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

[Jeff Layton]

On Wed, 25 Mar 2009 20:17:43 +0800
Wu Fengguang <fengguang.wu@xxxxxxxxx> 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@xxxxxxxxx> 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.

-- 
Jeff Layton <jlayton@xxxxxxxxxx>
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