docs: rename Documentation/vm to Documentation/mm

so it will be consistent with code mm directory and with
Documentation/admin-guide/mm and won't be confused with virtual machines.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Tested-by: Ira Weiny <ira.weiny@intel.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Wu XiangCheng <bobwxc@email.cn>

1 files changed, 554 insertions, 0 deletions

diff --git a/Documentation/mm/unevictable-lru.rst b/Documentation/mm/unevictable-lru.rst
new file mode 100644
index 000000000000..b280367d6a44
--- /dev/null
+++ b/Documentation/mm/unevictable-lru.rst
@@ -0,0 +1,554 @@
+.. _unevictable_lru:
+
+==============================
+Unevictable LRU Infrastructure
+==============================
+
+.. contents:: :local:
+
+
+Introduction
+============
+
+This document describes the Linux memory manager's "Unevictable LRU"
+infrastructure and the use of this to manage several types of "unevictable"
+pages.
+
+The document attempts to provide the overall rationale behind this mechanism
+and the rationale for some of the design decisions that drove the
+implementation.  The latter design rationale is discussed in the context of an
+implementation description.  Admittedly, one can obtain the implementation
+details - the "what does it do?" - by reading the code.  One hopes that the
+descriptions below add value by provide the answer to "why does it do that?".
+
+
+
+The Unevictable LRU
+===================
+
+The Unevictable LRU facility adds an additional LRU list to track unevictable
+pages and to hide these pages from vmscan.  This mechanism is based on a patch
+by Larry Woodman of Red Hat to address several scalability problems with page
+reclaim in Linux.  The problems have been observed at customer sites on large
+memory x86_64 systems.
+
+To illustrate this with an example, a non-NUMA x86_64 platform with 128GB of
+main memory will have over 32 million 4k pages in a single node.  When a large
+fraction of these pages are not evictable for any reason [see below], vmscan
+will spend a lot of time scanning the LRU lists looking for the small fraction
+of pages that are evictable.  This can result in a situation where all CPUs are
+spending 100% of their time in vmscan for hours or days on end, with the system
+completely unresponsive.
+
+The unevictable list addresses the following classes of unevictable pages:
+
+ * Those owned by ramfs.
+
+ * Those mapped into SHM_LOCK'd shared memory regions.
+
+ * Those mapped into VM_LOCKED [mlock()ed] VMAs.
+
+The infrastructure may also be able to handle other conditions that make pages
+unevictable, either by definition or by circumstance, in the future.
+
+
+The Unevictable LRU Page List
+-----------------------------
+
+The Unevictable LRU page list is a lie.  It was never an LRU-ordered list, but a
+companion to the LRU-ordered anonymous and file, active and inactive page lists;
+and now it is not even a page list.  But following familiar convention, here in
+this document and in the source, we often imagine it as a fifth LRU page list.
+
+The Unevictable LRU infrastructure consists of an additional, per-node, LRU list
+called the "unevictable" list and an associated page flag, PG_unevictable, to
+indicate that the page is being managed on the unevictable list.
+
+The PG_unevictable flag is analogous to, and mutually exclusive with, the
+PG_active flag in that it indicates on which LRU list a page resides when
+PG_lru is set.
+
+The Unevictable LRU infrastructure maintains unevictable pages as if they were
+on an additional LRU list for a few reasons:
+
+ (1) We get to "treat unevictable pages just like we treat other pages in the
+     system - which means we get to use the same code to manipulate them, the
+     same code to isolate them (for migrate, etc.), the same code to keep track
+     of the statistics, etc..." [Rik van Riel]
+
+ (2) We want to be able to migrate unevictable pages between nodes for memory
+     defragmentation, workload management and memory hotplug.  The Linux kernel
+     can only migrate pages that it can successfully isolate from the LRU
+     lists (or "Movable" pages: outside of consideration here).  If we were to
+     maintain pages elsewhere than on an LRU-like list, where they can be
+     detected by isolate_lru_page(), we would prevent their migration.
+
+The unevictable list does not differentiate between file-backed and anonymous,
+swap-backed pages.  This differentiation is only important while the pages are,
+in fact, evictable.
+
+The unevictable list benefits from the "arrayification" of the per-node LRU
+lists and statistics originally proposed and posted by Christoph Lameter.
+
+
+Memory Control Group Interaction
+--------------------------------
+
+The unevictable LRU facility interacts with the memory control group [aka
+memory controller; see Documentation/admin-guide/cgroup-v1/memory.rst] by
+extending the lru_list enum.
