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+==============
+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.
+
+movable_ops page migration
+==========================
+
+Selected typed, non-folio pages (e.g., pages inflated in a memory balloon,
+zsmalloc pages) can be migrated using the movable_ops migration framework.
+
+The "struct movable_operations" provide callbacks specific to a page type
+for isolating, migrating and un-isolating (putback) these pages.
+
+Once a page is indicated as having movable_ops, that condition must not
+change until the page was freed back to the buddy. This includes not
+changing/clearing the page type and not changing/clearing the
+PG_movable_ops page flag.
+
+Arbitrary drivers cannot currently make use of this framework, as it
+requires:
+
+(a) a page type
+(b) indicating them as possibly having movable_ops in page_has_movable_ops()
+    based on the page type
+(c) returning the movable_ops from page_movable_ops() based on the page
+    type
+(d) not reusing the PG_movable_ops and PG_movable_ops_isolated page flags
+    for other purposes
+
+For example, balloon drivers can make use of this framework through the
+balloon-compaction infrastructure residing in the core kernel.
+
+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