WSL2-Linux-Kernel/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.