mirror of https://github.com/Qortal/Brooklyn
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
554 lines
26 KiB
554 lines
26 KiB
.. _unevictable_lru: |
|
|
|
============================== |
|
Unevictable LRU Infrastructure |
|
============================== |
|
|
|
.. contents:: :local: |
|
|
|
|
|
Introduction |
|
============ |
|
|
|
This document describes the Linux memory manager's "Unevictable LRU" |
|
infrastructure and the use of this to manage several types of "unevictable" |
|
pages. |
|
|
|
The document attempts to provide the overall rationale behind this mechanism |
|
and the rationale for some of the design decisions that drove the |
|
implementation. The latter design rationale is discussed in the context of an |
|
implementation description. Admittedly, one can obtain the implementation |
|
details - the "what does it do?" - by reading the code. One hopes that the |
|
descriptions below add value by provide the answer to "why does it do that?". |
|
|
|
|
|
|
|
The Unevictable LRU |
|
=================== |
|
|
|
The Unevictable LRU facility adds an additional LRU list to track unevictable |
|
pages and to hide these pages from vmscan. This mechanism is based on a patch |
|
by Larry Woodman of Red Hat to address several scalability problems with page |
|
reclaim in Linux. The problems have been observed at customer sites on large |
|
memory x86_64 systems. |
|
|
|
To illustrate this with an example, a non-NUMA x86_64 platform with 128GB of |
|
main memory will have over 32 million 4k pages in a single node. When a large |
|
fraction of these pages are not evictable for any reason [see below], vmscan |
|
will spend a lot of time scanning the LRU lists looking for the small fraction |
|
of pages that are evictable. This can result in a situation where all CPUs are |
|
spending 100% of their time in vmscan for hours or days on end, with the system |
|
completely unresponsive. |
|
|
|
The unevictable list addresses the following classes of unevictable pages: |
|
|
|
* Those owned by ramfs. |
|
|
|
* Those mapped into SHM_LOCK'd shared memory regions. |
|
|
|
* Those mapped into VM_LOCKED [mlock()ed] VMAs. |
|
|
|
The infrastructure may also be able to handle other conditions that make pages |
|
unevictable, either by definition or by circumstance, in the future. |
|
|
|
|
|
The Unevictable LRU Page List |
|
----------------------------- |
|
|
|
The Unevictable LRU page list is a lie. It was never an LRU-ordered list, but a |
|
companion to the LRU-ordered anonymous and file, active and inactive page lists; |
|
and now it is not even a page list. But following familiar convention, here in |
|
this document and in the source, we often imagine it as a fifth LRU page list. |
|
|
|
The Unevictable LRU infrastructure consists of an additional, per-node, LRU list |
|
called the "unevictable" list and an associated page flag, PG_unevictable, to |
|
indicate that the page is being managed on the unevictable list. |
|
|
|
The PG_unevictable flag is analogous to, and mutually exclusive with, the |
|
PG_active flag in that it indicates on which LRU list a page resides when |
|
PG_lru is set. |
|
|
|
The Unevictable LRU infrastructure maintains unevictable pages as if they were |
|
on an additional LRU list for a few reasons: |
|
|
|
(1) We get to "treat unevictable pages just like we treat other pages in the |
|
system - which means we get to use the same code to manipulate them, the |
|
same code to isolate them (for migrate, etc.), the same code to keep track |
|
of the statistics, etc..." [Rik van Riel] |
|
|
|
(2) We want to be able to migrate unevictable pages between nodes for memory |
|
defragmentation, workload management and memory hotplug. The Linux kernel |
|
can only migrate pages that it can successfully isolate from the LRU |
|
lists (or "Movable" pages: outside of consideration here). If we were to |
|
maintain pages elsewhere than on an LRU-like list, where they can be |
|
detected by isolate_lru_page(), we would prevent their migration. |
|
|
|
The unevictable list does not differentiate between file-backed and anonymous, |
