[PATCH] ext4: Fix performance regression in ext4_writepages()

[Ming Lei]

Commit 4e7ea81db5(ext4: restructure writeback path) introduces another
performance regression on random write: The logic in
mpage_prepare_extent_to_map() always prepares at least one page for
mapping and the loop in ext4_writepages() doesn't terminate when
nr_to_write drops to zero if there is something to map. So we will still
try to write more that we should and we will do it in 1 page chunks.

Fix the problem by moving nr_to_write check in
mpage_prepare_extent_to_map() before preparing the first page. That way
mpage_prepare_extent_to_map() will return without preparing anything when
nr_to_write is exhausted and the loop in ext4_writepages() will be
terminated properly.

Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
 fs/ext4/inode.c | 32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index e274e9c1171f..7d12a38fbde4 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2334,6 +2334,22 @@ static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd)
 			if (mpd->map.m_len > 0 && mpd->next_page != page->index)
 				goto out;
 
+			if (mpd->map.m_len == 0)
+				mpd->first_page = page->index;
+			mpd->next_page = page->index + 1;
+			/*
+			 * Accumulated enough dirty pages? This doesn't apply
+			 * to WB_SYNC_ALL mode. For integrity sync we have to
+			 * keep going because someone may be concurrently
+			 * dirtying pages, and we might have synced a lot of
+			 * newly appeared dirty pages, but have not synced all
+			 * of the old dirty pages.
+			 */
+			if (mpd->wbc->sync_mode == WB_SYNC_NONE &&
+			    mpd->next_page - mpd->first_page >
+							mpd->wbc->nr_to_write)
+				goto out;
+
 			lock_page(page);
 			/*
 			 * If the page is no longer dirty, or its mapping no
@@ -2353,9 +2369,6 @@ static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd)
 			wait_on_page_writeback(page);
 			BUG_ON(PageWriteback(page));
 
-			if (mpd->map.m_len == 0)
-				mpd->first_page = page->index;
-			mpd->next_page = page->index + 1;
 			/* Add all dirty buffers to mpd */
 			lblk = ((ext4_lblk_t)page->index) <<
 				(PAGE_CACHE_SHIFT - blkbits);
@@ -2364,19 +2377,6 @@ static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd)
 			if (err <= 0)
 				goto out;
 			err = 0;
-
-			/*
-			 * Accumulated enough dirty pages? This doesn't apply
-			 * to WB_SYNC_ALL mode. For integrity sync we have to
-			 * keep going because someone may be concurrently
-			 * dirtying pages, and we might have synced a lot of
-			 * newly appeared dirty pages, but have not synced all
-			 * of the old dirty pages.
-			 */
-			if (mpd->wbc->sync_mode == WB_SYNC_NONE &&
-			    mpd->next_page - mpd->first_page >=
-							mpd->wbc->nr_to_write)
-				goto out;
 		}
 		pagevec_release(&pvec);
 		cond_resched();
-- 
1.8.1.4


--aM3YZ0Iwxop3KEKx--
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