Documentation/mm/page_migration.rst - linux - Git at Google

 ==============
 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 Documentation/mm/hmm.rst 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
 :ref:`CPUSETS <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()
 ================================

 1. 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.

 2. 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.

 3. 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:

 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 up-to-date) page immediately block 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
    or wait for the migration page table entry to be removed.

 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 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.

 17. 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.

 18. 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.

 1. 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.

 2. 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.

 3. THP_MIGRATION_SUCCESS: A THP was migrated without being split.

 4. THP_MIGRATION_FAIL: A THP could not be migrated nor it could be split.

 5. 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.

 .. kernel-doc:: include/linux/migrate.h
	==============
	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 Documentation/mm/hmm.rst 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
	:ref:`CPUSETS <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()
	================================

	1. 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.

	2. 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.

	3. 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:

	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 up-to-date) page immediately block 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
	or wait for the migration page table entry to be removed.

	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 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.

	17. 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.

	18. 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.

	1. 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.

	2. 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.

	3. THP_MIGRATION_SUCCESS: A THP was migrated without being split.

	4. THP_MIGRATION_FAIL: A THP could not be migrated nor it could be split.

	5. 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.

	.. kernel-doc:: include/linux/migrate.h