|
swap-backed pages. This differentiation is only important while the pages are, |
|
in fact, evictable. |
|
|
|
The unevictable list benefits from the "arrayification" of the per-node LRU |
|
lists and statistics originally proposed and posted by Christoph Lameter. |
|
|
|
|
|
Memory Control Group Interaction |
|
-------------------------------- |
|
|
|
The unevictable LRU facility interacts with the memory control group [aka |
|
memory controller; see Documentation/admin-guide/cgroup-v1/memory.rst] by |
|
extending the lru_list enum. |
|
|
|
The memory controller data structure automatically gets a per-node unevictable |
|
list as a result of the "arrayification" of the per-node LRU lists (one per |
|
lru_list enum element). The memory controller tracks the movement of pages to |
|
and from the unevictable list. |
|
|
|
When a memory control group comes under memory pressure, the controller will |
|
not attempt to reclaim pages on the unevictable list. This has a couple of |
|
effects: |
|
|
|
(1) Because the pages are "hidden" from reclaim on the unevictable list, the |
|
reclaim process can be more efficient, dealing only with pages that have a |
|
chance of being reclaimed. |
|
|
|
(2) On the other hand, if too many of the pages charged to the control group |
|
are unevictable, the evictable portion of the working set of the tasks in |
|
the control group may not fit into the available memory. This can cause |
|
the control group to thrash or to OOM-kill tasks. |
|
|
|
|
|
.. _mark_addr_space_unevict: |
|
|
|
Marking Address Spaces Unevictable |
|
---------------------------------- |
|
|
|
For facilities such as ramfs none of the pages attached to the address space |
|
may be evicted. To prevent eviction of any such pages, the AS_UNEVICTABLE |
|
address space flag is provided, and this can be manipulated by a filesystem |
|
using a number of wrapper functions: |
|
|
|
* ``void mapping_set_unevictable(struct address_space *mapping);`` |
|
|
|
Mark the address space as being completely unevictable. |
|
|
|
* ``void mapping_clear_unevictable(struct address_space *mapping);`` |
|
|
|
Mark the address space as being evictable. |
|
|
|
* ``int mapping_unevictable(struct address_space *mapping);`` |
|
|
|
Query the address space, and return true if it is completely |
|
unevictable. |
|
|
|
These are currently used in three places in the kernel: |
|
|
|
(1) By ramfs to mark the address spaces of its inodes when they are created, |
|
and this mark remains for the life of the inode. |
|
|
|
(2) By SYSV SHM to mark SHM_LOCK'd address spaces until SHM_UNLOCK is called. |
|
Note that SHM_LOCK is not required to page in the locked pages if they're |
|
swapped out; the application must touch the pages manually if it wants to |
|
ensure they're in memory. |
|
|
|
(3) By the i915 driver to mark pinned address space until it's unpinned. The |
|
amount of unevictable memory marked by i915 driver is roughly the bounded |
|
object size in debugfs/dri/0/i915_gem_objects. |
|
|
|
|
|
Detecting Unevictable Pages |
|
--------------------------- |
|
|
|
The function page_evictable() in mm/internal.h determines whether a page is |
|
evictable or not using the query function outlined above [see section |
|
:ref:`Marking address spaces unevictable <mark_addr_space_unevict>`] |
|
to check the AS_UNEVICTABLE flag. |
|
|
|
For address spaces that are so marked after being populated (as SHM regions |
|
might be), the lock action (e.g. SHM_LOCK) can be lazy, and need not populate |
|
the page tables for the region as does, for example, mlock(), nor need it make |
|
any special effort to push any pages in the SHM_LOCK'd area to the unevictable |
|
list. Instead, vmscan will do this if and when it encounters the pages during |
|
a reclamation scan. |
|
|
|
On an unlock action (such as SHM_UNLOCK), the unlocker (e.g. shmctl()) must scan |
|
the pages in the region and "rescue" them from the unevictable list if no other |
|
condition is keeping them unevictable. If an unevictable region is destroyed, |
|
