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diff --git a/Documentation/vm/unevictable-lru.txt b/Documentation/vm/unevictable-lru.txt new file mode 100644 index 000000000000..125eed560e5a --- /dev/null +++ b/Documentation/vm/unevictable-lru.txt @@ -0,0 +1,615 @@ + +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. + + |