doc: unevictable LRU and mlocked pages documentation

Documentation for unevictable lru list and its usage.
Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/vm/unevictable-lru.txt

+
+This document describes the Linux memory management "Unevictable LRU"
+infrastructure and the use of this infrastructure 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?".
+
+Unevictable LRU Infrastructure:
+
+The Unevictable LRU 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.  For example, a non-numal 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 LRU infrastructure addresses the following classes of
+unevictable pages:
+
++ page owned by ramfs
++ page mapped into SHM_LOCKed shared memory regions
++ page mapped into VM_LOCKED [mlock()ed] vmas
+
+The infrastructure might be able to handle other conditions that make pages
+unevictable, either by definition or by circumstance, in the future.
+
+
+The Unevictable LRU 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 list is source configurable based on the UNEVICTABLE_LRU
+Kconfig option.
+
+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 elsewise 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.
+
+
+The unevictable LRU list does not differentiate between file backed and swap
+backed [anon] pages.  This differentiation is only important while the pages
+are, in fact, evictable.
+
+The unevictable LRU 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.  The reason for this is to prevent 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.
+
+
+Unevictable LRU and Memory Controller Interaction
+
+The memory controller data structure automatically gets a per zone unevictable
+lru list as a result of the "arrayification" of the per-zone LRU lists.  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.  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.  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.
+
+
+Unevictable LRU:  Detecting Unevictable Pages
+
+The function page_evictable(page, vma) in vmscan.c determines whether a
+page is evictable or not.  For ramfs pages and pages in SHM_LOCKed regions,
+page_evictable() tests a new address space flag, AS_UNEVICTABLE, in the page's
+address space using a wrapper function.  Wrapper functions are used to set,
+clear and test the flag to reduce the requirement for #ifdef's throughout the
+source code.  AS_UNEVICTABLE is set on ramfs inode/mapping when it is created.
+This flag remains for the life of the inode.
+
+For shared memory regions, AS_UNEVICTABLE is set when an application
+successfully SHM_LOCKs the region and is removed when the region is
+SHM_UNLOCKed.  Note that shmctl(SHM_LOCK, ...) does not populate the page
+tables for the region as does, for example, mlock().   So, we make no special
+effort to push any pages in the SHM_LOCKed region to the unevictable list.
+Vmscan will do this when/if it encounters the pages during reclaim.  On
+SHM_UNLOCK, shmctl() scans the pages in the region and "rescues" them from the
+unevictable list if no other condition keeps them unevictable.  If a SHM_LOCKed
+region is destroyed, the pages are also "rescued" from the unevictable list in
+the process of freeing them.
+
+page_evictable() detects mlock()ed pages by testing an additional page flag,
+PG_mlocked via the PageMlocked() wrapper.  If the page is NOT mlocked, and a
+non-NULL vma is supplied, page_evictable() will check whether the vma is
+VM_LOCKED via is_mlocked_vma().  is_mlocked_vma() will SetPageMlocked() and
+update the appropriate statistics if the vma is VM_LOCKED.  This method allows
+efficient "culling" of pages in the fault path that are being faulted in to
+VM_LOCKED vmas.
+
+
+Unevictable Pages and Vmscan [shrink_*_list()]
+
+If unevictable pages are culled in the fault path, or moved to the unevictable
+list at mlock() or mmap() time, vmscan will never encounter the pages until
+they have become evictable again, for example, via munlock() 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 PageMlocked.  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.
+
+Note that for anonymous pages, shrink_page_list() attempts to add the page to
+the swap cache before it tries to unmap the page.  To avoid this unnecessary
+consumption of swap space, shrink_page_list() calls try_to_munlock() to check
+whether any VM_LOCKED vmas map the page without attempting to unmap the page.
+If try_to_munlock() returns SWAP_MLOCK, shrink_page_list() will cull the page
+without consuming swap space.  try_to_munlock() will be described below.
+
+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 lru
+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 Page:  Prior Work
+
+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.  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.
+
+
+Mlocked Pages:  Basic Management
+
+Mlocked pages--pages mapped into a VM_LOCKED vma--represent one class of
+unevictable pages.  When such a page has been "noticed" by the memory
+management subsystem, the page is marked with the PG_mlocked [PageMlocked()]
+flag.  A PageMlocked() page will be placed on the unevictable LRU list when
+it is added to the LRU.   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 mmap()ing a region with the
+   MAP_LOCKED flag, or mmap()ing a region in a task that has called
+   mlockall() with the MCL_FUTURE flag.  Both of these conditions result
+   in the VM_LOCKED flag being set for the vma.