the pages are also "rescued" from the unevictable list in the process of |
|
freeing them. |
|
|
|
page_evictable() also checks for mlocked pages by testing an additional page |
|
flag, PG_mlocked (as wrapped by PageMlocked()), which is set when a page is |
|
faulted into a VM_LOCKED VMA, or found in a VMA being VM_LOCKED. |
|
|
|
|
|
Vmscan's Handling of Unevictable Pages |
|
-------------------------------------- |
|
|
|
If unevictable pages are culled in the fault path, or moved to the unevictable |
|
list at mlock() or mmap() time, vmscan will not encounter the pages until they |
|
have become evictable again (via munlock() for example) and have been "rescued" |
|
from the unevictable list. However, there may be situations where we decide, |
|
for the sake of expediency, to leave an unevictable page on one of the regular |
|
active/inactive LRU lists for vmscan to deal with. vmscan checks for such |
|
pages in all of the shrink_{active|inactive|page}_list() functions and will |
|
"cull" such pages that it encounters: that is, it diverts those pages to the |
|
unevictable list for the memory cgroup and node being scanned. |
|
|
|
There may be situations where a page is mapped into a VM_LOCKED VMA, but the |
|
page is not marked as PG_mlocked. Such pages will make it all the way to |
|
shrink_active_list() or shrink_page_list() where they will be detected when |
|
vmscan walks the reverse map in page_referenced() or try_to_unmap(). The page |
|
is culled to the unevictable list when it is released by the shrinker. |
|
|
|
To "cull" an unevictable page, vmscan simply puts the page back on the LRU list |
|
using putback_lru_page() - the inverse operation to isolate_lru_page() - after |
|
dropping the page lock. Because the condition which makes the page unevictable |
|
may change once the page is unlocked, __pagevec_lru_add_fn() will recheck the |
|
unevictable state of a page before placing it on the unevictable list. |
|
|
|
|
|
MLOCKED Pages |
|
============= |
|
|
|
The unevictable page list is also useful for mlock(), in addition to ramfs and |
|
SYSV SHM. Note that mlock() is only available in CONFIG_MMU=y situations; in |
|
NOMMU situations, all mappings are effectively mlocked. |
|
|
|
|
|
History |
|
------- |
|
|
|
The "Unevictable mlocked Pages" infrastructure is based on work originally |
|
posted by Nick Piggin in an RFC patch entitled "mm: mlocked pages off LRU". |
|
Nick posted his patch as an alternative to a patch posted by Christoph Lameter |
|
to achieve the same objective: hiding mlocked pages from vmscan. |
|
|
|
In Nick's patch, he used one of the struct page LRU list link fields as a count |
|
of VM_LOCKED VMAs that map the page (Rik van Riel had the same idea three years |
|
earlier). But this use of the link field for a count prevented the management |
|
of the pages on an LRU list, and thus mlocked pages were not migratable as |
|
isolate_lru_page() could not detect them, and the LRU list link field was not |
|
available to the migration subsystem. |
|
|
|
Nick resolved this by putting mlocked pages back on the LRU list before |
|
attempting to isolate them, thus abandoning the count of VM_LOCKED VMAs. When |
|
Nick's patch was integrated with the Unevictable LRU work, the count was |
|
replaced by walking the reverse map when munlocking, to determine whether any |
|
other VM_LOCKED VMAs still mapped the page. |
|
|
|
However, walking the reverse map for each page when munlocking was ugly and |
|
inefficient, and could lead to catastrophic contention on a file's rmap lock, |
|
when many processes which had it mlocked were trying to exit. In 5.18, the |
|
idea of keeping mlock_count in Unevictable LRU list link field was revived and |
|
put to work, without preventing the migration of mlocked pages. This is why |
|
the "Unevictable LRU list" cannot be a linked list of pages now; but there was |
|
no use for that linked list anyway - though its size is maintained for meminfo. |
|
|
|
|
|
Basic Management |
|
---------------- |
|
|
|
mlocked pages - pages mapped into a VM_LOCKED VMA - are a class of unevictable |
