Date:	Wed, 25 Mar 2009 10:00:49 -0400
From:	Jeff Layton <jlayton@...hat.com>
To:	Ian Kent <raven@...maw.net>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Dave Chinner <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH] writeback: reset inode dirty time when adding it back
 to empty s_dirty list

On Wed, 25 Mar 2009 22:38:47 +0900
Ian Kent <raven@...maw.net> wrote:

> Ian Kent wrote:
> > Jeff Layton wrote:
> >> On Wed, 25 Mar 2009 20:17:43 +0800
> >> Wu Fengguang <fengguang.wu@...el.com> wrote:
> >>
> >>> On Wed, Mar 25, 2009 at 07:51:10PM +0800, Jeff Layton wrote:
> >>>> On Wed, 25 Mar 2009 10:50:37 +0800
> >>>> Wu Fengguang <fengguang.wu@...el.com> wrote:
> >>>>
> >>>>>> Given the right situation though (or maybe the right filesystem), it's
> >>>>>> not too hard to imagine this problem occurring even in current mainline
> >>>>>> code with an inode that's frequently being redirtied.
> >>>>> My reasoning with recent kernel is: for kupdate, s_dirty enqueues only
> >>>>> happen in __mark_inode_dirty() and redirty_tail().  Newly dirtied
> >>>>> inodes will be parked in s_dirty for 30s. During which time the
> >>>>> actively being-redirtied inodes, if their dirtied_when is an old stuck
> >>>>> value, will be retried for writeback and then re-inserted into a
> >>>>> non-empty s_dirty queue and have their dirtied_when refreshed.
> >>>>>
> >>>> Doesn't that assume that there are new inodes that are being dirtied?
> >>>> If you only have the same inodes being redirtied and never any new
> >>>> ones, the problem still occurs, right?
> >>> Yes. But will a production server run months without making one single
> >>> new dirtied inode? (Just out of curiosity. Not that I'm not willing to
> >>> fix this possible issue.:)
> >>>
> >> Yes. It's not that the box will run that long without creating a
> >> single new dirtied inode, but rather that it won't necessarily create
> >> one on all of its mounts. It's often the case that someone has a
> >> mountpoint for a dedicated purpose.
> >>
> >> Consider a host that has a mountpoint that contains logfiles that are
> >> being heavily written. There's nothing that says that they must rotate
> >> those logs over a particular period (assuming the fs has enough space,
> >> etc). If the same ones are constantly being redirtied and no new
> >> ones are created, then I think this problem can easily happen.
> >>
> >>>>>>> ...I see no obvious reasons against unconditionally resetting dirtied_when.
> >>>>>>>
> >>>>>>> (a) Delaying an inode's writeback for 30s maybe too long - its blocking
> >>>>>>> condition may well go away within 1s. (b) And it would be very undesirable
> >>>>>>> if one big file is repeatedly redirtied hence its writeback being
> >>>>>>> delayed considerably.
> >>>>>>>
> >>>>>>> However, redirty_tail() currently only tries to speedup writeback-after-redirty
> >>>>>>> in a _best effort_ way. It at best partially hides the above issues,
> >>>>>>> if there are any. In particular, if (b) is possible, the bug should
> >>>>>>> already show up at least in some situations.
> >>>>>>>
> >>>>>>> For XFS, immediately sync of redirtied inode is actually discouraged:
> >>>>>>>
> >>>>>>>         http://lkml.org/lkml/2008/1/16/491
> >>>>>>>
> >>>>>>>
> >>>>>> Ok, those are good points that I need to think about.
> >>>>>>
> >>>>>> Thanks for the help so far. I'd welcome any suggestions you have on
> >>>>>> how best to fix this.
> >>>>> For NFS, is it desirable to retry a redirtied inode after 30s, or
> >>>>> after a shorter 5s, or after 0.1~5s? Or the exact timing simply
> >>>>> doesn't matter?
> >>>>>
> >>>> I don't really consider NFS to be a special case here. It just happens
> >>>> to be where we saw the problem originally. Some of its characteristics
> >>>> might make it easier to hit this, but I'm not certain of that.
> >>> Now there are now two possible solutions:
> >>> - unconditionally update dirtied_when in redirty_tail();
> >>> - keep dirtied_when and redirty inodes to a new dedicated queue.
> >>> The first one involves less code, the second one allows more flexible timing.
> >>>
> >>> NFS/XFS could be a good starting point for discussing the
> >>> requirements, so that we can reach a suitable solution.
> >>>
> >> It sounds like it, yes. I saw that you posted some patches in January
> >> (including your s_more_io_wait patch). I'll give those a closer look.
> >> Adding the new s_more_io_wait queue is interesting and might sidestep
> >> this problem nicely.
> >>
> > 
> > Yes, I was looking at that bit of code but, so far, I think it won't be
> > called for the case we are trying to describe.
> 
> I take that back.
> As Jeff pointed out I haven't seen these patches and can't seem to find
> them in my fsdevel list folder, Wu can you send me a copy please?
> 

Actually, I think you were right. We still have this check in
generic_sync_sb_inodes() even with Wu's January 2008 patches:

	/* Was this inode dirtied after sync_sb_inodes was called? */
	if (time_after(inode->dirtied_when, start))
		break;

...this check is the crux of the problem. We're assuming that the
dirtied_when value will never appear to be in the future. If we change
this check so that it's checking that dirtied_when is between "start"
and "now", then this problem basically goes away.

We'll probably also need to change the test in move_expired_inodes
too, unless Wu's changes go in.

-- 
Jeff Layton <jlayton@...hat.com>
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