+
+The memory controller data structure automatically gets a per-node unevictable
+list as a result of the "arrayification" of the per-node LRU lists (one per
+lru_list enum element).  The memory controller tracks the movement of pages to
+and from the unevictable list.
+
+When a memory control group comes under memory pressure, the controller will
+not attempt to reclaim pages on the unevictable list.  This has a couple of
+effects:
+
+ (1) Because the pages are "hidden" from reclaim on the unevictable list, the
+     reclaim process can be more efficient, dealing only with pages that have a
+     chance of being reclaimed.
+
+ (2) On the other hand, if too many of the pages charged to the control group
+     are unevictable, the evictable portion of the working set of the tasks in
+     the control group may not fit into the available memory.  This can cause
+     the control group to thrash or to OOM-kill tasks.
+
+
+.. _mark_addr_space_unevict:
+
+Marking Address Spaces Unevictable
+----------------------------------
+
+For facilities such as ramfs none of the pages attached to the address space
+may be evicted.  To prevent eviction of any such pages, the AS_UNEVICTABLE
+address space flag is provided, and this can be manipulated by a filesystem
+using a number of wrapper functions:
+
+ * ``void mapping_set_unevictable(struct address_space *mapping);``
+
+	Mark the address space as being completely unevictable.
+
+ * ``void mapping_clear_unevictable(struct address_space *mapping);``
+
+	Mark the address space as being evictable.
+
+ * ``int mapping_unevictable(struct address_space *mapping);``
+
+	Query the address space, and return true if it is completely
+	unevictable.
+
+These are currently used in three places in the kernel:
+
+ (1) By ramfs to mark the address spaces of its inodes when they are created,
+     and this mark remains for the life of the inode.
+
+ (2) By SYSV SHM to mark SHM_LOCK'd address spaces until SHM_UNLOCK is called.
+     Note that SHM_LOCK is not required to page in the locked pages if they're
+     swapped out; the application must touch the pages manually if it wants to
+     ensure they're in memory.
+
+ (3) By the i915 driver to mark pinned address space until it's unpinned. The
+     amount of unevictable memory marked by i915 driver is roughly the bounded
+     object size in debugfs/dri/0/i915_gem_objects.
+
+
+Detecting Unevictable Pages
+---------------------------
+
+The function page_evictable() in mm/internal.h determines whether a page is
+evictable or not using the query function outlined above [see section
+:ref:`Marking address spaces unevictable <mark_addr_space_unevict>`]
+to check the AS_UNEVICTABLE flag.
+
+For address spaces that are so marked after being populated (as SHM regions
+might be), the lock action (e.g. SHM_LOCK) can be lazy, and need not populate
+the page tables for the region as does, for example, mlock(), nor need it make
+any special effort to push any pages in the SHM_LOCK'd area to the unevictable
+list.  Instead, vmscan will do this if and when it encounters the pages during
+a reclamation scan.
+
+On an unlock action (such as SHM_UNLOCK), the unlocker (e.g. shmctl()) must scan
+the pages in the region and "rescue" them from the unevictable list if no other
+condition is keeping them unevictable.  If an unevictable region is destroyed,
+the pages are also "rescued" from the unevictable list in the process of
+freeing them.
+
+page_evictable() also checks for mlocked pages by testing an additional page
+flag, PG_mlocked (as wrapped by PageMlocked()), which is set when a page is
+faulted into a VM_LOCKED VMA, or found in a VMA being VM_LOCKED.
+
+
+Vmscan's Handling of Unevictable Pages
+--------------------------------------
+
+If unevictable pages are culled in the fault path, or moved to the unevictable
+list at mlock() or mmap() time, vmscan will not encounter the pages until they
+have become evictable again (via munlock() for example) and have been "rescued"
+from the unevictable list.  However, there may be situations where we decide,
+for the sake of expediency, to leave an unevictable page on one of the regular
+active/inactive LRU lists for vmscan to deal with.  vmscan checks for such
+pages in all of the shrink_{active|inactive|page}_list() functions and will
+"cull" such pages that it encounters: that is, it diverts those pages to the
+unevictable list for the memory cgroup and node being scanned.
+
+There may be situations where a page is mapped into a VM_LOCKED VMA, but the
+page is not marked as PG_mlocked.  Such pages will make it all the way to
+shrink_active_list() or shrink_page_list() where they will be detected when
+vmscan walks the reverse map in page_referenced() or try_to_unmap().  The page
+is culled to the unevictable list when it is released by the shrinker.