+3) in the fault path, if mlocked pages are "culled" in the fault path,
+   and when a VM_LOCKED stack segment is expanded.
+4) as mentioned above, in vmscan:shrink_page_list() with attempting to
+   reclaim a page in a VM_LOCKED vma--via try_to_unmap() or try_to_munlock().
+
+Mlocked pages become unlocked and rescued from the unevictable list when:
+
+1) mapped in a range unlocked via the munlock()/munlockall() system calls.
+2) munmapped() 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 mmap()ed file.
+4) before a page is COWed in a VM_LOCKED vma.
+
+
+Mlocked Pages:  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 mlock()ing and munlock()ing 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--mlock_fixup() simply returns.
+
+If the vma passes some filtering described in "Mlocked Pages:  Filtering 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
+__mlock_vma_pages_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 OK.  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, __mlock_vma_pages_range() tests the 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.  Worse case, this results in 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(), called with the page locked [N.B., not "mlocked"], will
+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() is done.
+
+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
+putback the page--putback_lru_page()--which will notice that the page is now
+mlocked and divert the page to the zone's unevictable LRU list.  If
+mlock_vma_page() is unable to isolate the page from the LRU, vmscan will handle
+it later if/when it attempts to reclaim the page.
+
+
+Mlocked Pages:  Filtering Special Vmas
+
+mlock_fixup() filters several classes of "special" vmas:
+
+1) vmas with VM_IO|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 don't 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|VM_RESERVED are generally user space 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() or munmap()--for example, at task exit.  Neither does mlock_fixup()
+account these vmas against the task's "locked_vm".
+
+Mlocked Pages:  Downgrading the Mmap Semaphore.
+
+mlock_fixup() must be called with the mmap semaphore held for write, because
+it may have to merge or split vmas.  However, mlocking a large region of
+memory can take a long time--especially if vmscan must reclaim pages to
+satisfy the regions requirements.  Faulting in a large region with the mmap
+semaphore held for write can hold off other faults on the address space, in
+the case of a multi-threaded task.  It can also hold off scans of the task's
+address space via /proc.  While testing under heavy load, it was observed that
+the ps(1) command could be held off for many minutes while a large segment was
+mlock()ed down.
+
+To address this issue, and to make the system more responsive during mlock()ing
+of large segments, mlock_fixup() downgrades the mmap semaphore to read mode
+during the call to __mlock_vma_pages_range().  This works fine.  However, the
+callers of mlock_fixup() expect the semaphore to be returned in write mode.
+So, mlock_fixup() "upgrades" the semphore to write mode.  Linux does not
+support an atomic upgrade_sem() call, so mlock_fixup() must drop the semaphore
+and reacquire it in write mode.  In a multi-threaded task, it is possible for
+the task memory map to change while the semaphore is dropped.  Therefore,
+mlock_fixup() looks up the vma at the range start address after reacquiring
+the semaphore in write mode and verifies that it still covers the original
+range.  If not, mlock_fixup() returns an error [-EAGAIN].  All callers of
+mlock_fixup() have been changed to deal with this new error condition.
+
+Note:  when munlocking a region, all of the pages should already be resident--
+unless we have racing threads mlocking() and munlocking() regions.  So,
+unlocking should not have to wait for page allocations nor faults  of any kind.
+Therefore mlock_fixup() does not downgrade the semaphore for munlock().
+
+
+Mlocked Pages:  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 munlock()ed 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
+__mlock_vma_pages_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 some 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
+mlock()ing.
+
+For munlock(), __mlock_vma_pages_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 when/if vmscan tries to reclaim the
+page.  This should be relatively rare.
+
+Mlocked Pages:  Migrating Them...
+
+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 mapping
+[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 PageMlocked and PageUnevictable 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, 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.
+
+When completing 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.
+
+
+Mlocked Pages:  mmap(MAP_LOCKED) System Call Handling
+
+In addition the the mlock()/mlockall() system calls, an application can request
+that a region of memory be mlocked using the MAP_LOCKED flag with the mmap()
+call.  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
+mlock_vma_pages_range() specifying the vma and the address range to mlock.
+mlock_vma_pages_range() filters vmas like mlock_fixup(), as described above in
+"Mlocked Pages:  Filtering Vmas".  It will clear the VM_LOCKED flag, which will
+have already been set by the caller, in filtered vmas.  Thus these vma's need
+not be visited for munlock when the region is unmapped.
