diff options
Diffstat (limited to 'Documentation/vm')
| -rw-r--r-- | Documentation/vm/page_migration | 108 | ||||
| -rw-r--r-- | Documentation/vm/transhuge.txt | 128 | ||||
| -rw-r--r-- | Documentation/vm/unevictable-lru.txt | 21 |
3 files changed, 220 insertions, 37 deletions
diff --git a/Documentation/vm/page_migration b/Documentation/vm/page_migration index 344d2d3ad3cd..0478ae2ad44a 100644 --- a/Documentation/vm/page_migration +++ b/Documentation/vm/page_migration @@ -142,5 +142,111 @@ Steps: 20. The new page is moved to the LRU and can be scanned by the swapper etc again. -Christoph Lameter, May 8, 2006. +C. Non-LRU page migration +------------------------- + +Although original migration aimed for reducing the latency of memory access +for NUMA, compaction who want to create high-order page is also main customer. + +Current problem of the implementation is that it is designed to migrate only +*LRU* pages. However, there are potential non-lru pages which can be migrated +in drivers, for example, zsmalloc, virtio-balloon pages. + +For virtio-balloon pages, some parts of migration code path have been hooked +up and added virtio-balloon specific functions to intercept migration logics. +It's too specific to a driver so other drivers who want to make their pages +movable would have to add own specific hooks in migration path. + +To overclome the problem, VM supports non-LRU page migration which provides +generic functions for non-LRU movable pages without driver specific hooks +migration path. + +If a driver want to make own pages movable, it should define three functions +which are function pointers of struct address_space_operations. + +1. bool (*isolate_page) (struct page *page, isolate_mode_t mode); + +What VM expects on isolate_page function of driver is to return *true* +if driver isolates page successfully. On returing true, VM marks the page +as PG_isolated so concurrent isolation in several CPUs skip the page +for isolation. If a driver cannot isolate the page, it should return *false*. + +Once page is successfully isolated, VM uses page.lru fields so driver +shouldn't expect to preserve values in that fields. + +2. int (*migratepage) (struct address_space *mapping, + struct page *newpage, struct page *oldpage, enum migrate_mode); + +After isolation, VM calls migratepage of driver with isolated page. +The function of migratepage is to move content of the old page to new page +and set up fields of struct page newpage. Keep in mind that you should +indicate to the VM the oldpage is no longer movable via __ClearPageMovable() +under page_lock if you migrated the oldpage successfully and returns +MIGRATEPAGE_SUCCESS. If driver cannot migrate the page at the moment, driver +can return -EAGAIN. On -EAGAIN, VM will retry page migration in a short time +because VM interprets -EAGAIN as "temporal migration failure". On returning +any error except -EAGAIN, VM will give up the page migration without retrying +in this time. + +Driver shouldn't touch page.lru field VM using in the functions. + +3. void (*putback_page)(struct page *); + +If migration fails on isolated page, VM should return the isolated page +to the driver so VM calls driver's putback_page with migration failed page. +In this function, driver should put the isolated page back to the own data +structure. +4. non-lru movable page flags + +There are two page flags for supporting non-lru movable page. + +* PG_movable + +Driver should use the below function to make page movable under page_lock. + + void __SetPageMovable(struct page *page, struct address_space *mapping) + +It needs argument of address_space for registering migration family functions +which will be called by VM. Exactly speaking, PG_movable is not a real flag of +struct page. Rather than, VM reuses page->mapping's lower bits to represent it. + + #define PAGE_MAPPING_MOVABLE 0x2 + page->mapping = page->mapping | PAGE_MAPPING_MOVABLE; + +so driver shouldn't access page->mapping directly. Instead, driver should +use page_mapping which mask off the low two bits of page->mapping under +page lock so it can get right struct address_space. + +For testing of non-lru movable page, VM supports __PageMovable function. +However, it doesn't guarantee to identify non-lru movable page because +page->mapping field is unified with other variables in struct page. +As well, if driver releases the page after isolation by VM, page->mapping +doesn't have stable value although it has PAGE_MAPPING_MOVABLE +(Look at __ClearPageMovable). But __PageMovable is cheap to catch whether +page is LRU or non-lru movable once the page has been isolated. Because +LRU pages never can have PAGE_MAPPING_MOVABLE in page->mapping. It is also +good for just peeking to test non-lru movable pages before more expensive +checking with lock_page in pfn scanning to select victim. + +For guaranteeing non-lru movable page, VM provides PageMovable function. +Unlike __PageMovable, PageMovable functions validates page->mapping and +mapping->a_ops->isolate_page under lock_page. The lock_page prevents sudden +destroying of page->mapping. + +Driver using __SetPageMovable should clear the flag via __ClearMovablePage +under page_lock before the releasing the page. + +* PG_isolated + +To prevent concurrent isolation among several CPUs, VM marks isolated page +as PG_isolated under lock_page. So if a CPU encounters PG_isolated non-lru +movable page, it can skip it. Driver doesn't need to manipulate the