+
+To "cull" an unevictable page, vmscan simply puts the page back on the LRU list
+using putback_lru_page() - the inverse operation to isolate_lru_page() - after
+dropping the page lock.  Because the condition which makes the page unevictable
+may change once the page is unlocked, __pagevec_lru_add_fn() will recheck the
+unevictable state of a page before placing it on the unevictable list.
+
+
+MLOCKED Pages
+=============
+
+The unevictable page list is also useful for mlock(), in addition to ramfs and
+SYSV SHM.  Note that mlock() is only available in CONFIG_MMU=y situations; in
+NOMMU situations, all mappings are effectively mlocked.
+
+
+History
+-------
+
+The "Unevictable mlocked Pages" infrastructure is based on work originally
+posted by Nick Piggin in an RFC patch entitled "mm: mlocked pages off LRU".
+Nick posted his patch as an alternative to a patch posted by Christoph Lameter
+to achieve the same objective: hiding mlocked pages from vmscan.
+
+In Nick's patch, he used one of the struct page LRU list link fields as a count
+of VM_LOCKED VMAs that map the page (Rik van Riel had the same idea three years
+earlier).  But this use of the link field for a count prevented the management
+of the pages on an LRU list, and thus mlocked pages were not migratable as
+isolate_lru_page() could not detect them, and the LRU list link field was not
+available to the migration subsystem.
+
+Nick resolved this by putting mlocked pages back on the LRU list before
+attempting to isolate them, thus abandoning the count of VM_LOCKED VMAs.  When
+Nick's patch was integrated with the Unevictable LRU work, the count was
+replaced by walking the reverse map when munlocking, to determine whether any
+other VM_LOCKED VMAs still mapped the page.
+
+However, walking the reverse map for each page when munlocking was ugly and
+inefficient, and could lead to catastrophic contention on a file's rmap lock,
+when many processes which had it mlocked were trying to exit.  In 5.18, the
+idea of keeping mlock_count in Unevictable LRU list link field was revived and
+put to work, without preventing the migration of mlocked pages.  This is why
+the "Unevictable LRU list" cannot be a linked list of pages now; but there was
+no use for that linked list anyway - though its size is maintained for meminfo.
+
+
+Basic Management
+----------------
+
+mlocked pages - pages mapped into a VM_LOCKED VMA - are a class of unevictable
+pages.  When such a page has been "noticed" by the memory management subsystem,
+the page is marked with the PG_mlocked flag.  This can be manipulated using the
+PageMlocked() functions.
+
+A PG_mlocked page will be placed on the unevictable list when it is added to
+the LRU.  Such pages can be "noticed" by memory management in several places:
+
+ (1) in the mlock()/mlock2()/mlockall() system call handlers;
+
+ (2) in the mmap() system call handler when mmapping a region with the
+     MAP_LOCKED flag;
+
+ (3) mmapping a region in a task that has called mlockall() with the MCL_FUTURE
+     flag;
+
+ (4) in the fault path and when a VM_LOCKED stack segment is expanded; or
+
+ (5) as mentioned above, in vmscan:shrink_page_list() when attempting to
+     reclaim a page in a VM_LOCKED VMA by page_referenced() or try_to_unmap().
+
+mlocked pages become unlocked and rescued from the unevictable list when:
+
+ (1) mapped in a range unlocked via the munlock()/munlockall() system calls;
+
+ (2) munmap()'d out of the last VM_LOCKED VMA that maps the page, including
+     unmapping at task exit;
+
+ (3) when the page is truncated from the last VM_LOCKED VMA of an mmapped file;
+     or
+
+ (4) before a page is COW'd in a VM_LOCKED VMA.
+
+
+mlock()/mlock2()/mlockall() System Call Handling
+------------------------------------------------
+
+mlock(), mlock2() and mlockall() system call handlers proceed to mlock_fixup()
+for each VMA in the range specified by the call.  In the case of mlockall(),
+this is the entire active address space of the task.  Note that mlock_fixup()
+is used for both mlocking and munlocking a range of memory.  A call to mlock()
+an already VM_LOCKED VMA, or to munlock() a VMA that is not VM_LOCKED, is
+treated as a no-op and mlock_fixup() simply returns.