+
+For "normal" vmas, mlock_vma_pages_range() calls __mlock_vma_pages_range() to
+fault/allocate the pages and mlock them.  Again, like mlock_fixup(),
+mlock_vma_pages_range() downgrades the mmap semaphore to read mode before
+attempting to fault/allocate and mlock the pages; and "upgrades" the semaphore
+back to write mode before returning.
+
+The callers of mlock_vma_pages_range() will have already added the memory
+range to be mlocked to the task's "locked_vm".  To account for filtered vmas,
+mlock_vma_pages_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.
+
+
+Mlocked Pages:  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() hander and task address space tear down function call
+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.
+
+
+Mlocked Page:  try_to_unmap()
+
+[Note:  the code changes represented by this section are really quite small
+compared to the text to describe what happening and why, and to discuss the
+implications.]
+
+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 munlock()ing 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 "Unevictable Pages and Vmscan [shrink_*_list()]".  To
+handle this situation, try_to_unmap() has been enhanced to check 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.  BUG_ON()
+assertions enforce this requirement.  Separate functions handle anonymous and
+mapped file pages, as these types of pages have different reverse map
+mechanisms.
+
+	try_to_unmap_anon()
+
+To unmap anonymous pages, each vma in the list anchored in the anon_vma must be
+visited--at least until a VM_LOCKED vma is encountered.  If the page is being
+unmapped for migration, VM_LOCKED vmas do not stop the process because mlocked
+pages are migratable.  However, for reclaim, if the page is mapped into a
+VM_LOCKED vma, the scan stops.  try_to_unmap() attempts to acquire the mmap
+semphore of the mm_struct to which the vma belongs in read mode.  If this is
+successful, try_to_unmap() will mlock the page via mlock_vma_page()--we
+wouldn't have gotten to try_to_unmap() if the page were already mlocked--and
+will return SWAP_MLOCK, indicating that the page is unevictable.  If the
+mmap semaphore cannot be acquired, we are not sure whether the page is really
+unevictable or not.  In this case, try_to_unmap() will return SWAP_AGAIN.
+
+	try_to_unmap_file() -- linear mappings
+
+Unmapping of a mapped file page works the same, except that the scan visits
+all vmas that maps the page's index/page offset in the page's mapping's
+reverse map priority search tree.  It must also visit each vma in the page's
+mapping's non-linear list, if the list is non-empty.  As for anonymous pages,
+on encountering a VM_LOCKED vma for a mapped file page, try_to_unmap() will
+attempt to acquire the associated mm_struct's mmap semaphore to mlock the page,
+returning SWAP_MLOCK if this is successful, and SWAP_AGAIN, if not.
+
+	try_to_unmap_file() -- non-linear mappings
+
+If a page's mapping contains a non-empty non-linear mapping vma list, then
+try_to_un{map|lock}() must also visit each vma in that list to determine
+whether the page is mapped in a VM_LOCKED vma.  Again, the scan must visit
+all vmas in the non-linear list to ensure that the pages is not/should not be
+mlocked.  If a VM_LOCKED vma is found in the list, the scan could terminate.
+However, there is no easy way to determine whether the page is actually mapped
+in a given vma--either for unmapping or testing whether the VM_LOCKED vma
+actually pins the page.
+
+So, try_to_unmap_file() handles non-linear mappings by scanning a certain
+number of pages--a "cluster"--in each non-linear vma associated with the page's
+mapping, for each file mapped page that vmscan tries to unmap.  If this happens
+to unmap the page we're trying to unmap, try_to_unmap() will notice this on
+return--(page_mapcount(page) == 0)--and return SWAP_SUCCESS.  Otherwise, it
+will return SWAP_AGAIN, causing vmscan to recirculate this page.  We take
+advantage of the cluster scan in try_to_unmap_cluster() as follows:
+
+For each non-linear vma, try_to_unmap_cluster() attempts to acquire the mmap
+semaphore of the associated mm_struct for read without blocking.  If this
+attempt is successful and the vma is VM_LOCKED, try_to_unmap_cluster() will
+retain the mmap semaphore for the scan; otherwise it drops it here.  Then,
+for each page in the cluster, if we're holding the mmap semaphore for a locked
+vma, try_to_unmap_cluster() calls mlock_vma_page() to mlock the page.  This
+call is a no-op if the page is already locked, but will mlock any pages in
+the non-linear mapping that happen to be unlocked.  If one of the pages so
+mlocked is the page passed in to try_to_unmap(), try_to_unmap_cluster() will
+return SWAP_MLOCK, rather than the default SWAP_AGAIN.  This will allow vmscan
+to cull the page, rather than recirculating it on the inactive list.  Again,
+if try_to_unmap_cluster() cannot acquire the vma's mmap sem, it returns
+SWAP_AGAIN, indicating that the page is mapped by a VM_LOCKED vma, but
+couldn't be mlocked.
