Re: [PATCH 00/12] xfs: flush related error handling cleanups

[Brian Foster <bfoster@redhat.com>]

On Mon, Apr 20, 2020 at 08:53:06AM +1000, Dave Chinner wrote:
> On Fri, Apr 17, 2020 at 11:08:47AM -0400, Brian Foster wrote:
> > Hi all,
> > 
> > This actually started as what I intended to be a cleanup of xfsaild
> > error handling and the fact that unexpected errors are kind of lost in
> > the ->iop_push() handlers of flushable log items. Some discussion with
> > Dave on that is available here[1]. I was thinking of genericizing the
> > behavior, but I'm not so sure that is possible now given the error
> > handling requirements of the associated items.
> > 
> > While thinking through that, I ended up incorporating various cleanups
> > in the somewhat confusing and erratic error handling on the periphery of
> > xfsaild, such as the flush handlers. Most of these are straightforward
> > cleanups except for patch 9, which I think requires careful review and
> > is of debatable value. I have used patch 12 to run an hour or so of
> > highly concurrent fsstress load against it and will execute a longer run
> > over the weekend now that fstests has completed.
> > 
> > Thoughts, reviews, flames appreciated.
> 
> I'll need to do something thinking on this patchset - I have a
> patchset that touches a lot of the same code I'm working on right
> now to pin inode cluster buffers in memory when the inode is dirtied
> so we don't get RMW cycles in AIL flushing.
> 
> That code gets rid of xfs_iflush() completely, removes dirty inodes
> from the AIL and tracks only ordered cluster buffers in the AIL for
> inode writeback (i.e. reduces AIL tracked log items by up to 30x).
> It also only does inode writeback from the ordered cluster buffers.
> 

Ok. I could see that being reason enough to drop the iflush iodone
patch, given that it depends on a bit of a rework/hack. A cleaner
solution requires more thought and it might not be worth the time if the
code is going away. Most of the rest are straightforward cleanups though
so I wouldn't expect complex conflict resolution. It's hard to say
for sure without seeing the code, of course..

> The idea behind this is to make inode flushing completely
> non-blocking, and to simply inode cluster flushing to simply iterate
> all the dirty inodes attached to the buffer. This gets rid of radix
> tree lookups and races with reclaim, and gets rid of having to
> special case a locked inode in the cluster iteration code.
> 

Sounds interesting, but it's not really clear to me what the general
flushing dynamic looks like in this model. I.e., you mention
xfs_iflush() goes away, but cluster flushing still exists in some form,
so I can't really tell if xfs_iflush() going away is tied to a
functional change or primarily a refactoring/cleanup. Anyways, no need
to go into the weeds if the code will eventually clarify..

> I was looking at this as the model to then apply to dquot flushing,
> too, because it currently does not have cluster flushing, and hence
> flushes dquots individually, even though there can be multiple dirty
> dquots per buffer. Some of this patchset moves the dquot flushing a
> bit closer to the inode code, so those parts are going to be useful
> regardless of everything else....
> 

Makes sense.

> Do you have a git tree I could pull this from to see how bad the
> conflicts are?
> 

I don't have a public tree. I suppose I could look into getting
kernel.org access if somebody could point me in the right direction for
that. :) In the meantime I could make a private tree accessible to you
directly if that's helpful..

Brian

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
>