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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.

1	Page migration
2	--------------
4	Page migration allows the moving of the physical location of pages between
5	nodes in a numa system while the process is running. This means that the
6	virtual addresses that the process sees do not change. However, the
7	system rearranges the physical location of those pages.
9	The main intend of page migration is to reduce the latency of memory access
10	by moving pages near to the processor where the process accessing that memory
11	is running.
13	Page migration allows a process to manually relocate the node on which its
14	pages are located through the MF_MOVE and MF_MOVE_ALL options while setting
15	a new memory policy via mbind(). The pages of process can also be relocated
16	from another process using the sys_migrate_pages() function call. The
17	migrate_pages function call takes two sets of nodes and moves pages of a
18	process that are located on the from nodes to the destination nodes.
19	Page migration functions are provided by the numactl package by Andi Kleen
20	(a version later than 0.9.3 is required. Get it from
21	ftp://oss.sgi.com/www/projects/libnuma/download/). numactl provides libnuma
22	which provides an interface similar to other numa functionality for page
23	migration.  cat /proc/<pid>/numa_maps allows an easy review of where the
24	pages of a process are located. See also the numa_maps documentation in the
25	proc(5) man page.
27	Manual migration is useful if for example the scheduler has relocated
28	a process to a processor on a distant node. A batch scheduler or an
29	administrator may detect the situation and move the pages of the process
30	nearer to the new processor. The kernel itself does only provide
31	manual page migration support. Automatic page migration may be implemented
32	through user space processes that move pages. A special function call
33	"move_pages" allows the moving of individual pages within a process.
34	A NUMA profiler may f.e. obtain a log showing frequent off node
35	accesses and may use the result to move pages to more advantageous
36	locations.
38	Larger installations usually partition the system using cpusets into
39	sections of nodes. Paul Jackson has equipped cpusets with the ability to
40	move pages when a task is moved to another cpuset (See
41	Documentation/cgroup-v1/cpusets.txt).
42	Cpusets allows the automation of process locality. If a task is moved to
43	a new cpuset then also all its pages are moved with it so that the
44	performance of the process does not sink dramatically. Also the pages
45	of processes in a cpuset are moved if the allowed memory nodes of a
46	cpuset are changed.
48	Page migration allows the preservation of the relative location of pages
49	within a group of nodes for all migration techniques which will preserve a
50	particular memory allocation pattern generated even after migrating a
51	process. This is necessary in order to preserve the memory latencies.
52	Processes will run with similar performance after migration.
54	Page migration occurs in several steps. First a high level
55	description for those trying to use migrate_pages() from the kernel
56	(for userspace usage see the Andi Kleen's numactl package mentioned above)
57	and then a low level description of how the low level details work.
59	A. In kernel use of migrate_pages()
60	-----------------------------------
62	1. Remove pages from the LRU.
64	   Lists of pages to be migrated are generated by scanning over
65	   pages and moving them into lists. This is done by
66	   calling isolate_lru_page().
67	   Calling isolate_lru_page increases the references to the page
68	   so that it cannot vanish while the page migration occurs.
69	   It also prevents the swapper or other scans to encounter
70	   the page.
72	2. We need to have a function of type new_page_t that can be
73	   passed to migrate_pages(). This function should figure out
74	   how to allocate the correct new page given the old page.
76	3. The migrate_pages() function is called which attempts
77	   to do the migration. It will call the function to allocate
78	   the new page for each page that is considered for
79	   moving.
81	B. How migrate_pages() works
82	----------------------------
84	migrate_pages() does several passes over its list of pages. A page is moved
85	if all references to a page are removable at the time. The page has
86	already been removed from the LRU via isolate_lru_page() and the refcount
87	is increased so that the page cannot be freed while page migration occurs.
89	Steps:
91	1. Lock the page to be migrated
93	2. Insure that writeback is complete.
95	3. Lock the new page that we want to move to. It is locked so that accesses to
96	   this (not yet uptodate) page immediately lock while the move is in progress.
98	4. All the page table references to the page are converted to migration
99	   entries. This decreases the mapcount of a page. If the resulting
100	   mapcount is not zero then we do not migrate the page. All user space
101	   processes that attempt to access the page will now wait on the page lock.
103	5. The radix tree lock is taken. This will cause all processes trying
104	   to access the page via the mapping to block on the radix tree spinlock.
106	6. The refcount of the page is examined and we back out if references remain
107	   otherwise we know that we are the only one referencing this page.
109	7. The radix tree is checked and if it does not contain the pointer to this
110	   page then we back out because someone else modified the radix tree.
112	8. The new page is prepped with some settings from the old page so that
113	   accesses to the new page will discover a page with the correct settings.
115	9. The radix tree is changed to point to the new page.
117	10. The reference count of the old page is dropped because the radix tree
118	    reference is gone. A reference to the new page is established because
119	    the new page is referenced to by the radix tree.
121	11. The radix tree lock is dropped. With that lookups in the mapping
122	    become possible again. Processes will move from spinning on the tree_lock
123	    to sleeping on the locked new page.
125	12. The page contents are copied to the new page.
127	13. The remaining page flags are copied to the new page.
129	14. The old page flags are cleared to indicate that the page does
130	    not provide any information anymore.
132	15. Queued up writeback on the new page is triggered.
