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/fs/ext4/inode.c b/fs/ext4/inode.c
index a5763e3505ba..8558b6c3450a 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2741,7 +2741,7 @@ static int write_cache_pages_da(struct address_space *mapping,
         index = wbc->range_start >> PAGE_CACHE_SHIFT;
         end = wbc->range_end >> PAGE_CACHE_SHIFT;
 
-        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;
@@ -2973,7 +2973,7 @@ static int ext4_da_writepages(struct address_space *mapping,
         }
 
 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);
 
         while (!ret && wbc->nr_to_write > 0) {