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Diffstat (limited to 'Documentation/vm')
| -rw-r--r-- | Documentation/vm/page_migration | 107 |
1 files changed, 106 insertions, 1 deletions
diff --git a/Documentation/vm/page_migration b/Documentation/vm/page_migration index fea5c0864170..18d37c7ac50b 100644 --- a/Documentation/vm/page_migration +++ b/Documentation/vm/page_migration @@ -142,5 +142,110 @@ 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 0. +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. |