[PATCH] mm, vmscan: Do not wait for page writeback for GFP_NOFS

Nikolay has reported a hang when a memcg reclaim got stuck with the following backtrace: PID: 18308 TASK: ffff883d7c9b0a30 CPU: 1 COMMAND: "rsync" #0 [ffff88177374ac60] __schedule at ffffffff815ab152 #1 [ffff88177374acb0] schedule at ffffffff815ab76e #2 [ffff88177374acd0] schedule_timeout at ffffffff815ae5e5 #3 [ffff88177374ad70] io_schedule_timeout at ffffffff815aad6a #4 [ffff88177374ada0] bit_wait_io at ffffffff815abfc6 #5 [ffff88177374adb0] __wait_on_bit at ffffffff815abda5 torvalds#6 [ffff88177374ae00] wait_on_page_bit at ffffffff8111fd4f torvalds#7 [ffff88177374ae50] shrink_page_list at ffffffff81135445 torvalds#8 [ffff88177374af50] shrink_inactive_list at ffffffff81135845 torvalds#9 [ffff88177374b060] shrink_lruvec at ffffffff81135ead torvalds#10 [ffff88177374b150] shrink_zone at ffffffff811360c3 torvalds#11 [ffff88177374b220] shrink_zones at ffffffff81136eff torvalds#12 [ffff88177374b2a0] do_try_to_free_pages at ffffffff8113712f torvalds#13 [ffff88177374b300] try_to_free_mem_cgroup_pages at ffffffff811372be torvalds#14 [ffff88177374b380] try_charge at ffffffff81189423 torvalds#15 [ffff88177374b430] mem_cgroup_try_charge at ffffffff8118c6f5 torvalds#16 [ffff88177374b470] __add_to_page_cache_locked at ffffffff8112137d torvalds#17 [ffff88177374b4e0] add_to_page_cache_lru at ffffffff81121618 torvalds#18 [ffff88177374b510] pagecache_get_page at ffffffff8112170b torvalds#19 [ffff88177374b560] grow_dev_page at ffffffff811c8297 torvalds#20 [ffff88177374b5c0] __getblk_slow at ffffffff811c91d6 torvalds#21 [ffff88177374b600] __getblk_gfp at ffffffff811c92c1 torvalds#22 [ffff88177374b630] ext4_ext_grow_indepth at ffffffff8124565c torvalds#23 [ffff88177374b690] ext4_ext_create_new_leaf at ffffffff81246ca8 torvalds#24 [ffff88177374b6e0] ext4_ext_insert_extent at ffffffff81246f09 torvalds#25 [ffff88177374b750] ext4_ext_map_blocks at ffffffff8124a848 torvalds#26 [ffff88177374b870] ext4_map_blocks at ffffffff8121a5b7 torvalds#27 [ffff88177374b910] mpage_map_one_extent at ffffffff8121b1fa torvalds#28 [ffff88177374b950] mpage_map_and_submit_extent at ffffffff8121f07b torvalds#29 [ffff88177374b9b0] ext4_writepages at ffffffff8121f6d5 torvalds#30 [ffff88177374bb20] do_writepages at ffffffff8112c490 torvalds#31 [ffff88177374bb30] __filemap_fdatawrite_range at ffffffff81120199 torvalds#32 [ffff88177374bb80] filemap_flush at ffffffff8112041c torvalds#33 [ffff88177374bb90] ext4_alloc_da_blocks at ffffffff81219da1 torvalds#34 [ffff88177374bbb0] ext4_rename at ffffffff81229b91 torvalds#35 [ffff88177374bcd0] ext4_rename2 at ffffffff81229e32 torvalds#36 [ffff88177374bce0] vfs_rename at ffffffff811a08a5 torvalds#37 [ffff88177374bd60] SYSC_renameat2 at ffffffff811a3ffc torvalds#38 [ffff88177374bf60] sys_renameat2 at ffffffff811a408e torvalds#39 [ffff88177374bf70] sys_rename at ffffffff8119e51e torvalds#40 [ffff88177374bf80] system_call_fastpath at ffffffff815afa89 Dave Chinner has properly pointed out that this is a deadlock in the reclaim code because ext4 doesn't submit pages which are marked by PG_writeback right away. The heuristic was introduced by e62e384 ("memcg: prevent OOM with too many dirty pages") and it was applied only when may_enter_fs was specified. The code has been changed by c3b94f4 ("memcg: further prevent OOM with too many dirty pages") which has removed the __GFP_FS restriction with a reasoning that we do not get into the fs code. But this is not sufficient apparently because the fs doesn't necessarily submit pages marked PG_writeback for IO right away. ext4_bio_write_page calls io_submit_add_bh but that doesn't necessarily submit the bio. Instead it tries to map more pages into the