Adding upstream version 4.19.249.upstream/4.19.249

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. _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 zone.  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 Page List
+-------------------------
+
+The Unevictable LRU infrastructure consists of an additional, per-zone, 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 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.  If we were to maintain pages elsewhere than on an LRU-like list,
+     where they can be found by isolate_lru_page(), we would prevent their
+     migration, unless we reworked migration code to find the unevictable pages
+     itself.
+
+
+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-zone LRU
+lists and statistics originally proposed and posted by Christoph Lameter.
+
+The unevictable list does not use the LRU pagevec mechanism. Rather,
+unevictable pages are placed directly on the page's zone's unevictable list
+under the zone lru_lock.  This allows us to prevent the stranding of pages on
+the unevictable list when one task has the page isolated from the LRU and other
+tasks are changing the "evictability" state of the page.
+
+
+Memory Control Group Interaction
+--------------------------------
+
+The unevictable LRU facility interacts with the memory control group [aka
+memory controller; see Documentation/cgroup-v1/memory.txt] by extending the
+lru_list enum.
+
+The memory controller data structure automatically gets a per-zone unevictable
+list as a result of the "arrayification" of the per-zone 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 two 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.
+
+
+Detecting Unevictable Pages
+---------------------------
+
+The function page_evictable() in vmscan.c 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 (eg: 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 (eg: 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 a 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 zone 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_page_list() where they will be detected when vmscan walks the reverse
+map in try_to_unmap().  If try_to_unmap() returns SWAP_MLOCK,
+shrink_page_list() will cull the page at that point.
+
+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, putback_lru_page() will recheck the
+unevictable state of a page that it places on the unevictable list.  If the
+page has become unevictable, putback_lru_page() removes it from the list and
+retries, including the page_unevictable() test.  Because such a race is a rare
+event and movement of pages onto the unevictable list should be rare, these
+extra evictabilty checks should not occur in the majority of calls to
+putback_lru_page().
+
+
+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.  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 find 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 to determine whether any VM_LOCKED VMAs
+mapped the page.  More on this below.
+
+
+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()/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, if mlocked pages are "culled" 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 via try_to_unmap()
+
+all of which result in the VM_LOCKED flag being set for the VMA if it doesn't
+already have it set.
+
+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()/mlockall() System Call Handling
+---------------------------------------
+
+Both [do\_]mlock() and [do\_]mlockall() system call handlers call 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.  Once the
+VMA has been merged or split or neither, mlock_fixup() will call
+populate_vma_page_range() to fault in the pages via get_user_pages() and to
+mark the pages as mlocked via mlock_vma_page().
+
+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, we'll handle them in the
+fault path or in vmscan.
+
+Also note that a page returned by get_user_pages() could be truncated or
+migrated out from under us, while we're trying to mlock it.  To detect this,
+populate_vma_page_range() checks page_mapping() after acquiring the page lock.
+If the page is still associated with its mapping, we'll go ahead and call
+mlock_vma_page().  If the mapping is gone, we just unlock the page and move on.
+In the worst case, this will result in a page mapped in a VM_LOCKED VMA
+remaining on a normal LRU list without being PageMlocked().  Again, vmscan will
+detect and cull such pages.
+
+mlock_vma_page() will call TestSetPageMlocked() for each page returned by
+get_user_pages().  We use TestSetPageMlocked() because the page might already
+be mlocked by another task/VMA and we don't want to do extra work.  We
+especially do not want to count an mlocked page more than once in the
+statistics.  If the page was already mlocked, mlock_vma_page() need do nothing
+more.
+
+If the page was NOT already mlocked, mlock_vma_page() attempts to isolate the
+page from the LRU, as it is likely on the appropriate active or inactive list
+at that time.  If the isolate_lru_page() succeeds, mlock_vma_page() will put
+back the page - by calling putback_lru_page() - which will notice that the page
+is now mlocked and divert the page to the zone's unevictable list.  If
+mlock_vma_page() is unable to isolate the page from the LRU, vmscan will handle
+it later if and when it attempts to reclaim the page.
+
+
+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.  However, to preserve the
+   prior behavior of mlock() - before the unevictable/mlock changes -
+   mlock_fixup() will call make_pages_present() in the hugetlbfs VMA range to
+   allocate the huge pages and populate 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.
+   mlock_fixup() treats these VMAs the same as hugetlbfs VMAs.  It calls
+   make_pages_present() to populate the ptes.
+
+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_handling:
+
+munlock()/munlockall() System Call Handling
+-------------------------------------------
+
+The munlock() and munlockall() system calls are handled by the same functions -
+do_mlock[all]() - as the mlock() and mlockall() system calls with the unlock vs
+lock operation indicated by an argument.  So, these system calls are also
+handled by mlock_fixup().  Again, if called for 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, these VMAs will be
+ignored for munlock.
