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>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index c09ef5219cbe36f267a37f55d6fc670815082522..a803f5e33471fe6092a11cf233fc29d354d9c2b4 100644 (file)
--- 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);