Re: [PATCH 2/5] writeback: stop periodic/background work on seeingsync works

From: Wu Fengguang
Date: Tue Aug 03 2010 - 09:45:20 EST

On Tue, Aug 03, 2010 at 09:22:16PM +0800, Wu Fengguang wrote:
> > > Fengguang, how about merging also the attached simple patch together with
> > > my fix? With these two patches, I'm not able to trigger any sync livelock
> > > while without one of them I hit them quite easily...
> >
> > This looks OK. However note that redirty_tail() can modify
> > dirtied_when unexpectedly. So the more we rely on wb_start, the more
> > possibility an inode is (wrongly) skipped by sync. I have a bunch of
> > patches to remove redirty_tail(). However they may not be good
> > candidates for 2.6.36..
> It looks that setting wb_start at the beginning of
> writeback_inodes_wb() won't be easily affected by redirty_tail().

Except for this redirty_tail(), which may mess up the dirtied_when
ordering in b_dirty and later on break the assumption of
inode_dirtied_after(inode, wbc->wb_start).

It can be replaced by a requeue_io() for now. Christoph mentioned a
patchset to introduce sb->s_wb, which should be a better solution.


diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index a178828..e56e68b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -457,12 +457,7 @@ int generic_writeback_sb_inodes(struct super_block *sb,

if (inode->i_sb != sb) {
if (only_this_sb) {
- /*
- * We only want to write back data for this
- * superblock, move all inodes not belonging
- * to it back onto the dirty list.
- */
- redirty_tail(inode);
+ requeue_io(inode);

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