+
+If the VMA is VM_LOCKED, mlock_fixup() again attempts to merge or split off the
+specified range.  The range is then munlocked via the function
+populate_vma_page_range() - the same function used to mlock a VMA range -
+passing a flag to indicate that munlock() is being performed.
+
+Because the VMA access protections could have been changed to PROT_NONE after
+faulting in and mlocking pages, get_user_pages() was unreliable for visiting
+these pages for munlocking.  Because we don't want to leave pages mlocked,
+get_user_pages() was enhanced to accept a flag to ignore the permissions when
+fetching the pages - all of which should be resident as a result of previous
+mlocking.
+
+For munlock(), populate_vma_page_range() unlocks individual pages by calling
+munlock_vma_page().  munlock_vma_page() unconditionally clears the PG_mlocked
+flag using TestClearPageMlocked().  As with mlock_vma_page(),
+munlock_vma_page() use the Test*PageMlocked() function to handle the case where
+the page might have already been unlocked by another task.  If the page was
+mlocked, munlock_vma_page() updates that zone statistics for the number of
+mlocked pages.  Note, however, that at this point we haven't checked whether
+the page is mapped by other VM_LOCKED VMAs.
+
+We can't call try_to_munlock(), the function that walks the reverse map to
+check for other VM_LOCKED VMAs, without first isolating the page from the LRU.
+try_to_munlock() is a variant of try_to_unmap() and thus requires that the page
+not be on an LRU list [more on these below].  However, the call to
+isolate_lru_page() could fail, in which case we couldn't try_to_munlock().  So,
+we go ahead and clear PG_mlocked up front, as this might be the only chance we
+have.  If we can successfully isolate the page, we go ahead and
+try_to_munlock(), which will restore the PG_mlocked flag and update the zone
+page statistics if it finds another VMA holding the page mlocked.  If we fail
+to isolate the page, we'll have left a potentially mlocked page on the LRU.
+This is fine, because we'll catch it later if and if vmscan tries to reclaim
+the page.  This should be relatively rare.
+
+
+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.  This involves simply moving the
+PG_mlocked and PG_unevictable states from the old page to the new page.
+
+Note that page migration can race with mlocking or munlocking of the same page.
+This has been discussed from the mlock/munlock perspective in the respective
+sections above.  Both processes (migration and m[un]locking) hold the page
+locked.  This provides the first level of synchronization.  Page migration
+zeros out the page_mapping of the old page before unlocking it, so m[un]lock
+can skip these pages by testing the page mapping under page lock.
+
+To complete page migration, we place the new and old pages back onto the LRU
+after dropping the page lock.  The "unneeded" page - old page on success, new
+page on failure - will be freed when the reference count held by the migration
+process is released.  To ensure that we don't strand pages on the unevictable
+list because of a race between munlock and migration, page migration uses the
+putback_lru_page() function to add migrated pages back to the LRU.
+
+
+Compacting MLOCKED Pages
+------------------------
+
+The unevictable LRU can be scanned for compactable regions and the default
+behavior is to do so.  /proc/sys/vm/compact_unevictable_allowed controls
+this behavior (see Documentation/sysctl/vm.txt).  Once scanning of the
+unevictable LRU is enabled, 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, 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 new intermittent failure mode for the syscall is undesirable.
+
+We handle this by keeping PTE-mapped huge pages on normal LRU lists: the
+PMD on border of VM_LOCKED VMA will be split into 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 unevictable LRU and the rest can be reclaimed.
+
+See also comment in follow_trans_huge_pmd().
+
+mmap(MAP_LOCKED) System Call Handling
+-------------------------------------
+
+In addition the mlock()/mlockall() system calls, an application can request
+that a region of memory be mlocked 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 - aka. 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.
+
+The callers of populate_vma_page_range() will have already added the memory range
+to be mlocked to the task's "locked_vm".  To account for filtered VMAs,
+populate_vma_page_range() returns the number of pages NOT mlocked.  All of the
+callers then subtract a non-negative return value from the task's locked_vm.  A
+negative return value represent an error - for example, from get_user_pages()
+attempting to fault in a VMA with PROT_NONE access.  In this case, we leave the
+memory range accounted as locked_vm, as the protections could be changed later
+and pages allocated into that region.
+
+
+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.
+
+To munlock a range of memory under the unevictable/mlock infrastructure, the
+munmap() handler and task address space call tear down function
+munlock_vma_pages_all().  The name reflects the observation that one always
+specifies the entire VMA range when munlock()ing during unmap of a region.
+Because of the VMA filtering when mlocking() regions, only "normal" VMAs that
+actually contain mlocked pages will be passed to munlock_vma_pages_all().