+
+
+Mlocked pages:  try_to_munlock() Reverse Map Scan
+
+TODO/FIXME:  a better name might be page_mlocked()--analogous to the
+page_referenced() reverse map walker--especially if we continue to call this
+from shrink_page_list().  See related TODO/FIXME below.
+
+When munlock_vma_page()--see "Mlocked Pages:  munlock()/munlockall() System
+Call Handling" above--tries to munlock a page, or when shrink_page_list()
+encounters an anonymous page that is not yet in the swap cache, they need 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 pages with an additional argument specifing unlock versus unmap
+processing.  Again, these functions walk the respective reverse maps looking
+for VM_LOCKED vmas.  When such a vma is found for anonymous pages and file
+pages mapped in linear VMAs, as in the try_to_unmap() case, the functions
+attempt to acquire the associated mmap semphore, mlock the page via
+mlock_vma_page() and return SWAP_MLOCK.  This effectively undoes the
+pre-clearing of the page's PG_mlocked done by munlock_vma_page() and informs
+shrink_page_list() that the anonymous page should be culled rather than added
+to the swap cache in preparation for a try_to_unmap() that will almost
+certainly fail.
+
+If try_to_unmap() is unable to acquire a VM_LOCKED vma's associated mmap
+semaphore, it will return SWAP_AGAIN.  This will allow shrink_page_list()
+to recycle the page on the inactive list and hope that it has better luck
+with the page next time.
+
+For file pages mapped into non-linear vmas, the try_to_munlock() logic works
+slightly differently.  On encountering a VM_LOCKED non-linear vma that might
+map the page, try_to_munlock() returns SWAP_AGAIN without actually mlocking
+the page.  munlock_vma_page() will just leave the page unlocked and let
+vmscan deal with it--the usual fallback position.
+
+Note that try_to_munlock()'s reverse map walk must visit every vma in a pages'
+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 and can
+successfully acquire the vma's mmap semphore for read and mlock the page.
+Although try_to_munlock() can be called many [very many!] times when
+munlock()ing a large region or tearing down a large address space that has been
+mlocked via mlockall(), overall this is a fairly rare event.  In addition,
+although shrink_page_list() calls try_to_munlock() for every anonymous page that
+it handles that is not yet in the swap cache, on average anonymous pages will
+have very short reverse map lists.
+
+Mlocked Page:  Page Reclaim in shrink_*_list()
+
+shrink_active_list() culls any obviously unevictable pages--i.e.,
+!page_evictable(page, NULL)--diverting these to the unevictable lru
+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_LOCKed 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_LOCKing
+   the segment.
+
+3) Mlocked pages that could not be isolated from the lru and moved to the
+   unevictable list in mlock_vma_page().
+
+3) Pages mapped into multiple VM_LOCKED vmas, but try_to_munlock() couldn't
+   acquire the vma's mmap semaphore to test the flags and set PageMlocked.
+   munlock_vma_page() was forced to let the page back on to the normal
+   LRU list for vmscan to handle.
+
+shrink_inactive_list() also culls any unevictable pages that it finds
+on the inactive lists, again diverting them to the appropriate zone's unevictable
+lru list.  shrink_inactive_list() should only see SHM_LOCKed pages that became
+SHM_LOCKed 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().  As already discussed,
+shrink_page_list() proactively looks for anonymous pages that should have
+PG_mlocked set but don't--these would not be detected by page_evictable()--to
+avoid adding them to the swap cache unnecessarily.  File 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.
+
+TODO/FIXME:  If we can enhance the swap cache to reliably remove entries
+with page_count(page) > 2, as long as all ptes are mapped to the page and
+not the swap entry, we can probably remove the call to try_to_munlock() in
+shrink_page_list() and just remove the page from the swap cache when
+try_to_unmap() returns SWAP_MLOCK.   Currently, remove_exclusive_swap_page()
+doesn't seem to allow that.
+
+