|
pages. When such a page has been "noticed" by the memory management subsystem, |
|
the page is marked with the PG_mlocked flag. This can be manipulated using the |
|
PageMlocked() functions. |
|
|
|
A PG_mlocked page will be placed on the unevictable list when it is added to |
|
the LRU. Such pages can be "noticed" by memory management in several places: |
|
|
|
(1) in the mlock()/mlock2()/mlockall() system call handlers; |
|
|
|
(2) in the mmap() system call handler when mmapping a region with the |
|
MAP_LOCKED flag; |
|
|
|
(3) mmapping a region in a task that has called mlockall() with the MCL_FUTURE |
|
flag; |
|
|
|
(4) in the fault path and when a VM_LOCKED stack segment is expanded; or |
|
|
|
(5) as mentioned above, in vmscan:shrink_page_list() when attempting to |
|
reclaim a page in a VM_LOCKED VMA by page_referenced() or try_to_unmap(). |
|
|
|
mlocked pages become unlocked and rescued from the unevictable list when: |
|
|
|
(1) mapped in a range unlocked via the munlock()/munlockall() system calls; |
|
|
|
(2) munmap()'d out of the last VM_LOCKED VMA that maps the page, including |
|
unmapping at task exit; |
|
|
|
(3) when the page is truncated from the last VM_LOCKED VMA of an mmapped file; |
|
or |
|
|
|
(4) before a page is COW'd in a VM_LOCKED VMA. |
|
|
|
|
|
mlock()/mlock2()/mlockall() System Call Handling |
|
------------------------------------------------ |
|
|
|
mlock(), mlock2() and mlockall() system call handlers proceed to mlock_fixup() |
|
for each VMA in the range specified by the call. In the case of mlockall(), |
|
this is the entire active address space of the task. Note that mlock_fixup() |
|
is used for both mlocking and munlocking a range of memory. A call to mlock() |
|
an already VM_LOCKED VMA, or to munlock() a VMA that is not VM_LOCKED, is |
|
treated as a no-op and mlock_fixup() simply returns. |
|
|
|
If the VMA passes some filtering as described in "Filtering Special VMAs" |
|
below, mlock_fixup() will attempt to merge the VMA with its neighbors or split |
|
off a subset of the VMA if the range does not cover the entire VMA. Any pages |
|
already present in the VMA are then marked as mlocked by mlock_page() via |
|
mlock_pte_range() via walk_page_range() via mlock_vma_pages_range(). |
|
|
|
Before returning from the system call, do_mlock() or mlockall() will call |
|
__mm_populate() to fault in the remaining pages via get_user_pages() and to |
|
mark those pages as mlocked as they are faulted. |
|
|
|
Note that the VMA being mlocked might be mapped with PROT_NONE. In this case, |
|
get_user_pages() will be unable to fault in the pages. That's okay. If pages |
|
do end up getting faulted into this VM_LOCKED VMA, they will be handled in the |
|
fault path - which is also how mlock2()'s MLOCK_ONFAULT areas are handled. |
|
|
|
For each PTE (or PMD) being faulted into a VMA, the page add rmap function |
|
calls mlock_vma_page(), which calls mlock_page() when the VMA is VM_LOCKED |
|
(unless it is a PTE mapping of a part of a transparent huge page). Or when |
|
it is a newly allocated anonymous page, lru_cache_add_inactive_or_unevictable() |
|
calls mlock_new_page() instead: similar to mlock_page(), but can make better |
|
judgments, since this page is held exclusively and known not to be on LRU yet. |
|
|
|
mlock_page() sets PageMlocked immediately, then places the page on the CPU's |
|
mlock pagevec, to batch up the rest of the work to be done under lru_lock by |
|
__mlock_page(). __mlock_page() sets PageUnevictable, initializes mlock_count |
|
and moves the page to unevictable state ("the unevictable LRU", but with |
|
mlock_count in place of LRU threading). Or if the page was already PageLRU |
|
and PageUnevictable and PageMlocked, it simply increments the mlock_count. |
|
|
|
But in practice that may not work ideally: the page may not yet be on an LRU, or |
|
it may have been temporarily isolated from LRU. In such cases the mlock_count |
|
field cannot be touched, but will be set to 0 later when __pagevec_lru_add_fn() |
|
returns the page to "LRU". Races prohibit mlock_count from being set to 1 then: |
