[PATCH 2/9] btrfs: don't wait for writeback on clean pages in extent_write_cache_pages

[Christoph Hellwig <hch@lst.de>]

__extent_writepage can have started on more pages than the one it was
called for.  This happens regularly for zoned file systems, and in theory
could happen for compressed I/O if the worker thread was executed very
quicky. For such pages extent_write_cache_pages waits for writeback
to complete before moving on to the next page, which is highly inefficient
as it blocks the flusher thread

Port over the PageDirty check that was added to write_cache_pages in
commit 515f4a037fb ("mm: write_cache_pages optimise page cleaning") to
fix this.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/btrfs/extent_io.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index cb8c5d06fe2304..2ae3badaf36df6 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2075,6 +2075,12 @@ static int extent_write_cache_pages(struct address_space *mapping,
 				continue;
 			}
 
+			if (!folio_test_dirty(folio)) {
+				/* someone wrote it for us */
+				folio_unlock(folio);
+				continue;
+			}
+
 			if (wbc->sync_mode != WB_SYNC_NONE) {
 				if (folio_test_writeback(folio))
 					submit_write_bio(bio_ctrl, 0);
-- 
2.39.2