mm: compaction: determine if dirty pages can be migrated without blocking within ->migratepage

Asynchronous compaction is used when allocating transparent hugepages to
avoid blocking for long periods of time.  Due to reports of stalling,
there was a debate on disabling synchronous compaction but this severely
impacted allocation success rates.  Part of the reason was that many dirty
pages are skipped in asynchronous compaction by the following check;

	if (PageDirty(page) && !sync &&
		mapping->a_ops->migratepage != migrate_page)
			rc = -EBUSY;

This skips over all mapping aops using buffer_migrate_page() even though
it is possible to migrate some of these pages without blocking.  This
patch updates the ->migratepage callback with a "sync" parameter.  It is
the responsibility of the callback to fail gracefully if migration would
block.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Andy Isaacson <adi@hexapodia.org>
Cc: Nai Xia <nai.xia@gmail.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index a7409bc157e0..b92b73d0b2b9 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -609,9 +609,12 @@ struct address_space_operations {
                         loff_t offset, unsigned long nr_segs);
         int (*get_xip_mem)(struct address_space *, pgoff_t, int,
                                                 void **, unsigned long *);
-        /* migrate the contents of a page to the specified target */
+        /*
+         * migrate the contents of a page to the specified target. If sync
+         * is false, it must not block.
+         */
         int (*migratepage) (struct address_space *,
-                        struct page *, struct page *);
+                        struct page *, struct page *, bool);
         int (*launder_page) (struct page *);
         int (*is_partially_uptodate) (struct page *, read_descriptor_t *,
                                         unsigned long);
@@ -2537,7 +2540,7 @@ extern int generic_check_addressable(unsigned, u64);
 
 #ifdef CONFIG_MIGRATION
 extern int buffer_migrate_page(struct address_space *,
-                                struct page *, struct page *);
+                                struct page *, struct page *, bool);
 #else
 #define buffer_migrate_page NULL
 #endif