|
rather than risk stranding a page indefinitely as unevictable, always err with |
|
mlock_count on the low side, so that when munlocked the page will be rescued to |
|
an evictable LRU, then perhaps be mlocked again later if vmscan finds it in a |
|
VM_LOCKED VMA. |
|
|
|
|
|
Filtering Special VMAs |
|
---------------------- |
|
|
|
mlock_fixup() filters several classes of "special" VMAs: |
|
|
|
1) VMAs with VM_IO or VM_PFNMAP set are skipped entirely. The pages behind |
|
these mappings are inherently pinned, so we don't need to mark them as |
|
mlocked. In any case, most of the pages have no struct page in which to so |
|
mark the page. Because of this, get_user_pages() will fail for these VMAs, |
|
so there is no sense in attempting to visit them. |
|
|
|
2) VMAs mapping hugetlbfs page are already effectively pinned into memory. We |
|
neither need nor want to mlock() these pages. But __mm_populate() includes |
|
hugetlbfs ranges, allocating the huge pages and populating the PTEs. |
|
|
|
3) VMAs with VM_DONTEXPAND are generally userspace mappings of kernel pages, |
|
such as the VDSO page, relay channel pages, etc. These pages are inherently |
|
unevictable and are not managed on the LRU lists. __mm_populate() includes |
|
these ranges, populating the PTEs if not already populated. |
|
|
|
4) VMAs with VM_MIXEDMAP set are not marked VM_LOCKED, but __mm_populate() |
|
includes these ranges, populating the PTEs if not already populated. |
|
|
|
Note that for all of these special VMAs, mlock_fixup() does not set the |
|
VM_LOCKED flag. Therefore, we won't have to deal with them later during |
|
munlock(), munmap() or task exit. Neither does mlock_fixup() account these |
|
VMAs against the task's "locked_vm". |
|
|
|
|
|
munlock()/munlockall() System Call Handling |
|
------------------------------------------- |
|
|
|
The munlock() and munlockall() system calls are handled by the same |
|
mlock_fixup() function as mlock(), mlock2() and mlockall() system calls are. |
|
If called to munlock an already munlocked VMA, mlock_fixup() simply returns. |
|
Because of the VMA filtering discussed above, VM_LOCKED will not be set in |
|
any "special" VMAs. So, those VMAs will be ignored for munlock. |
|
|
|
If the VMA is VM_LOCKED, mlock_fixup() again attempts to merge or split off the |
|
specified range. All pages in the VMA are then munlocked by munlock_page() via |
|
mlock_pte_range() via walk_page_range() via mlock_vma_pages_range() - the same |
|
function used when mlocking a VMA range, with new flags for the VMA indicating |
|
that it is munlock() being performed. |
|
|
|
munlock_page() uses the mlock pagevec to batch up work to be done under |
|
lru_lock by __munlock_page(). __munlock_page() decrements the page's |
|
mlock_count, and when that reaches 0 it clears PageMlocked and clears |
|
PageUnevictable, moving the page from unevictable state to inactive LRU. |
|
|
|
But in practice that may not work ideally: the page may not yet have reached |
|
"the unevictable LRU", or it may have been temporarily isolated from it. In |
|
those cases its mlock_count field is unusable and must be assumed to be 0: so |
|
that the page will be rescued to an evictable LRU, then perhaps be mlocked |
|
again later if vmscan finds it in a VM_LOCKED VMA. |
|
|
|
|
|
Migrating MLOCKED Pages |
|
----------------------- |
|
|
|
A page that is being migrated has been isolated from the LRU lists and is held |
|
locked across unmapping of the page, updating the page's address space entry |
|
and copying the contents and state, until the page table entry has been |
|
replaced with an entry that refers to the new page. Linux supports migration |
|
of mlocked pages and other unevictable pages. PG_mlocked is cleared from the |
|
the old page when it is unmapped from the last VM_LOCKED VMA, and set when the |
|
new page is mapped in place of migration entry in a VM_LOCKED VMA. If the page |
|
was unevictable because mlocked, PG_unevictable follows PG_mlocked; but if the |
|
