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

[Christoph Hellwig]

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.