mm: migrate: lock buffers before migrate_page_move_mapping()

Lock buffers before calling into migrate_page_move_mapping() so that that
function doesn't have to know about buffers (which is somewhat unexpected
anyway) and all the buffer head logic is in buffer_migrate_page().

Link: http://lkml.kernel.org/r/20181211172143.7358-3-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 26 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 94c9ebf1f33e..e0bc03e15e74 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -487,20 +487,6 @@ int migrate_page_move_mapping(struct address_space *mapping,
         }
 
         /*
-         * In the async migration case of moving a page with buffers, lock the
-         * buffers using trylock before the mapping is moved. If the mapping
-         * was moved, we later failed to lock the buffers and could not move
-         * the mapping back due to an elevated page count, we would have to
-         * block waiting on other references to be dropped.
-         */
-        if (mode == MIGRATE_ASYNC && head &&
-                        !buffer_migrate_lock_buffers(head, mode)) {
-                page_ref_unfreeze(page, expected_count);
-                xas_unlock_irq(&xas);
-                return -EAGAIN;
-        }
-
-        /*
          * Now we know that no one else is looking at the page:
          * no turning back from here.
          */
@@ -775,24 +761,23 @@ int buffer_migrate_page(struct address_space *mapping,
 {
         struct buffer_head *bh, *head;
         int rc;
+        int expected_count;
 
         if (!page_has_buffers(page))
                 return migrate_page(mapping, newpage, page, mode);
 
-        head = page_buffers(page);
+        /* Check whether page does not have extra refs before we do more work */
+        expected_count = expected_page_refs(page);
+        if (page_count(page) != expected_count)
+                return -EAGAIN;
 
-        rc = migrate_page_move_mapping(mapping, newpage, page, head, mode, 0);
+        head = page_buffers(page);
+        if (!buffer_migrate_lock_buffers(head, mode))
+                return -EAGAIN;
 
+        rc = migrate_page_move_mapping(mapping, newpage, page, NULL, mode, 0);
         if (rc != MIGRATEPAGE_SUCCESS)
-                return rc;
-
-        /*
-         * In the async case, migrate_page_move_mapping locked the buffers
-         * with an IRQ-safe spinlock held. In the sync case, the buffers
-         * need to be locked now
-         */
-        if (mode != MIGRATE_ASYNC)
-                BUG_ON(!buffer_migrate_lock_buffers(head, mode));
+                goto unlock_buffers;
 
         ClearPagePrivate(page);
         set_page_private(newpage, page_private(page));
@@ -814,6 +799,8 @@ int buffer_migrate_page(struct address_space *mapping,
         else
                 migrate_page_states(newpage, page);
 
+        rc = MIGRATEPAGE_SUCCESS;
+unlock_buffers:
         bh = head;
         do {
                 unlock_buffer(bh);
@@ -822,7 +809,7 @@ int buffer_migrate_page(struct address_space *mapping,
 
         } while (bh != head);
 
-        return MIGRATEPAGE_SUCCESS;
+        return rc;
 }
 EXPORT_SYMBOL(buffer_migrate_page);
 #endif