[PATCH] mm: write_cache_pages be more sequential

[Adrian Hunter]

If a file is written to sequentially, then writeback
should write the pages sequentially also.  However,
that does not always happen.  For example:

1) user writes pages 0, 1 and 2 but 2 is incomplete
2) write_cache_pages writes pages 0, 1 and 2 and sets
writeback_index to 3
3) user finishes writing page 2 and writes pages 3 and 4
4) write_cache_pages writes pages 3 and 4, and then cycles
back and writes page 2 again.

So the pages are written out in the order 0, 1, 2, 3 ,4 ,2
instead of 0, 1, 2, 2, 3, 4.

This situation was noticed on UBIFS because it writes
directly from writepage.  Hence if there is an unexpected
power-loss, a file will end up with a hole even though
the file was written sequentially by the user.

Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
---
 mm/page-writeback.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 81627eb..7410b7a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -960,6 +960,8 @@ int write_cache_pages(struct address_space *mapping,
 	pagevec_init(&pvec, 0);
 	if (wbc->range_cyclic) {
 		writeback_index = mapping->writeback_index; /* prev offset */
+		if (writeback_index)
+			writeback_index -= 1;
 		index = writeback_index;
 		if (index == 0)
 			cycled = 1;
-- 
1.5.6.3
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