mm/migrate: __unmap_and_move() push good newpage to LRU

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>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

diff --git a/mm/migrate.c b/mm/migrate.c
index 7c4223ce250012e6cdfa7a124dba0a277465a944..f4bcf1541b629dadd1cd8c857cfb59b7b7f12e37 100644 (file)
--- 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 @@ out_unlock:
        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;
 }