[PATCH 08/13] mm/migrate: __unmap_and_move() push good newpage to LRU

Hugh Dickins
2022-02-06T21:43:59Z

Compaction, NUMA page movement, THP collapse/split, and memory failure
do isolate unevictable pages from their "LRU", losing the record of
mlock_count in doing so (isolators are likely to use page->lru for their
own private lists, so mlock_count has to be presumed lost).

That's unfortunate, and we should put in some work to correct that: one
can imagine a function to build up the mlock_count again - but it would
require i_mmap_rwsem for read, so be careful where it's called. Or
page_referenced_one() and try_to_unmap_one() might do that extra work.

But one place that can very easily be improved is page migration's
__unmap_and_move(): a small adjustment to where the successful new page
is put back on LRU, and its mlock_count (if any) is built back up by
remove_migration_ptes().

Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/migrate.c | 31 +++++++++++++++++++------------
1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 7c4223ce2500..f4bcf1541b62 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1032,6 +1032,21 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
if (!page_mapped(page))
rc = move_to_new_page(newpage, page, mode);

+ /*
+ * When successful, push newpage to LRU immediately: so that if it
+ * turns out to be an mlocked page, remove_migration_ptes() will
+ * automatically build up the correct newpage->mlock_count for it.
+ *
+ * We would like to do something similar for the old page, when
+ * unsuccessful, and other cases when a page has been temporarily
+ * isolated from the unevictable LRU: but this case is the easiest.
+ */
+ if (rc == MIGRATEPAGE_SUCCESS) {
+ lru_cache_add(newpage);
+ if (page_was_mapped)
+ lru_add_drain();
+ }
+
if (page_was_mapped)
remove_migration_ptes(page,
rc == MIGRATEPAGE_SUCCESS ? newpage : page, false);
@@ -1045,20 +1060,12 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
unlock_page(page);
out:
/*
- * If migration is successful, decrease refcount of the newpage
+ * If migration is successful, decrease refcount of the newpage,
* which will not free the page because new page owner increased
- * refcounter. As well, if it is LRU page, add the page to LRU
- * list in here. Use the old state of the isolated source page to
- * determine if we migrated a LRU page. newpage was already unlocked
- * and possibly modified by its owner - don't rely on the page
- * state.
+ * refcounter.
*/
- if (rc == MIGRATEPAGE_SUCCESS) {
- if (unlikely(!is_lru))
- put_page(newpage);
- else
- putback_lru_page(newpage);
- }
+ if (rc == MIGRATEPAGE_SUCCESS)
+ put_page(newpage);

return rc;
}
--
2.34.1