[PATCH] Make mapping->writeback_index to point to the last written page

[Jan Kara <jack@suse.cz>]

From: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>

For range-cyclic writeback (e.g. kupdate), the writeback code sets a
continuation point of the next writeback to mapping->writeback_index which is
set the page after the last written page. This happens so that we evenly write
the whole file even if pages in it get continuously redirtied.

However, in some cases, sequential writer is writing in the middle of the page
and it just redirties the last written page by continuing from that. For
example with an application which uses a file as a big ring buffer we see:
[1st writeback session]
       ...
       flush-8:0-2743  4571: block_bio_queue: 8,0 W 94898514 + 8
       flush-8:0-2743  4571: block_bio_queue: 8,0 W 94898522 + 8
       flush-8:0-2743  4571: block_bio_queue: 8,0 W 94898530 + 8
       flush-8:0-2743  4571: block_bio_queue: 8,0 W 94898538 + 8
       flush-8:0-2743  4571: block_bio_queue: 8,0 W 94898546 + 8
     kworker/0:1-11    4571: block_rq_issue: 8,0 W 0 () 94898514 + 40
>>     flush-8:0-2743  4571: block_bio_queue: 8,0 W 94898554 + 8
>>     flush-8:0-2743  4571: block_rq_issue: 8,0 W 0 () 94898554 + 8

[2nd writeback session after 35sec]
       flush-8:0-2743  4606: block_bio_queue: 8,0 W 94898562 + 8
       flush-8:0-2743  4606: block_bio_queue: 8,0 W 94898570 + 8
       flush-8:0-2743  4606: block_bio_queue: 8,0 W 94898578 + 8
       ...
     kworker/0:1-11    4606: block_rq_issue: 8,0 W 0 () 94898562 + 640
     kworker/0:1-11    4606: block_rq_issue: 8,0 W 0 () 94899202 + 72
       ...
       flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899962 + 8
       flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899970 + 8
       flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899978 + 8
       flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899986 + 8
       flush-8:0-2743  4606: block_bio_queue: 8,0 W 94899994 + 8
     kworker/0:1-11    4606: block_rq_issue: 8,0 W 0 () 94899962 + 40
>>     flush-8:0-2743  4606: block_bio_queue: 8,0 W 94898554 + 8
>>     flush-8:0-2743  4606: block_rq_issue: 8,0 W 0 () 94898554 + 8

So we seeked back to 94898554 after we wrote all the pages at the end of
the file.

This extra seek seems unnecessary. If we continue writeback from the last
written page, we can avoid it and do not cause harm to other cases. The
original intent of even writeout over the whole file is preserved and if the
page does not get redirtied pagevec_lookup_tag() just skips it.

As an exceptional case, when I/O error happens, set done_index to
the next page as the comment in the code suggests.

Tested-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Jan Kara <jack@suse.cz>
---
Andrew, would you merge this patch please?

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2cb01f6..beae3ed 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -927,7 +927,7 @@ retry:
 				break;
 			}
 
-			done_index = page->index + 1;
+			done_index = page->index;
 
 			lock_page(page);
 
@@ -977,6 +977,7 @@ continue_unlock:
 					 * not be suitable for data integrity
 					 * writeout).
 					 */
+					done_index = page->index + 1;
 					done = 1;
 					break;
 				}