bio and mpage_map_one_extent might trigger memcg charge which might end up waiting on a page which is marked PG_writeback but hasn't been submitted yet so we would end up waiting for something that never finishes. Fix this issue by replacing __GFP_IO by __GFP_FS check (for case 2) before we go to wait on the writeback. The page fault path, which is the only path that triggers memcg oom killer since 3.12, shouldn't require GFP_NOFS and so we shouldn't reintroduce the premature OOM killer issue which was originally addressed by the heuristic. As per David Chinner the xfs is doing similar thing since 2.6.15 already so ext4 is not the only affected filesystem. Moreover he notes: : For example: IO completion might require unwritten extent conversion : which executes filesystem transactions and GFP_NOFS allocations. The : writeback flag on the pages can not be cleared until unwritten : extent conversion completes. Hence memory reclaim cannot wait on : page writeback to complete in GFP_NOFS context because it is not : safe to do so, memcg reclaim or otherwise. Cc: stable # 3.6+ [tytso@mit.edu: check for __GFP_FS rather than __GFP_IO] Fixes: c3b94f4 ("memcg: further prevent OOM with too many dirty pages") Reported-by: Nikolay Borisov <kernel@kyup.com> Signed-off-by: Michal Hocko <mhocko@suse.cz> --- mm/vmscan.c | 24 ++++++++++-------------- 1 file changed, 10 insertions(+), 14 deletions(-)
kernelOfTruth · Jul 11, 2015 · 042e907 · 042e907
1 parent b953c0d
commit 042e907
Showing 1 changed file with 10 additions and 14 deletions.
diff --git a/mm/vmscan.c b/mm/vmscan.c
@@ -937,21 +937,17 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		 *
 		 * 2) Global reclaim encounters a page, memcg encounters a
 		 *    page that is not marked for immediate reclaim or
-		 *    the caller does not have __GFP_IO. In this case mark
+		 *    the caller does not have __GFP_FS. In this case mark
 		 *    the page for immediate reclaim and continue scanning.
 		 *
-		 *    __GFP_IO is checked  because a loop driver thread might
-		 *    enter reclaim, and deadlock if it waits on a page for
-		 *    which it is needed to do the write (loop masks off
+		 *    Require __GFP_FS even though we are not entering fs
+		 *    because we are waiting for a fs activity and we might
+		 *    be in the middle of the writeout. Moreover a loop driver
+		 *    might enter reclaim, and deadlock of it waits on a page
+		 *    for which it is needed to do the write (loop masks off
 		 *    __GFP_IO|__GFP_FS for this reason); but more thought
 		 *    would probably show more reasons.
 		 *
-		 *    Don't require __GFP_FS, since we're not going into the
-		 *    FS, just waiting on its writeback completion. Worryingly,
-		 *    ext4 gfs2 and xfs allocate pages with
-		 *    grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so testing
-		 *    may_enter_fs here is liable to OOM on them.
-		 *
 		 * 3) memcg encounters a page that is not already marked
 		 *    PageReclaim. memcg does not have any dirty pages
 		 *    throttling so we could easily OOM just because too many
@@ -968,7 +964,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 
 			/* Case 2 above */
 			} else if (global_reclaim(sc) ||
-			    !PageReclaim(page) || !(sc->gfp_mask & __GFP_IO)) {
+			    !PageReclaim(page) || !(sc->gfp_mask & __GFP_FS)) {
 				/*
 				 * This is slightly racy - end_page_writeback()
 				 * might have just cleared PageReclaim, then
@@ -985,10 +981,10 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 
 				goto keep_locked;
 
-			/* Case 3 above */
-			} else {
-				wait_on_page_writeback(page);
 			}
+
+			/* Case 3 above */
+			wait_on_page_writeback(page);
 		}
 
 		if (!force_reclaim)