/vmstat unevictable_pgs_cleared,
526which is usually 0).
527
528
529Page Reclaim in shrink_*_list()
530-------------------------------
531
532vmscan's shrink_active_list() culls any obviously unevictable pages -
533i.e. !page_evictable(page) pages - diverting those to the unevictable list.
534However, shrink_active_list() only sees unevictable pages that made it onto the
535active/inactive LRU lists.  Note that these pages do not have PG_unevictable
536set - otherwise they would be on the unevictable list and shrink_active_list()
537would never see them.
538
539Some examples of these unevictable pages on the LRU lists are:
540
541 (1) ramfs pages that have been placed on the LRU lists when first allocated.
542
543 (2) SHM_LOCK'd shared memory pages.  shmctl(SHM_LOCK) does not attempt to
544     allocate or fault in the pages in the shared memory region.  This happens
545     when an application accesses the page the first time after SHM_LOCK'ing
546     the segment.
547
548 (3) pages still mapped into VM_LOCKED VMAs, which should be marked mlocked,
549     but events left mlock_count too low, so they were munlocked too early.
550
551vmscan's shrink_inactive_list() and shrink_page_list() also divert obviously
552unevictable pages found on the inactive lists to the appropriate memory cgroup
553and node unevictable list.
554
555rmap's folio_referenced_one(), called via vmscan's shrink_active_list() or
556shrink_page_list(), and rmap's try_to_unmap_one() called via shrink_page_list(),
557check for (3) pages still mapped into VM_LOCKED VMAs, and call mlock_vma_folio()
558to correct them.  Such pages are culled to the unevictable list when released
559by the shrinker.
560