ext4: implement writeback livelock avoidance using page tagging

This is analogous to Jan Kara's commit,
f446daaea9d4a420d16c606f755f3689dcb2d0ce
mm: implement writeback livelock avoidance using page tagging

but since we forked write_cache_pages, we need to reimplement
it there (and in ext4_da_writepages, since range_cyclic handling
was moved to there)

If you start a large buffered IO to a file, and then set
fsync after it, you'll find that fsync does not complete
until the other IO stops.

If you continue re-dirtying the file (say, putting dd
with conv=notrunc in a loop), when fsync finally completes
(after all IO is done), it reports via tracing that
it has written many more pages than the file contains;
in other words it has synced and re-synced pages in
the file multiple times.

This then leads to problems with our writeback_index
update, since it advances it by pages written, and
essentially sets writeback_index off the end of the
file...

With the following patch, we only sync as much as was
dirty at the time of the sync.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 15 insertions, 3 deletions

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 6671fcbb5293..c9ea95ba5fde 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2809,16 +2809,21 @@ static int write_cache_pages_da(struct address_space *mapping,
         pgoff_t index;
         pgoff_t end;            /* Inclusive */
         long nr_to_write = wbc->nr_to_write;
+        int tag;
 
         pagevec_init(&pvec, 0);
         index = wbc->range_start >> PAGE_CACHE_SHIFT;
         end = wbc->range_end >> PAGE_CACHE_SHIFT;
 
+        if (wbc->sync_mode == WB_SYNC_ALL)
+                tag = PAGECACHE_TAG_TOWRITE;
+        else
+                tag = PAGECACHE_TAG_DIRTY;
+
         while (!done && (index <= end)) {
                 int i;
 
-                nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
-                              PAGECACHE_TAG_DIRTY,
+                nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
                               min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
                 if (nr_pages == 0)
                         break;
@@ -2923,6 +2928,7 @@ static int ext4_da_writepages(struct address_space *mapping,
         long desired_nr_to_write, nr_to_writebump = 0;
         loff_t range_start = wbc->range_start;
         struct ext4_sb_info *sbi = EXT4_SB(mapping->host->i_sb);
+        pgoff_t end;
 
         trace_ext4_da_writepages(inode, wbc);
 
@@ -2958,8 +2964,11 @@ static int ext4_da_writepages(struct address_space *mapping,
                 wbc->range_start = index << PAGE_CACHE_SHIFT;
                 wbc->range_end  = LLONG_MAX;
                 wbc->range_cyclic = 0;
-        } else
+                end = -1;
+        } else {
                 index = wbc->range_start >> PAGE_CACHE_SHIFT;
+                end = wbc->range_end >> PAGE_CACHE_SHIFT;
+        }
 
         /*
          * This works around two forms of stupidity.  The first is in
@@ -3000,6 +3009,9 @@ static int ext4_da_writepages(struct address_space *mapping,
         pages_skipped = wbc->pages_skipped;
 
 retry:
+        if (wbc->sync_mode == WB_SYNC_ALL)
+                tag_pages_for_writeback(mapping, index, end);
+
         while (!ret && wbc->nr_to_write > 0) {
 
                 /*