Page migration

网址：https://www.kernel.org/doc/html/latest/vm/page_migration.html

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.

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.

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. 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.

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