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Message-ID: <20150820061224.GG17933@dhcp-13-216.nay.redhat.com>
Date: Thu, 20 Aug 2015 14:12:24 +0800
From: Eryu Guan <eguan@...hat.com>
To: Dave Chinner <david@...morbit.com>
Cc: Tejun Heo <tj@...nel.org>, Jens Axboe <axboe@...nel.dk>,
Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
xfs@....sgi.com, axboe@...com, Jan Kara <jack@...e.com>,
linux-fsdevel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH block/for-linus] writeback: fix syncing of I_DIRTY_TIME
inodes
On Wed, Aug 19, 2015 at 07:56:11AM +1000, Dave Chinner wrote:
> On Tue, Aug 18, 2015 at 12:54:39PM -0700, Tejun Heo wrote:
> > Hello,
> >
> > On Tue, Aug 18, 2015 at 10:47:18AM -0700, Tejun Heo wrote:
> > > Hmm... the only possibility I can think of is tot_write_bandwidth
> > > being zero when it shouldn't be. I've been staring at the code for a
> > > while now but nothing rings a bell. Time for another debug patch, I
> > > guess.
> >
> > So, I can now reproduce the bug (it takes a lot of trials but lowering
> > the number of tested files helps quite a bit) and instrumented all the
> > early exit paths w/o the fix patch. bdi_has_dirty_io() and
> > wb_has_dirty_io() are never out of sync with the actual dirty / io
> > lists even when the test 048 fails, so the bug at least is not caused
> > by writeback skipping due to buggy bdi/wb_has_dirty_io() result.
> > Whenever it skips, all the lists are actually empty (verified while
> > holding list_lock).
> >
> > One suspicion I have is that this could be a subtle timing issue which
> > is being exposed by the new short-cut path. Anything which adds delay
> > seems to make the issue go away. Dave, does anything ring a bell?
>
> No, it doesn't. The data writeback mechanisms XFS uses are all
> generic. It marks inodes I_DIRTY_PAGES and lets the generic code
> take care of everything else. Yes, we do delayed allocation during
> writeback, and we log the inode size updates during IO completion,
> so if inode sizes are not getting updated, then Occam's Razor
> suggests that writeback is not happening.
>
> I'd suggest looking at some of the XFS tracepoints during the test:
>
> tracepoint trigger
> xfs_file_buffered_write once per write syscall
> xfs_file_sync once per fsync per inode
> xfs_vm_writepage every ->writepage call
> xfs_setfilesize every IO completion that updates inode size
I gave the tracepoints a try, but my root fs is xfs so I got many
noises. I'll try to install a new vm with ext4 as root fs. But I'm not
sure if the new vm could reproduce the failure, will see.
BTW, I guess xfs_vm_writepage should be xfs_writepage, and xfs_file_sync
should be xfs_file_fsync?
Thanks,
Eryu
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