+
+munlock_vma_pages_all() clears the VM_LOCKED VMA flag and, like mlock_fixup()
+for the munlock case, calls __munlock_vma_pages_range() to walk the page table
+for the VMA's memory range and munlock_vma_page() each resident page mapped by
+the VMA.  This effectively munlocks the page, only if this is the last
+VM_LOCKED VMA that maps the page.
+
+
+try_to_unmap()
+--------------
+
+Pages can, of course, be mapped into multiple VMAs.  Some of these VMAs may
+have VM_LOCKED flag set.  It is possible for a page mapped into one or more
+VM_LOCKED VMAs not to have the PG_mlocked flag set and therefore reside on one
+of the active or inactive LRU lists.  This could happen if, for example, a task
+in the process of munlocking the page could not isolate the page from the LRU.
+As a result, vmscan/shrink_page_list() might encounter such a page as described
+in section "vmscan's handling of unevictable pages".  To handle this situation,
+try_to_unmap() checks for VM_LOCKED VMAs while it is walking a page's reverse
+map.
+
+try_to_unmap() is always called, by either vmscan for reclaim or for page
+migration, with the argument page locked and isolated from the LRU.  Separate
+functions handle anonymous and mapped file and KSM pages, as these types of
+pages have different reverse map lookup mechanisms, with different locking.
+In each case, whether rmap_walk_anon() or rmap_walk_file() or rmap_walk_ksm(),
+it will call try_to_unmap_one() for every VMA which might contain the page.
+
+When trying to reclaim, if try_to_unmap_one() finds the page in a VM_LOCKED
+VMA, it will then mlock the page via mlock_vma_page() instead of unmapping it,
+and return SWAP_MLOCK to indicate that the page is unevictable: and the scan
+stops there.
+
+mlock_vma_page() is called while holding the page table's lock (in addition
+to the page lock, and the rmap lock): to serialize against concurrent mlock or
+munlock or munmap system calls, mm teardown (munlock_vma_pages_all), reclaim,
+holepunching, and truncation of file pages and their anonymous COWed pages.
+
+
+try_to_munlock() Reverse Map Scan
+---------------------------------
+
+.. warning::
+   [!] TODO/FIXME: a better name might be page_mlocked() - analogous to the
+   page_referenced() reverse map walker.
+
+When munlock_vma_page() [see section :ref:`munlock()/munlockall() System Call
+Handling <munlock_munlockall_handling>` above] tries to munlock a
+page, it needs to determine whether or not the page is mapped by any
+VM_LOCKED VMA without actually attempting to unmap all PTEs from the
+page.  For this purpose, the unevictable/mlock infrastructure
+introduced a variant of try_to_unmap() called try_to_munlock().
+
+try_to_munlock() calls the same functions as try_to_unmap() for anonymous and
+mapped file and KSM pages with a flag argument specifying unlock versus unmap
+processing.  Again, these functions walk the respective reverse maps looking
+for VM_LOCKED VMAs.  When such a VMA is found, as in the try_to_unmap() case,
+the functions mlock the page via mlock_vma_page() and return SWAP_MLOCK.  This
+undoes the pre-clearing of the page's PG_mlocked done by munlock_vma_page.
+
+Note that try_to_munlock()'s reverse map walk must visit every VMA in a page's
+reverse map to determine that a page is NOT mapped into any VM_LOCKED VMA.
+However, the scan can terminate when it encounters a VM_LOCKED VMA.
+Although try_to_munlock() might be called a great many times when munlocking a
+large region or tearing down a large address space that has been mlocked via
+mlockall(), overall this is a fairly rare event.
+
+
+Page Reclaim in shrink_*_list()
+-------------------------------
+
+shrink_active_list() culls any obviously unevictable pages - i.e.
+!page_evictable(page) - diverting these 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) mlocked pages that could not be isolated from the LRU and moved to the
+     unevictable list in mlock_vma_page().
+
+shrink_inactive_list() also diverts any unevictable pages that it finds on the
+inactive lists to the appropriate zone's unevictable list.
+
+shrink_inactive_list() should only see SHM_LOCK'd pages that became SHM_LOCK'd
+after shrink_active_list() had moved them to the inactive list, or pages mapped
+into VM_LOCKED VMAs that munlock_vma_page() couldn't isolate from the LRU to
+recheck via try_to_munlock().  shrink_inactive_list() won't notice the latter,
+but will pass on to shrink_page_list().
+
+shrink_page_list() again culls obviously unevictable pages that it could
+encounter for similar reason to shrink_inactive_list().  Pages mapped into
+VM_LOCKED VMAs but without PG_mlocked set will make it all the way to
+try_to_unmap().  shrink_page_list() will divert them to the unevictable list
+when try_to_unmap() returns SWAP_MLOCK, as discussed above.