flag +because VM will set/clear it automatically. Keep in mind that if driver +sees PG_isolated page, it means the page have been isolated by VM so it +shouldn't touch page.lru field. +PG_isolated is alias with PG_reclaim flag so driver shouldn't use the flag +for own purpose. + +Christoph Lameter, May 8, 2006. +Minchan Kim, Mar 28, 2016. diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt index 7c871d6beb63..2ec6adb5a4ce 100644 --- a/Documentation/vm/transhuge.txt +++ b/Documentation/vm/transhuge.txt @@ -9,8 +9,8 @@ using huge pages for the backing of virtual memory with huge pages that supports the automatic promotion and demotion of page sizes and without the shortcomings of hugetlbfs. -Currently it only works for anonymous memory mappings but in the -future it can expand over the pagecache layer starting with tmpfs. +Currently it only works for anonymous memory mappings and tmpfs/shmem. +But in the future it can expand to other filesystems. The reason applications are running faster is because of two factors. The first factor is almost completely irrelevant and it's not @@ -57,10 +57,6 @@ miss is going to run faster. feature that applies to all dynamic high order allocations in the kernel) -- this initial support only offers the feature in the anonymous memory - regions but it'd be ideal to move it to tmpfs and the pagecache - later - Transparent Hugepage Support maximizes the usefulness of free memory if compared to the reservation approach of hugetlbfs by allowing all unused memory to be used as cache or other movable (or even unmovable @@ -94,21 +90,21 @@ madvise(MADV_HUGEPAGE) on their critical mmapped regions. == sysfs == -Transparent Hugepage Support can be entirely disabled (mostly for -debugging purposes) or only enabled inside MADV_HUGEPAGE regions (to -avoid the risk of consuming more memory resources) or enabled system -wide. This can be achieved with one of: +Transparent Hugepage Support for anonymous memory can be entirely disabled +(mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE +regions (to avoid the risk of consuming more memory resources) or enabled +system wide. This can be achieved with one of: echo always >/sys/kernel/mm/transparent_hugepage/enabled echo madvise >/sys/kernel/mm/transparent_hugepage/enabled echo never >/sys/kernel/mm/transparent_hugepage/enabled It's also possible to limit defrag efforts in the VM to generate -hugepages in case they're not immediately free to madvise regions or -to never try to defrag memory and simply fallback to regular pages -unless hugepages are immediately available. Clearly if we spend CPU -time to defrag memory, we would expect to gain even more by the fact -we use hugepages later instead of regular pages. This isn't always +anonymous hugepages in case they're not immediately free to madvise +regions or to never try to defrag memory and simply fallback to regular +pages unless hugepages are immediately available. Clearly if we spend CPU +time to defrag memory, we would expect to gain even more by the fact we +use hugepages later instead of regular pages. This isn't always guaranteed, but it may be more likely in case the allocation is for a MADV_HUGEPAGE region. @@ -133,9 +129,9 @@ that are have used madvise(MADV_HUGEPAGE). This is the default behaviour. "never" should be self-explanatory. -By default kernel tries to use huge zero page on read page fault. -It's possible to disable huge zero page by writing 0 or enable it -back by writing 1: +By default kernel tries to use huge zero page on read page fault to +anonymous mapping. It's possible to disable huge zero page by writing 0 +or enable it back by writing 1: echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page @@ -204,21 +200,67 @@ Support by passing the parameter "transparent_hugepage=always" or "transparent_hugepage=madvise" or "transparent_hugepage=never" (without "") to the kernel command line. +== Hugepages in tmpfs/shmem == + +You can control hugepage allocation policy in tmpfs with mount option +"huge=". It can have following values: + + - "always": + Attempt to allocate huge pages every time we need a new page; + + - "never": + Do not allocate huge pages; + + - "within_size": + Only allocate huge page if it will be fully within i_size. + Also respect fadvise()/madvise() hints; + + - "advise: + Only allocate huge pages if requested with fadvise()/madvise(); + +The default policy is "never". + +"mount -o remount,huge= /mountpoint" works fine after mount: remounting +huge=never will not attempt to break up huge pages at all, just stop more +from being allocated. + +There's also sysfs knob to control hugepage allocation policy for internal +shmem mount: /sys/kernel/mm/transparent_hugepage/shmem_enabled. The mount +is used for SysV SHM, memfds, shared anonymous mmaps (of /dev/zero or +MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem. + +In addition to policies listed above, shmem_enabled allows two further +values: + + - "deny": + For use in emergencies, to force the huge option off from + all mounts; + - "force": + Force the huge option on for all - very useful for testing; + == Need of application restart == -The transparent_hugepage/enabled values only affect future -behavior. So to make them effective you need to restart any -application that could have been using hugepages. This also applies to -the regions registered in khugepaged. +The transparent_hugepage/enabled values and tmpfs mount option only affect +future behavior. So to make them effective you need to restart any +application that could have been using hugepages. This also applies to the +regions registered in khugepaged. == Monitoring usage == -The number of transparent huge pages currently used by the system is -available by reading the AnonHugePages field in /proc/meminfo. To -identify what applications are using transparent huge pages, it is -necessary to read /proc/PID/smaps and count the AnonHugePages fields -for each mapping. Note that reading the smaps file is expensive and -reading it frequently will incur overhead. +The number of anonymous transparent huge pages currently used by the +system is available by reading the AnonHugePages field in /proc/meminfo. +To identify what applications are using anonymous transparent huge pages, +it is necessary to read /proc/PID/smaps and count the AnonHugePages fields +for each mapping. + +The number of file transparent huge pages mapped to userspace is available +by reading ShmemPmdMapped and ShmemHugePages fields in /proc/meminfo. +To identify what applications are mapping file transparent huge pages, it +is necessary to read /proc/PID/smaps and count the FileHugeMapped fields +for each mapping. + +Note that reading the smaps file is expensive and reading it +frequently will incur overhead. There are a number of counters in /proc/vmstat that may be used to monitor how successfully the system is providing huge pages for use. @@ -238,6 +280,12 @@ thp_collapse_alloc_failed is incremented if khugepaged found a range of pages that should be collapsed into one huge page but failed the allocation. +thp_file_alloc is incremented every time a file huge page is successfully +i allocated. + +thp_file_mapped is incremented every time a file huge page is mapped into + user address space. + thp_split_page is incremented every time a huge page is split into base pages. This can happen for a variety of reasons but a common reason is that a huge page is old and is being reclaimed. @@ -403,19 +451,27 @@ pages: on relevant sub-page of the compound page. - map/unmap of the whole compound page accounted in compound_mapcount - (stored in first tail page). + (stored in first tail page). For file huge pages, we also increment + ->_mapcount of all sub-pages in order to have race-free detection of + last unmap of subpages. -PageDoubleMap() indicates that ->_mapcount in all subpages is offset up by one. -This additional reference is required to get race-free detection of unmap of -subpages when we have them mapped with both PMDs and PTEs. +PageDoubleMap() indicates that the page is *possibly* mapped with PTEs. + +For anonymous pages PageDoubleMap() also indicates ->_mapcount in all +subpages is offset up by one. This additional reference is required to +get race-free detection of unmap of subpages when we have them mapped with +both PMDs and PTEs. This is optimization required to lower overhead of per-subpage mapcount tracking. The alternative is alter ->_mapcount in all subpages on each map/unmap of the whole compound page. -We set PG_double_map when a PMD of the page got split for the first time, -but still have PMD mapping. The additional references go away with last -compound_mapcount. +For anonymous pages, we set PG_double_map when a PMD of the page got split +for the first time, but still have PMD mapping. The additional references +go away with last compound_mapcount. + +File pages get PG_double_map set on first map of the page with PTE and +goes away when the page gets evicted from page cache. split_huge_page internally has to distribute the refcounts in the head page to the tail pages before clearing all PG_head/tail bits from the page @@ -427,7 +483,7 @@ sum of mapcount of all sub-pages plus one (split_huge_page caller must have reference for head page). split_huge_page uses migration entries to stabilize page->_refcount and -page->_mapcount. +page->_mapcount of anonymous pages. File pages just got unmapped. We safe against physical memory scanners too: the only legitimate way scanner can get reference to a page is get_page_unless_zero(). diff --git a/Documentation/vm/unevictable-lru.txt b/Documentation/vm/unevictable-lru.txt index 4e565ed09218..e14718572476 100644 --- a/Documentation/vm/unevictable-lru.txt +++ b/Documentation/vm/unevictable-lru.txt @@ -461,6 +461,27 @@ unevictable LRU is enabled, the work of compaction is mostly handled by the page migration code and the same work flow as described in MIGRATING MLOCKED PAGES will apply. +MLOCKING TRANSPARENT HUGE PAGES +------------------------------- + +A transparent huge page is represented by a single entry on an LRU list. +Therefore, we can only make unevictable an entire compound page, not +individual subpages. + +If a user tries to mlock() part of a huge page, we want the rest of the +page to be reclaimable. + +We cannot just split the page on partial mlock() as split_huge_page() can +fail and new intermittent failure mode for the syscall is undesirable. + +We handle this by keeping PTE-mapped huge pages on normal LRU lists: the +PMD on border of VM_LOCKED VMA will be split into PTE table. + +This way the huge page is accessible for vmscan. Under memory pressure the +page will be split, subpages which belong to VM_LOCKED VMAs will be moved +to unevictable LRU and the rest can be reclaimed. + +See also comment in follow_trans_huge_pmd(). mmap(MAP_LOCKED) SYSTEM CALL HANDLING ------------------------------------- |