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+Page migration
+--------------
+
+Page migration allows the moving of 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.
+
+The main intend of page migration is to reduce the latency of memory access
+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 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
+ftp://oss.sgi.com/www/projects/libnuma/download/). 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 does only provide
+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.
+A NUMA profiler may f.e. 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
+Documentation/cgroups/cpusets.txt).
+Cpusets allows 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.
+
+A. In kernel use of migrate_pages()
+-----------------------------------
+
+1. Remove pages from the LRU.
+
+   Lists of pages to be migrated are generated by scanning over
+   pages and moving them into lists. This is done by
+   calling isolate_lru_page().
+   Calling isolate_lru_page increases the references to the page
+   so that it cannot vanish while the page migration occurs.
+   It also prevents the swapper or other scans to encounter
+   the page.
+
+2. We need to have a function of type new_page_t that can be
+   passed to migrate_pages(). This function should figure out
+   how to allocate the correct new page given the old page.
+
+3. The migrate_pages() function is called which attempts
+   to do the migration. It will call the function to allocate
+   the new page for each page that is considered for
+   moving.
+
+B. How migrate_pages() works
+----------------------------
+
+migrate_pages() does several passes over its list of pages. A page is moved
+if all references to a page are removable at the time. The page has
+already been removed from the LRU via isolate_lru_page() and the refcount
+is increased so that the page cannot be freed while page migration occurs.
+
+Steps:
+
+1. Lock the page to be migrated
+
+2. Insure that writeback is complete.
+
+3. Prep the new page that we want to move to. It is locked
+   and set to not being uptodate so that all accesses to the new
+   page immediately lock while the move is in progress.
+
+4. 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.
+
+5. All the page table references to the page are converted
+   to migration entries or dropped (nonlinear vmas).
+   This decrease 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.
+
+6. The radix tree lock is taken. This will cause all processes trying
+   to access the page via the mapping to block on the radix tree spinlock.
+
+7. 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.
+
+8. 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.
+
+9. The radix tree is changed to point to the new page.
+
+10. The reference count of the old page is dropped because the radix tree
+    reference is gone. A reference to the new page is established because
+    the new page is referenced to by the radix tree.
+
+11. The radix tree lock is dropped. With that lookups in the mapping
+    become possible again. Processes will move from spinning on the tree_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 page then replace them with real ptes. Doing
+    so will enable access for user space processes not already waiting for
+    the page lock.
+
+19. 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.
+
+20. The new page is moved to the LRU and can be scanned by the swapper
+    etc again.
+
+Christoph Lameter, May 8, 2006.
+