Page migration¶

Page migration allows moving 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.

Also see Heterogeneous Memory Management (HMM) for migrating pages to or from device private memory.

The main intent of page migration is to reduce the latency of memory accesses 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 a 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 https://github.com/numactl/numactl.git). 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 only provides 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. For example, A NUMA profiler may 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 CPUSETS). Cpusets allow 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()¶

Remove folios from the LRU.

Lists of folios to be migrated are generated by scanning over folios and moving them into lists. This is done by calling folio_isolate_lru(). Calling folio_isolate_lru() increases the references to the folio so that it cannot vanish while the folio migration occurs. It also prevents the swapper or other scans from encountering the folio.
We need to have a function of type new_folio_t that can be passed to migrate_pages(). This function should figure out how to allocate the correct new folio given the old folio.
The migrate_pages() function is called which attempts to do the migration. It will call the function to allocate the new folio for each folio that is considered for moving.

How migrate_pages() works¶

migrate_pages() does several passes over its list of folios. A folio is moved if all references to a folio are removable at the time. The folio has already been removed from the LRU via folio_isolate_lru() and the refcount is increased so that the folio cannot be freed while folio migration occurs.

Steps:

Lock the page to be migrated.
Ensure that writeback is complete.
Lock the new page that we want to move to. It is locked so that accesses to this (not yet up-to-date) page immediately block while the move is in progress.
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 or wait for the migration page table entry to be removed.
The i_pages lock is taken. This will cause all processes trying to access the page via the mapping to block on the spinlock.
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.
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.
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.
The radix tree is changed to point to the new page.
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.
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.
The page contents are copied to the new page.
The remaining page flags are copied to the new page.
The old page flags are cleared to indicate that the page does not provide any information anymore.
Queued up writeback on the new page is triggered.
If migration entries were inserted into the page table, then replace them with real ptes. Doing so will enable access for user space processes not already waiting for the page lock.
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.
The new page is moved to the LRU and can be scanned by the swapper, etc. again.

Non-LRU page migration¶

Although migration originally aimed for reducing the latency of memory accesses for NUMA, compaction also uses migration to create high-order pages. For compaction purposes, it is also useful to be able to move non-LRU pages, such as zsmalloc and virtio-balloon pages.

If a driver wants to make its pages movable, it should define a struct movable_operations. It then needs to call __SetPageMovable() on each page that it may be able to move. This uses the page->mapping field, so this field is not available for the driver to use for other purposes.

Monitoring Migration¶

The following events (counters) can be used to monitor page migration.

PGMIGRATE_SUCCESS: Normal page migration success. Each count means that a page was migrated. If the page was a non-THP and non-hugetlb page, then this counter is increased by one. If the page was a THP or hugetlb, then this counter is increased by the number of THP or hugetlb subpages. For example, migration of a single 2MB THP that has 4KB-size base pages (subpages) will cause this counter to increase by 512.
PGMIGRATE_FAIL: Normal page migration failure. Same counting rules as for PGMIGRATE_SUCCESS, above: this will be increased by the number of subpages, if it was a THP or hugetlb.
THP_MIGRATION_SUCCESS: A THP was migrated without being split.
THP_MIGRATION_FAIL: A THP could not be migrated nor it could be split.
THP_MIGRATION_SPLIT: A THP was migrated, but not as such: first, the THP had to be split. After splitting, a migration retry was used for its sub-pages.

THP_MIGRATION_* events also update the appropriate PGMIGRATE_SUCCESS or PGMIGRATE_FAIL events. For example, a THP migration failure will cause both THP_MIGRATION_FAIL and PGMIGRATE_FAIL to increase.

Christoph Lameter, May 8, 2006. Minchan Kim, Mar 28, 2016.

struct movable_operations¶: Driver page migration

Definition:

struct movable_operations {
    bool (*isolate_page)(struct page *, isolate_mode_t);
    int (*migrate_page)(struct page *dst, struct page *src, enum migrate_mode);
    void (*putback_page)(struct page *);
};

Members

isolate_page

The VM calls this function to prepare the page to be moved. The page is locked and the driver should not unlock it. The driver should return true if the page is movable and false if it is not currently movable. After this function returns, the VM uses the page->lru field, so the driver must preserve any information which is usually stored here.

migrate_page

After isolation, the VM calls this function with the isolated src page. The driver should copy the contents of the src page to the dst page and set up the fields of dst page. Both pages are locked. If page migration is successful, the driver should call __ClearPageMovable(src) and return MIGRATEPAGE_SUCCESS. If the driver cannot migrate the page at the moment, it can return -EAGAIN. The VM interprets this as a temporary migration failure and will retry it later. Any other error value is a permanent migration failure and migration will not be retried. The driver shouldn’t touch the src->lru field while in the migrate_page() function. It may write to dst->lru.

putback_page

If migration fails on the isolated page, the VM informs the driver that the page is no longer a candidate for migration by calling this function. The driver should put the isolated page back into its own data structure.