page was unevictable for other reasons, PG_unevictable is copied explicitly. |
|
|
|
Note that page migration can race with mlocking or munlocking of the same page. |
|
There is mostly no problem since page migration requires unmapping all PTEs of |
|
the old page (including munlock where VM_LOCKED), then mapping in the new page |
|
(including mlock where VM_LOCKED). The page table locks provide sufficient |
|
synchronization. |
|
|
|
However, since mlock_vma_pages_range() starts by setting VM_LOCKED on a VMA, |
|
before mlocking any pages already present, if one of those pages were migrated |
|
before mlock_pte_range() reached it, it would get counted twice in mlock_count. |
|
To prevent that, mlock_vma_pages_range() temporarily marks the VMA as VM_IO, |
|
so that mlock_vma_page() will skip it. |
|
|
|
To complete page migration, we place the old and new pages back onto the LRU |
|
afterwards. The "unneeded" page - old page on success, new page on failure - |
|
is freed when the reference count held by the migration process is released. |
|
|
|
|
|
Compacting MLOCKED Pages |
|
------------------------ |
|
|
|
The memory map can be scanned for compactable regions and the default behavior |
|
is to let unevictable pages be moved. /proc/sys/vm/compact_unevictable_allowed |
|
controls this behavior (see Documentation/admin-guide/sysctl/vm.rst). The work |
|
of compaction is mostly handled by the page migration code and the same work |
|
flow as described in Migrating MLOCKED Pages will apply. |
|
|
|
|
|
MLOCKING Transparent Huge Pages |
|
------------------------------- |
|
|
|
A transparent huge page is represented by a single entry on an LRU list. |
|
Therefore, we can only make unevictable an entire compound page, not |
|
individual subpages. |
|
|
|
If a user tries to mlock() part of a huge page, and no user mlock()s the |
|
whole of the huge page, we want the rest of the page to be reclaimable. |
|
|
|
We cannot just split the page on partial mlock() as split_huge_page() can |
|
fail and a new intermittent failure mode for the syscall is undesirable. |
|
|
|
We handle this by keeping PTE-mlocked huge pages on evictable LRU lists: |
|
the PMD on the border of a VM_LOCKED VMA will be split into a PTE table. |
|
|
|
This way the huge page is accessible for vmscan. Under memory pressure the |
|
page will be split, subpages which belong to VM_LOCKED VMAs will be moved |
|
to the unevictable LRU and the rest can be reclaimed. |
|
|
|
/proc/meminfo's Unevictable and Mlocked amounts do not include those parts |
|
of a transparent huge page which are mapped only by PTEs in VM_LOCKED VMAs. |
|
|
|
|
|
mmap(MAP_LOCKED) System Call Handling |
|
------------------------------------- |
|
|
|
In addition to the mlock(), mlock2() and mlockall() system calls, an application |
|
can request that a region of memory be mlocked by supplying the MAP_LOCKED flag |
|
to the mmap() call. There is one important and subtle difference here, though. |
|
mmap() + mlock() will fail if the range cannot be faulted in (e.g. because |
|
mm_populate fails) and returns with ENOMEM while mmap(MAP_LOCKED) will not fail. |
|
The mmaped area will still have properties of the locked area - pages will not |
|
get swapped out - but major page faults to fault memory in might still happen. |
|
|
|
Furthermore, any mmap() call or brk() call that expands the heap by a task |
|
that has previously called mlockall() with the MCL_FUTURE flag will result |
|
in the newly mapped memory being mlocked. Before the unevictable/mlock |
|
changes, the kernel simply called make_pages_present() to allocate pages |
|
and populate the page table. |
|
|
|
To mlock a range of memory under the unevictable/mlock infrastructure, |
|
the mmap() handler and task address space expansion functions call |
|
populate_vma_page_range() specifying the vma and the address range to mlock. |
|
|
|
|
|
munmap()/exit()/exec() System Call Handling |
|
------------------------------------------- |
|
|
|
When unmapping an mlocked region of memory, whether by an explicit call to |
|
munmap() or via an internal unmap from exit() or exec() processing, we must |
