[PATCH 5/5] writeback: check skipped pages on WB_SYNC_ALL

[Wu Fengguang]

In WB_SYNC_ALL mode, filesystems are not expected to skip dirty pages on
temporal lock contentions or non fatal errors, otherwise sync() will
return without actually syncing the skipped pages. Add a check to
catch possible redirty_page_for_writepage() callers that violate this
expectation.

I'd recommend to keep this check in -mm tree for some time and fixup the
possible warnings before pushing it to upstream.

If some FS triggers this warning and it's non-trivial to fix the FS,
we'll have to work out a sync retry scheme for skipped pages.

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 fs/fs-writeback.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- linux-next.orig/fs/fs-writeback.c	2010-11-10 07:04:43.000000000 +0800
+++ linux-next/fs/fs-writeback.c	2010-11-10 07:11:03.000000000 +0800
@@ -527,6 +527,12 @@ static int writeback_sb_inodes(struct su
 			 * buffers.  Skip this inode for now.
 			 */
 			redirty_tail(inode);
+			/*
+			 * There's no logic to retry skipped pages for sync(),
+			 * filesystems are assumed not to skip dirty pages on
+			 * temporal lock contentions or non fatal errors.
+			 */
+			WARN_ON_ONCE(wbc->sync_mode == WB_SYNC_ALL);
 		}
 		spin_unlock(&inode_lock);
 		iput(inode);


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