7 Page migration allows moving the physical location of pages between
8 nodes in a NUMA system while the process is running. This means that the
9 virtual addresses that the process sees do not change. However, the
10 system rearranges the physical location of those pages.
12 Also see :ref:`Heterogeneous Memory Management (HMM) <hmm>`
13 for migrating pages to or from device private memory.
15 The main intent of page migration is to reduce the latency of memory accesses
16 by moving pages near to the processor where the process accessing that memory
19 Page migration allows a process to manually relocate the node on which its
20 pages are located through the MF_MOVE and MF_MOVE_ALL options while setting
21 a new memory policy via mbind(). The pages of a process can also be relocated
22 from another process using the sys_migrate_pages() function call. The
23 migrate_pages() function call takes two sets of nodes and moves pages of a
24 process that are located on the from nodes to the destination nodes.
25 Page migration functions are provided by the numactl package by Andi Kleen
26 (a version later than 0.9.3 is required. Get it from
27 https://github.com/numactl/numactl.git). numactl provides libnuma
28 which provides an interface similar to other NUMA functionality for page
29 migration. cat ``/proc/<pid>/numa_maps`` allows an easy review of where the
30 pages of a process are located. See also the numa_maps documentation in the
33 Manual migration is useful if for example the scheduler has relocated
34 a process to a processor on a distant node. A batch scheduler or an
35 administrator may detect the situation and move the pages of the process
36 nearer to the new processor. The kernel itself only provides
37 manual page migration support. Automatic page migration may be implemented
38 through user space processes that move pages. A special function call
39 "move_pages" allows the moving of individual pages within a process.
40 For example, A NUMA profiler may obtain a log showing frequent off-node
41 accesses and may use the result to move pages to more advantageous
44 Larger installations usually partition the system using cpusets into
45 sections of nodes. Paul Jackson has equipped cpusets with the ability to
46 move pages when a task is moved to another cpuset (See
47 :ref:`CPUSETS <cpusets>`).
48 Cpusets allow the automation of process locality. If a task is moved to
49 a new cpuset then also all its pages are moved with it so that the
50 performance of the process does not sink dramatically. Also the pages
51 of processes in a cpuset are moved if the allowed memory nodes of a
54 Page migration allows the preservation of the relative location of pages
55 within a group of nodes for all migration techniques which will preserve a
56 particular memory allocation pattern generated even after migrating a
57 process. This is necessary in order to preserve the memory latencies.
58 Processes will run with similar performance after migration.
60 Page migration occurs in several steps. First a high level
61 description for those trying to use migrate_pages() from the kernel
62 (for userspace usage see the Andi Kleen's numactl package mentioned above)
63 and then a low level description of how the low level details work.
65 In kernel use of migrate_pages()
66 ================================
68 1. Remove pages from the LRU.
70 Lists of pages to be migrated are generated by scanning over
71 pages and moving them into lists. This is done by
72 calling isolate_lru_page().
73 Calling isolate_lru_page() increases the references to the page
74 so that it cannot vanish while the page migration occurs.
75 It also prevents the swapper or other scans from encountering
78 2. We need to have a function of type new_page_t that can be
79 passed to migrate_pages(). This function should figure out
80 how to allocate the correct new page given the old page.
82 3. The migrate_pages() function is called which attempts
83 to do the migration. It will call the function to allocate
84 the new page for each page that is considered for
87 How migrate_pages() works
88 =========================
90 migrate_pages() does several passes over its list of pages. A page is moved
91 if all references to a page are removable at the time. The page has
92 already been removed from the LRU via isolate_lru_page() and the refcount
93 is increased so that the page cannot be freed while page migration occurs.
97 1. Lock the page to be migrated.
99 2. Ensure that writeback is complete.
101 3. Lock the new page that we want to move to. It is locked so that accesses to
102 this (not yet up-to-date) page immediately block while the move is in progress.
104 4. All the page table references to the page are converted to migration
105 entries. This decreases the mapcount of a page. If the resulting
106 mapcount is not zero then we do not migrate the page. All user space
107 processes that attempt to access the page will now wait on the page lock
108 or wait for the migration page table entry to be removed.
110 5. The i_pages lock is taken. This will cause all processes trying
111 to access the page via the mapping to block on the spinlock.
113 6. The refcount of the page is examined and we back out if references remain.
114 Otherwise, we know that we are the only one referencing this page.
116 7. The radix tree is checked and if it does not contain the pointer to this
117 page then we back out because someone else modified the radix tree.
119 8. The new page is prepped with some settings from the old page so that
120 accesses to the new page will discover a page with the correct settings.
122 9. The radix tree is changed to point to the new page.
124 10. The reference count of the old page is dropped because the address space
125 reference is gone. A reference to the new page is established because
126 the new page is referenced by the address space.
128 11. The i_pages lock is dropped. With that lookups in the mapping
129 become possible again. Processes will move from spinning on the lock
130 to sleeping on the locked new page.
132 12. The page contents are copied to the new page.
134 13. The remaining page flags are copied to the new page.
136 14. The old page flags are cleared to indicate that the page does
137 not provide any information anymore.
139 15. Queued up writeback on the new page is triggered.
141 16. If migration entries were inserted into the page table, then replace them
142 with real ptes. Doing so will enable access for user space processes not
143 already waiting for the page lock.
145 17. The page locks are dropped from the old and new page.
146 Processes waiting on the page lock will redo their page faults
147 and will reach the new page.
149 18. The new page is moved to the LRU and can be scanned by the swapper,
152 Non-LRU page migration
153 ======================
155 Although migration originally aimed for reducing the latency of memory
156 accesses for NUMA, compaction also uses migration to create high-order
157 pages. For compaction purposes, it is also useful to be able to move
158 non-LRU pages, such as zsmalloc and virtio-balloon pages.
160 If a driver wants to make its pages movable, it should define a struct
161 movable_operations. It then needs to call __SetPageMovable() on each
162 page that it may be able to move. This uses the ``page->mapping`` field,
163 so this field is not available for the driver to use for other purposes.
166 =====================
168 The following events (counters) can be used to monitor page migration.
170 1. PGMIGRATE_SUCCESS: Normal page migration success. Each count means that a
171 page was migrated. If the page was a non-THP and non-hugetlb page, then
172 this counter is increased by one. If the page was a THP or hugetlb, then
173 this counter is increased by the number of THP or hugetlb subpages.
174 For example, migration of a single 2MB THP that has 4KB-size base pages
175 (subpages) will cause this counter to increase by 512.
177 2. PGMIGRATE_FAIL: Normal page migration failure. Same counting rules as for
178 PGMIGRATE_SUCCESS, above: this will be increased by the number of subpages,
179 if it was a THP or hugetlb.
181 3. THP_MIGRATION_SUCCESS: A THP was migrated without being split.
183 4. THP_MIGRATION_FAIL: A THP could not be migrated nor it could be split.
185 5. THP_MIGRATION_SPLIT: A THP was migrated, but not as such: first, the THP had
186 to be split. After splitting, a migration retry was used for it's sub-pages.
188 THP_MIGRATION_* events also update the appropriate PGMIGRATE_SUCCESS or
189 PGMIGRATE_FAIL events. For example, a THP migration failure will cause both
190 THP_MIGRATION_FAIL and PGMIGRATE_FAIL to increase.
192 Christoph Lameter, May 8, 2006.
193 Minchan Kim, Mar 28, 2016.
195 .. kernel-doc:: include/linux/migrate.h