|
munlock the pages if we're removing the last VM_LOCKED VMA that maps the pages. |
|
Before the unevictable/mlock changes, mlocking did not mark the pages in any |
|
way, so unmapping them required no processing. |
|
|
|
For each PTE (or PMD) being unmapped from a VMA, page_remove_rmap() calls |
|
munlock_vma_page(), which calls munlock_page() when the VMA is VM_LOCKED |
|
(unless it was a PTE mapping of a part of a transparent huge page). |
|
|
|
munlock_page() uses the mlock pagevec to batch up work to be done under |
|
lru_lock by __munlock_page(). __munlock_page() decrements the page's |
|
mlock_count, and when that reaches 0 it clears PageMlocked and clears |
|
PageUnevictable, moving the page from unevictable state to inactive LRU. |
|
|
|
But in practice that may not work ideally: the page may not yet have reached |
|
"the unevictable LRU", or it may have been temporarily isolated from it. In |
|
those cases its mlock_count field is unusable and must be assumed to be 0: so |
|
that the page will be rescued to an evictable LRU, then perhaps be mlocked |
|
again later if vmscan finds it in a VM_LOCKED VMA. |
|
|
|
|
|
Truncating MLOCKED Pages |
|
------------------------ |
|
|
|
File truncation or hole punching forcibly unmaps the deleted pages from |
|
userspace; truncation even unmaps and deletes any private anonymous pages |
|
which had been Copied-On-Write from the file pages now being truncated. |
|
|
|
Mlocked pages can be munlocked and deleted in this way: like with munmap(), |
|
for each PTE (or PMD) being unmapped from a VMA, page_remove_rmap() calls |
|
munlock_vma_page(), which calls munlock_page() when the VMA is VM_LOCKED |
|
(unless it was a PTE mapping of a part of a transparent huge page). |
|
|
|
However, if there is a racing munlock(), since mlock_vma_pages_range() starts |
|
munlocking by clearing VM_LOCKED from a VMA, before munlocking all the pages |
|
present, if one of those pages were unmapped by truncation or hole punch before |
|
mlock_pte_range() reached it, it would not be recognized as mlocked by this VMA, |
|
and would not be counted out of mlock_count. In this rare case, a page may |
|
still appear as PageMlocked after it has been fully unmapped: and it is left to |
|
release_pages() (or __page_cache_release()) to clear it and update statistics |
|
before freeing (this event is counted in /proc/vmstat unevictable_pgs_cleared, |
|
which is usually 0). |
|
|
|
|
|
Page Reclaim in shrink_*_list() |
|
------------------------------- |
|
|
|
vmscan's shrink_active_list() culls any obviously unevictable pages - |
|
i.e. !page_evictable(page) pages - diverting those to the unevictable list. |
|
However, shrink_active_list() only sees unevictable pages that made it onto the |
|
active/inactive LRU lists. Note that these pages do not have PageUnevictable |
|
set - otherwise they would be on the unevictable list and shrink_active_list() |
|
would never see them. |
|
|
|
Some examples of these unevictable pages on the LRU lists are: |
|
|
|
(1) ramfs pages that have been placed on the LRU lists when first allocated. |
|
|
|
(2) SHM_LOCK'd shared memory pages. shmctl(SHM_LOCK) does not attempt to |
|
allocate or fault in the pages in the shared memory region. This happens |
|
when an application accesses the page the first time after SHM_LOCK'ing |
|
the segment. |
|
|
|
(3) pages still mapped into VM_LOCKED VMAs, which should be marked mlocked, |
|
but events left mlock_count too low, so they were munlocked too early. |
|
|
|
vmscan's shrink_inactive_list() and shrink_page_list() also divert obviously |
|
unevictable pages found on the inactive lists to the appropriate memory cgroup |
|
and node unevictable list. |
|
|
|
rmap's page_referenced_one(), called via vmscan's shrink_active_list() or |
|
shrink_page_list(), and rmap's try_to_unmap_one() called via shrink_page_list(), |
|
check for (3) pages still mapped into VM_LOCKED VMAs, and call mlock_vma_page() |
|
to correct them. Such pages are culled to the unevictable list when released |
|
by the shrinker.
|
|
|