+
+If the VMA passes some filtering as described in "Filtering Special VMAs"
+below, mlock_fixup() will attempt to merge the VMA with its neighbors or split
+off a subset of the VMA if the range does not cover the entire VMA.  Any pages
+already present in the VMA are then marked as mlocked by mlock_page() via
+mlock_pte_range() via walk_page_range() via mlock_vma_pages_range().
+
+Before returning from the system call, do_mlock() or mlockall() will call
+__mm_populate() to fault in the remaining pages via get_user_pages() and to
+mark those pages as mlocked as they are faulted.
+
+Note that the VMA being mlocked might be mapped with PROT_NONE.  In this case,
+get_user_pages() will be unable to fault in the pages.  That's okay.  If pages
+do end up getting faulted into this VM_LOCKED VMA, they will be handled in the
+fault path - which is also how mlock2()'s MLOCK_ONFAULT areas are handled.
+
+For each PTE (or PMD) being faulted into a VMA, the page add rmap function
+calls mlock_vma_page(), which calls mlock_page() when the VMA is VM_LOCKED
+(unless it is a PTE mapping of a part of a transparent huge page).  Or when
+it is a newly allocated anonymous page, lru_cache_add_inactive_or_unevictable()
+calls mlock_new_page() instead: similar to mlock_page(), but can make better
+judgments, since this page is held exclusively and known not to be on LRU yet.
+
+mlock_page() sets PageMlocked immediately, then places the page on the CPU's
+mlock pagevec, to batch up the rest of the work to be done under lru_lock by
+__mlock_page().  __mlock_page() sets PageUnevictable, initializes mlock_count
+and moves the page to unevictable state ("the unevictable LRU", but with
+mlock_count in place of LRU threading).  Or if the page was already PageLRU
+and PageUnevictable and PageMlocked, it simply increments the mlock_count.
+
+But in practice that may not work ideally: the page may not yet be on an LRU, or
+it may have been temporarily isolated from LRU.  In such cases the mlock_count
+field cannot be touched, but will be set to 0 later when __pagevec_lru_add_fn()
+returns the page to "LRU".  Races prohibit mlock_count from being set to 1 then:
+rather than risk stranding a page indefinitely as unevictable, always err with
+mlock_count on the low side, so that when munlocked the page will be rescued to
+an evictable LRU, then perhaps be mlocked again later if vmscan finds it in a
+VM_LOCKED VMA.
+
+
+Filtering Special VMAs
+----------------------
+
+mlock_fixup() filters several classes of "special" VMAs:
+
+1) VMAs with VM_IO or VM_PFNMAP set are skipped entirely.  The pages behind
+   these mappings are inherently pinned, so we don't need to mark them as
+   mlocked.  In any case, most of the pages have no struct page in which to so
+   mark the page.  Because of this, get_user_pages() will fail for these VMAs,
+   so there is no sense in attempting to visit them.
+
+2) VMAs mapping hugetlbfs page are already effectively pinned into memory.  We
+   neither need nor want to mlock() these pages.  But __mm_populate() includes
+   hugetlbfs ranges, allocating the huge pages and populating the PTEs.
+
+3) VMAs with VM_DONTEXPAND are generally userspace mappings of kernel pages,
+   such as the VDSO page, relay channel pages, etc.  These pages are inherently
+   unevictable and are not managed on the LRU lists.  __mm_populate() includes
+   these ranges, populating the PTEs if not already populated.
+
+4) VMAs with VM_MIXEDMAP set are not marked VM_LOCKED, but __mm_populate()
+   includes these ranges, populating the PTEs if not already populated.
+
+Note that for all of these special VMAs, mlock_fixup() does not set the
+VM_LOCKED flag.  Therefore, we won't have to deal with them later during
+munlock(), munmap() or task exit.  Neither does mlock_fixup() account these
+VMAs against the task's "locked_vm".
+
+
+munlock()/munlockall() System Call Handling
+-------------------------------------------
+
+The munlock() and munlockall() system calls are handled by the same
+mlock_fixup() function as mlock(), mlock2() and mlockall() system calls are.
+If called to munlock an already munlocked VMA, mlock_fixup() simply returns.
+Because of the VMA filtering discussed above, VM_LOCKED will not be set in
+any "special" VMAs.  So, those VMAs will be ignored for munlock.
+
+If the VMA is VM_LOCKED, mlock_fixup() again attempts to merge or split off the
+specified range.  All pages in the VMA are then munlocked by munlock_page() via
+mlock_pte_range() via walk_page_range() via mlock_vma_pages_range() - the same
+function used when mlocking a VMA range, with new flags for the VMA indicating
+that it is munlock() being performed.
