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.
