Re: [PATCH] writeback: reset inode dirty time when adding it back to empty s_dirty list

[Wu Fengguang <fengguang.wu-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

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-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> 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.:)

> > > > ...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.

Thanks,
Fengguang

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