linux-kernel - [PATCH 3/3] Fix fsync-vs-write misbehavior

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Message-ID: <Pine.LNX.4.64.0810051814540.23248@hs20-bc2-1.build.redhat.com>
Date:	Sun, 5 Oct 2008 18:16:59 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	linux-kernel@...r.kernel.org, agk@...hat.com, mbroz@...hat.com,
	chris@...chsys.com
Subject: [PATCH 3/3] Fix fsync-vs-write misbehavior

Fix violation of sync()/fsync() semantics. Previous code walked up to
mapping->nrpages * 2 pages. Because pages could be created while
__filemap_fdatawrite_range was in progress, it could lead to a
misbehavior.  Example: there are two pages in address space with indices
4, 5. Both are dirty.  Someone calls __filemap_fdatawrite_range, it sets
.nr_to_write = 4.  Meanwhile, some other process creates dirty pages 0,
1, 2, 3.  __filemap_fdatawrite_range writes pages 0, 1, 2, 3, finds out
that it reached the limit and exits.

Result: pages that were dirty before __filemap_fdatawrite_range was
invoked were not written.

With starvation protection from the previous patch, this
mapping->nrpages * 2 logic is no longer needed.

Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>

---
 mm/filemap.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Index: linux-2.6.27-rc7-devel/mm/filemap.c
===================================================================
--- linux-2.6.27-rc7-devel.orig/mm/filemap.c	2008-09-24 14:47:01.000000000 +0200
+++ linux-2.6.27-rc7-devel/mm/filemap.c	2008-09-24 15:01:23.000000000 +0200
@@ -202,6 +202,11 @@ static int sync_page_killable(void *word
  * opposed to a regular memory cleansing writeback.  The difference between
  * these two operations is that if a dirty page/buffer is encountered, it must
  * be waited upon, and not just skipped over.
+ *
+ * Because new pages dirty can be created while this is executing, that
+ * mapping->nrpages * 2 condition is unsafe. If we are doing data integrity
+ * write, we must write all the pages. AS_STARVATION bit will eventually prevent
+ * creating more dirty pages to avoid starvation.
  */
 int __filemap_fdatawrite_range(struct address_space *mapping, loff_t start,
 				loff_t end, int sync_mode)
@@ -209,7 +214,7 @@ int __filemap_fdatawrite_range(struct ad
 	int ret;
 	struct writeback_control wbc = {
 		.sync_mode = sync_mode,
-		.nr_to_write = mapping->nrpages * 2,
+		.nr_to_write = sync_mode == WB_SYNC_NONE ? mapping->nrpages * 2 : LONG_MAX,
 		.range_start = start,
 		.range_end = end,
 	};
--
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