Re: [PATCH] iomap: Fix the write_count in iomap_add_to_ioend().

[Brian Foster]

On Thu, Sep 17, 2020 at 09:04:55AM +0100, Christoph Hellwig wrote:
> On Wed, Sep 16, 2020 at 09:07:14AM -0400, Brian Foster wrote:
> > Dave described the main purpose earlier in this thread [1]. The initial
> > motivation is that we've had downstream reports of soft lockup problems
> > in writeback bio completion down in the bio -> bvec loop of
> > iomap_finish_ioend() that has to finish writeback on each individual
> > page of insanely large bios and/or chains. We've also had an upstream
> > reports of a similar problem on linux-xfs [2].
> > 
> > The magic number itself was just pulled out of a hat. I picked it
> > because it seemed conservative enough to still allow large contiguous
> > bios (1GB w/ 4k pages) while hopefully preventing I/O completion
> > problems, but was hoping for some feedback on that bit if the general
> > approach was acceptable. I was also waiting for some feedback on either
> > of the two users who reported the problem but I don't think I've heard
> > back on that yet...
> 
> I think the saner answer is to always run large completions in the
> workqueue, and add a bunch of cond_resched() calls, rather than
> arbitrarily breaking up the I/O size.
> 

That wouldn't address the latency concern Dave brought up. That said, I
have no issue with this as a targeted solution for the softlockup issue.
iomap_finish_ioend[s]() is common code for both the workqueue and
->bi_end_io() contexts so that would require either some kind of context
detection (and my understanding is in_atomic() is unreliable/frowned
upon) or a new "atomic" parameter through iomap_finish_ioend[s]() to
indicate whether it's safe to reschedule. Preference?

Brian