mm: migrate: make buffer_migrate_page_norefs() actually succeed

Currently, buffer_migrate_page_norefs() was constantly failing because
buffer_migrate_lock_buffers() grabbed reference on each buffer.  In
fact, there's no reason for buffer_migrate_lock_buffers() to grab any
buffer references as the page is locked during all our operation and
thus nobody can reclaim buffers from the page.

So remove grabbing of buffer references which also makes
buffer_migrate_page_norefs() succeed.

Link: http://lkml.kernel.org/r/20190116131217.7226-1-jack@suse.cz
Fixes: 89cb0888 "mm: migrate: provide buffer_migrate_page_norefs()"
Signed-off-by: NJan Kara <jack@suse.cz>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Zi Yan <zi.yan@cs.rutgers.edu>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -709,7 +709,6 @@ static bool buffer_migrate_lock_buffers(struct buffer_head *head,
 	/* Simple case, sync compaction */
 	if (mode != MIGRATE_ASYNC) {
 		do {
-			get_bh(bh);
 			lock_buffer(bh);
 			bh = bh->b_this_page;
 
@@ -720,18 +719,15 @@ static bool buffer_migrate_lock_buffers(struct buffer_head *head,
 
 	/* async case, we cannot block on lock_buffer so use trylock_buffer */
 	do {
-		get_bh(bh);
 		if (!trylock_buffer(bh)) {
 			/*
 			 * We failed to lock the buffer and cannot stall in
 			 * async migration. Release the taken locks
 			 */
 			struct buffer_head *failed_bh = bh;
-			put_bh(failed_bh);
 			bh = head;
 			while (bh != failed_bh) {
 				unlock_buffer(bh);
-				put_bh(bh);
 				bh = bh->b_this_page;
 			}
 			return false;
@@ -818,7 +814,6 @@ static int __buffer_migrate_page(struct address_space *mapping,
 	bh = head;
 	do {
 		unlock_buffer(bh);
-		put_bh(bh);
 		bh = bh->b_this_page;
 
 	} while (bh != head);