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diff --git a/Documentation/vm/page_migration.rst b/Documentation/vm/page_migration.rst deleted file mode 100644 index 1d6cd7db4e43..000000000000 --- a/Documentation/vm/page_migration.rst +++ /dev/null @@ -1,257 +0,0 @@ -.. _page_migration: - -============== -Page migration -============== - -Page migration allows the moving of the physical location of pages between -nodes in a numa system while the process is running. This means that the -virtual addresses that the process sees do not change. However, the -system rearranges the physical location of those pages. - -The main intend of page migration is to reduce the latency of memory access -by moving pages near to the processor where the process accessing that memory -is running. - -Page migration allows a process to manually relocate the node on which its -pages are located through the MF_MOVE and MF_MOVE_ALL options while setting -a new memory policy via mbind(). The pages of process can also be relocated -from another process using the sys_migrate_pages() function call. The -migrate_pages function call takes two sets of nodes and moves pages of a -process that are located on the from nodes to the destination nodes. -Page migration functions are provided by the numactl package by Andi Kleen -(a version later than 0.9.3 is required. Get it from -ftp://oss.sgi.com/www/projects/libnuma/download/). numactl provides libnuma -which provides an interface similar to other numa functionality for page -migration. cat ``/proc/<pid>/numa_maps`` allows an easy review of where the -pages of a process are located. See also the numa_maps documentation in the -proc(5) man page. - -Manual migration is useful if for example the scheduler has relocated -a process to a processor on a distant node. A batch scheduler or an -administrator may detect the situation and move the pages of the process -nearer to the new processor. The kernel itself does only provide -manual page migration support. Automatic page migration may be implemented -through user space processes that move pages. A special function call -"move_pages" allows the moving of individual pages within a process. -A NUMA profiler may f.e. obtain a log showing frequent off node -accesses and may use the result to move pages to more advantageous -locations. - -Larger installations usually partition the system using cpusets into -sections of nodes. Paul Jackson has equipped cpusets with the ability to -move pages when a task is moved to another cpuset (See -Documentation/admin-guide/cgroup-v1/cpusets.rst). -Cpusets allows the automation of process locality. If a task is moved to -a new cpuset then also all its pages are moved with it so that the -performance of the process does not sink dramatically. Also the pages -of processes in a cpuset are moved if the allowed memory nodes of a -cpuset are changed. - -Page migration allows the preservation of the relative location of pages -within a group of nodes for all migration techniques which will preserve a -particular memory allocation pattern generated even after migrating a -process. This is necessary in order to preserve the memory latencies. -Processes will run with similar performance after migration. - -Page migration occurs in several steps. First a high level -description for those trying to use migrate_pages() from the kernel -(for userspace usage see the Andi Kleen's numactl package mentioned above) -and then a low level description of how the low level details work. - -In kernel use of migrate_pages() -================================ - -1. Remove pages from the LRU. - - Lists of pages to be migrated are generated by scanning over - pages and moving them into lists. This is done by - calling isolate_lru_page(). - Calling isolate_lru_page increases the references to the page - so that it cannot vanish while the page migration occurs. - It also prevents the swapper or other scans to encounter - the page. - -2. We need to have a function of type new_page_t that can be - passed to migrate_pages(). This function should figure out - how to allocate the correct new page given the old page. - -3. The migrate_pages() function is called which attempts - to do the migration. It will call the function to allocate - the new page for each page that is considered for - moving. - -How migrate_pages() works -========================= - -migrate_pages() does several passes over its list of pages. A page is moved -if all references to a page are removable at the time. The page has -already been removed from the LRU via isolate_lru_page() and the refcount -is increased so that the page cannot be freed while page migration occurs. - -Steps: - -1. Lock the page to be migrated - -2. Ensure that writeback is complete. - -3. Lock the new page that we want to move to. It is locked so that accesses to - this (not yet uptodate) page immediately lock while the move is in progress. - -4. All the page table references to the page are converted to migration - entries. This decreases the mapcount of a page. If the resulting - mapcount is not zero then we do not migrate the page. All user space - processes that attempt to access the page will now wait on the page lock. - -5. The i_pages lock is taken. This will cause all processes trying - to access the page via the mapping to block on the spinlock. - -6. The refcount of the page is examined and we back out if references remain - otherwise we know that we are the only one referencing this page. - -7. The radix tree is checked and if it does not contain the pointer to this - page then we back out because someone else modified the radix tree. - -8. The new page is prepped with some settings from the old page so that - accesses to the new page will discover a page with the correct settings. - -9. The radix tree is changed to point to the new page. - -10. The reference count of the old page is dropped because the address space - reference is gone. A reference to the new page is established because - the new page is referenced by the address space. - -11. The i_pages lock is dropped. With that lookups in the mapping - become possible again. Processes will move from spinning on the lock - to sleeping on the locked new page. - -12. The page contents are copied to the new page. - -13. The remaining page flags are copied to the new page. - -14. The old page flags are cleared to indicate that the page does - not provide any information anymore. - -15. Queued up writeback on the new page is triggered. - -16. If migration entries were page then replace them with real ptes. Doing - so will enable access for user space processes not already waiting for - the page lock. - -19. The page locks are dropped from the old and new page. - Processes waiting on the page lock will redo their page faults - and will reach the new page. - -20. The new page is moved to the LRU and can be scanned by the swapper - etc again. - -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. |