134	16. If migration entries were page then replace them with real ptes. Doing
135	    so will enable access for user space processes not already waiting for
136	    the page lock.
138	19. The page locks are dropped from the old and new page.
139	    Processes waiting on the page lock will redo their page faults
140	    and will reach the new page.
142	20. The new page is moved to the LRU and can be scanned by the swapper
143	    etc again.
145	C. Non-LRU page migration
146	-------------------------
148	Although original migration aimed for reducing the latency of memory access
149	for NUMA, compaction who want to create high-order page is also main customer.
151	Current problem of the implementation is that it is designed to migrate only
152	*LRU* pages. However, there are potential non-lru pages which can be migrated
153	in drivers, for example, zsmalloc, virtio-balloon pages.
155	For virtio-balloon pages, some parts of migration code path have been hooked
156	up and added virtio-balloon specific functions to intercept migration logics.
157	It's too specific to a driver so other drivers who want to make their pages
158	movable would have to add own specific hooks in migration path.
160	To overclome the problem, VM supports non-LRU page migration which provides
161	generic functions for non-LRU movable pages without driver specific hooks
162	migration path.
164	If a driver want to make own pages movable, it should define three functions
165	which are function pointers of struct address_space_operations.
167	1. bool (*isolate_page) (struct page *page, isolate_mode_t mode);
169	What VM expects on isolate_page function of driver is to return *true*
170	if driver isolates page successfully. On returing true, VM marks the page
171	as PG_isolated so concurrent isolation in several CPUs skip the page
172	for isolation. If a driver cannot isolate the page, it should return *false*.
174	Once page is successfully isolated, VM uses page.lru fields so driver
175	shouldn't expect to preserve values in that fields.
177	2. int (*migratepage) (struct address_space *mapping,
178			struct page *newpage, struct page *oldpage, enum migrate_mode);
180	After isolation, VM calls migratepage of driver with isolated page.
181	The function of migratepage is to move content of the old page to new page
182	and set up fields of struct page newpage. Keep in mind that you should
183	indicate to the VM the oldpage is no longer movable via __ClearPageMovable()
184	under page_lock if you migrated the oldpage successfully and returns
185	MIGRATEPAGE_SUCCESS. If driver cannot migrate the page at the moment, driver
186	can return -EAGAIN. On -EAGAIN, VM will retry page migration in a short time
187	because VM interprets -EAGAIN as "temporal migration failure". On returning
188	any error except -EAGAIN, VM will give up the page migration without retrying
189	in this time.
191	Driver shouldn't touch page.lru field VM using in the functions.
193	3. void (*putback_page)(struct page *);
195	If migration fails on isolated page, VM should return the isolated page
196	to the driver so VM calls driver's putback_page with migration failed page.
197	In this function, driver should put the isolated page back to the own data
198	structure.
200	4. non-lru movable page flags
202	There are two page flags for supporting non-lru movable page.
204	* PG_movable
206	Driver should use the below function to make page movable under page_lock.
208		void __SetPageMovable(struct page *page, struct address_space *mapping)
210	It needs argument of address_space for registering migration family functions
211	which will be called by VM. Exactly speaking, PG_movable is not a real flag of
212	struct page. Rather than, VM reuses page->mapping's lower bits to represent it.
214		#define PAGE_MAPPING_MOVABLE 0x2
215		page->mapping = page->mapping | PAGE_MAPPING_MOVABLE;
217	so driver shouldn't access page->mapping directly. Instead, driver should
218	use page_mapping which mask off the low two bits of page->mapping under
219	page lock so it can get right struct address_space.
221	For testing of non-lru movable page, VM supports __PageMovable function.
222	However, it doesn't guarantee to identify non-lru movable page because
223	page->mapping field is unified with other variables in struct page.
224	As well, if driver releases the page after isolation by VM, page->mapping
225	doesn't have stable value although it has PAGE_MAPPING_MOVABLE
226	(Look at __ClearPageMovable). But __PageMovable is cheap to catch whether
227	page is LRU or non-lru movable once the page has been isolated. Because
228	LRU pages never can have PAGE_MAPPING_MOVABLE in page->mapping. It is also
229	good for just peeking to test non-lru movable pages before more expensive
230	checking with lock_page in pfn scanning to select victim.
232	For guaranteeing non-lru movable page, VM provides PageMovable function.
233	Unlike __PageMovable, PageMovable functions validates page->mapping and
234	mapping->a_ops->isolate_page under lock_page. The lock_page prevents sudden
235	destroying of page->mapping.
237	Driver using __SetPageMovable should clear the flag via __ClearMovablePage
238	under page_lock before the releasing the page.
240	* PG_isolated
242	To prevent concurrent isolation among several CPUs, VM marks isolated page
243	as PG_isolated under lock_page. So if a CPU encounters PG_isolated non-lru
244	movable page, it can skip it. Driver doesn't need to manipulate the flag
245	because VM will set/clear it automatically. Keep in mind that if driver
246	sees PG_isolated page, it means the page have been isolated by VM so it
247	shouldn't touch page.lru field.
248	PG_isolated is alias with PG_reclaim flag so driver shouldn't use the flag
249	for own purpose.
251	Christoph Lameter, May 8, 2006.
252	Minchan Kim, Mar 28, 2016.
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