writeback: introduce .tagged_writepages for the WB_SYNC_NONE sync stage

sync(2) is performed in two stages: the WB_SYNC_NONE sync and the
WB_SYNC_ALL sync. Identify the first stage with .tagged_writepages and
do livelock prevention for it, too.

Jan's commit f446daaea9 ("mm: implement writeback livelock avoidance
using page tagging") is a partial fix in that it only fixed the
WB_SYNC_ALL phase livelock.

Although ext4 is tested to no longer livelock with commit f446daaea9,
it may due to some "redirty_tail() after pages_skipped" effect which
is by no means a guarantee for _all_ the file systems.

Note that writeback_inodes_sb() is called by not only sync(), they are
treated the same because the other callers also need livelock prevention.

Impact:  It changes the order in which pages/inodes are synced to disk.
Now in the WB_SYNC_NONE stage, it won't proceed to write the next inode
until finished with the current inode.

Acked-by: Jan Kara <jack@suse.cz>
CC: Dave Chinner <david@fromorbit.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 31f698862420..955fe35d01e0 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -892,12 +892,12 @@ int write_cache_pages(struct address_space *mapping,
                         range_whole = 1;
                 cycled = 1; /* ignore range_cyclic tests */
         }
-        if (wbc->sync_mode == WB_SYNC_ALL)
+        if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
                 tag = PAGECACHE_TAG_TOWRITE;
         else
                 tag = PAGECACHE_TAG_DIRTY;
 retry:
-        if (wbc->sync_mode == WB_SYNC_ALL)
+        if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
                 tag_pages_for_writeback(mapping, index, end);
         done_index = index;
         while (!done && (index <= end)) {