writeback: write_cache_pages doesn't terminate at nr_to_write <= 0

I noticed XFS writeback in 2.6.36-rc1 was much slower than it should have
been. Enabling writeback tracing showed:

    flush-253:16-8516  [007] 1342952.351608: wbc_writepage: bdi 253:16: towrt=1024 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
    flush-253:16-8516  [007] 1342952.351654: wbc_writepage: bdi 253:16: towrt=1023 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
    flush-253:16-8516  [000] 1342952.369520: wbc_writepage: bdi 253:16: towrt=0 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
    flush-253:16-8516  [000] 1342952.369542: wbc_writepage: bdi 253:16: towrt=-1 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
    flush-253:16-8516  [000] 1342952.369549: wbc_writepage: bdi 253:16: towrt=-2 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0

Writeback is not terminating in background writeback if ->writepage is
returning with wbc->nr_to_write == 0, resulting in sub-optimal single page
writeback on XFS.

Fix the write_cache_pages loop to terminate correctly when this situation
occurs and so prevent this sub-optimal background writeback pattern. This
improves sustained sequential buffered write performance from around
250MB/s to 750MB/s for a 100GB file on an XFS filesystem on my 8p test VM.

Cc:<stable@kernel.org>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 10 insertions, 16 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index c09ef5219cbe..a803f5e33471 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -985,22 +985,16 @@ continue_unlock:
                                 }
                         }
 
-                        if (wbc->nr_to_write > 0) {
-                                if (--wbc->nr_to_write == 0 &&
-                                    wbc->sync_mode == WB_SYNC_NONE) {
-                                        /*
-                                         * We stop writing back only if we are
-                                         * not doing integrity sync. In case of
-                                         * 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.
-                                         */
-                                        done = 1;
-                                        break;
-                                }
+                        /*
+                         * We stop writing back only if we are not doing
+                         * integrity sync. In case of integrity sync we have to
+                         * keep going until we have written all the pages
+                         * we tagged for writeback prior to entering this loop.
+                         */
+                        if (--wbc->nr_to_write <= 0 &&
+                            wbc->sync_mode == WB_SYNC_NONE) {
+                                done = 1;
+                                break;
                         }
                 }
                 pagevec_release(&pvec);