+
+munlock_page() uses the mlock pagevec to batch up work to be done under
+lru_lock by  __munlock_page().  __munlock_page() decrements the page's
+mlock_count, and when that reaches 0 it clears PageMlocked and clears
+PageUnevictable, moving the page from unevictable state to inactive LRU.
+
+But in practice that may not work ideally: the page may not yet have reached
+"the unevictable LRU", or it may have been temporarily isolated from it.  In
+those cases its mlock_count field is unusable and must be assumed to be 0: so
+that the page will be rescued to an evictable LRU, then perhaps be mlocked
+again later if vmscan finds it in a VM_LOCKED VMA.
+
+
+Migrating MLOCKED Pages
+-----------------------
+
+A page that is being migrated has been isolated from the LRU lists and is held
+locked across unmapping of the page, updating the page's address space entry
+and copying the contents and state, until the page table entry has been
+replaced with an entry that refers to the new page.  Linux supports migration
+of mlocked pages and other unevictable pages.  PG_mlocked is cleared from the
+the old page when it is unmapped from the last VM_LOCKED VMA, and set when the
+new page is mapped in place of migration entry in a VM_LOCKED VMA.  If the page
+was unevictable because mlocked, PG_unevictable follows PG_mlocked; but if the
+page was unevictable for other reasons, PG_unevictable is copied explicitly.
+
+Note that page migration can race with mlocking or munlocking of the same page.
+There is mostly no problem since page migration requires unmapping all PTEs of
+the old page (including munlock where VM_LOCKED), then mapping in the new page
+(including mlock where VM_LOCKED).  The page table locks provide sufficient
+synchronization.
+
+However, since mlock_vma_pages_range() starts by setting VM_LOCKED on a VMA,
+before mlocking any pages already present, if one of those pages were migrated
+before mlock_pte_range() reached it, it would get counted twice in mlock_count.
+To prevent that, mlock_vma_pages_range() temporarily marks the VMA as VM_IO,
+so that mlock_vma_page() will skip it.
+
+To complete page migration, we place the old and new pages back onto the LRU
+afterwards.  The "unneeded" page - old page on success, new page on failure -
+is freed when the reference count held by the migration process is released.
+
+
+Compacting MLOCKED Pages
+------------------------
+
+The memory map can be scanned for compactable regions and the default behavior
+is to let unevictable pages be moved.  /proc/sys/vm/compact_unevictable_allowed
+controls this behavior (see Documentation/admin-guide/sysctl/vm.rst).  The work
+of compaction is mostly handled by the page migration code and the same work
+flow as described in Migrating MLOCKED Pages will apply.
+
+
+MLOCKING Transparent Huge Pages
+-------------------------------
+
+A transparent huge page is represented by a single entry on an LRU list.
+Therefore, we can only make unevictable an entire compound page, not
+individual subpages.
+
+If a user tries to mlock() part of a huge page, and no user mlock()s the
+whole of the huge page, we want the rest of the page to be reclaimable.
+
+We cannot just split the page on partial mlock() as split_huge_page() can
+fail and a new intermittent failure mode for the syscall is undesirable.
+
+We handle this by keeping PTE-mlocked huge pages on evictable LRU lists:
+the PMD on the border of a VM_LOCKED VMA will be split into a PTE table.
+
+This way the huge page is accessible for vmscan.  Under memory pressure the
+page will be split, subpages which belong to VM_LOCKED VMAs will be moved
+to the unevictable LRU and the rest can be reclaimed.
+
+/proc/meminfo's Unevictable and Mlocked amounts do not include those parts
+of a transparent huge page which are mapped only by PTEs in VM_LOCKED VMAs.
+
+
+mmap(MAP_LOCKED) System Call Handling
+-------------------------------------
+
+In addition to the mlock(), mlock2() and mlockall() system calls, an application
+can request that a region of memory be mlocked by supplying the MAP_LOCKED flag
+to the mmap() call.  There is one important and subtle difference here, though.
+mmap() + mlock() will fail if the range cannot be faulted in (e.g. because
+mm_populate fails) and returns with ENOMEM while mmap(MAP_LOCKED) will not fail.
+The mmaped area will still have properties of the locked area - pages will not
+get swapped out - but major page faults to fault memory in might still happen.
