|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
|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
|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
|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
|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.
|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
|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
|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.