+
+Furthermore, any mmap() call or brk() call that expands the heap by a task
+that has previously called mlockall() with the MCL_FUTURE flag will result
+in the newly mapped memory being mlocked.  Before the unevictable/mlock
+changes, the kernel simply called make_pages_present() to allocate pages
+and populate the page table.
+
+To mlock a range of memory under the unevictable/mlock infrastructure,
+the mmap() handler and task address space expansion functions call
+populate_vma_page_range() specifying the vma and the address range to mlock.
+
+
+munmap()/exit()/exec() System Call Handling
+-------------------------------------------
+
+When unmapping an mlocked region of memory, whether by an explicit call to
+munmap() or via an internal unmap from exit() or exec() processing, we must
+munlock the pages if we're removing the last VM_LOCKED VMA that maps the pages.
+Before the unevictable/mlock changes, mlocking did not mark the pages in any
+way, so unmapping them required no processing.
+
+For each PTE (or PMD) being unmapped from a VMA, page_remove_rmap() calls
+munlock_vma_page(), which calls munlock_page() when the VMA is VM_LOCKED
+(unless it was a PTE mapping of a part of a transparent huge page).
+
+munlock_page() uses the mlock pagevec to batch up work to be done under
+lru_lock by  __munlock_page().  __munlock_page() decrements the page's
+mlock_count, and when that reaches 0 it clears PageMlocked and clears
+PageUnevictable, moving the page from unevictable state to inactive LRU.
+
+But in practice that may not work ideally: the page may not yet have reached
+"the unevictable LRU", or it may have been temporarily isolated from it.  In
+those cases its mlock_count field is unusable and must be assumed to be 0: so
+that the page will be rescued to an evictable LRU, then perhaps be mlocked
+again later if vmscan finds it in a VM_LOCKED VMA.
+
+
+Truncating MLOCKED Pages
+------------------------
+
+File truncation or hole punching forcibly unmaps the deleted pages from
+userspace; truncation even unmaps and deletes any private anonymous pages
+which had been Copied-On-Write from the file pages now being truncated.
+
+Mlocked pages can be munlocked and deleted in this way: like with munmap(),
+for each PTE (or PMD) being unmapped from a VMA, page_remove_rmap() calls
+munlock_vma_page(), which calls munlock_page() when the VMA is VM_LOCKED
+(unless it was a PTE mapping of a part of a transparent huge page).
+
+However, if there is a racing munlock(), since mlock_vma_pages_range() starts
+munlocking by clearing VM_LOCKED from a VMA, before munlocking all the pages
+present, if one of those pages were unmapped by truncation or hole punch before
+mlock_pte_range() reached it, it would not be recognized as mlocked by this VMA,
+and would not be counted out of mlock_count.  In this rare case, a page may
+still appear as PageMlocked after it has been fully unmapped: and it is left to
+release_pages() (or __page_cache_release()) to clear it and update statistics
+before freeing (this event is counted in /proc/vmstat unevictable_pgs_cleared,
+which is usually 0).
+
+
+Page Reclaim in shrink_*_list()
+-------------------------------
+
+vmscan's shrink_active_list() culls any obviously unevictable pages -
+i.e. !page_evictable(page) pages - diverting those to the unevictable list.
+However, shrink_active_list() only sees unevictable pages that made it onto the
+active/inactive LRU lists.  Note that these pages do not have PageUnevictable
+set - otherwise they would be on the unevictable list and shrink_active_list()
+would never see them.
+
+Some examples of these unevictable pages on the LRU lists are:
+
+ (1) ramfs pages that have been placed on the LRU lists when first allocated.
+
+ (2) SHM_LOCK'd shared memory pages.  shmctl(SHM_LOCK) does not attempt to
+     allocate or fault in the pages in the shared memory region.  This happens
+     when an application accesses the page the first time after SHM_LOCK'ing
+     the segment.
+
+ (3) pages still mapped into VM_LOCKED VMAs, which should be marked mlocked,
+     but events left mlock_count too low, so they were munlocked too early.
+
+vmscan's shrink_inactive_list() and shrink_page_list() also divert obviously
+unevictable pages found on the inactive lists to the appropriate memory cgroup
+and node unevictable list.
+
+rmap's page_referenced_one(), called via vmscan's shrink_active_list() or
+shrink_page_list(), and rmap's try_to_unmap_one() called via shrink_page_list(),
+check for (3) pages still mapped into VM_LOCKED VMAs, and call mlock_vma_page()
+to correct them.  Such pages are culled to the unevictable list when released
+by the shrinker.