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

[Ritesh Harjani]

On 8/21/20 11:30 AM, Christoph Hellwig wrote:
> On Fri, Aug 21, 2020 at 10:15:33AM +0530, Ritesh Harjani wrote:
> > Please correct me here, but as I see, bio has only these two limits
> > which it checks for adding page to bio. It doesn't check for limits
> > of /sys/block/<dev>/queue/* no? I guess then it could be checked
> > by block layer below b4 submitting the bio?
>
> The bio does not, but the blk-mq code will split the bios when mapping
> it to requests, take a look at blk_mq_submit_bio and __blk_queue_split.

Thanks :)

> But while the default limits are quite low, they can be increased
> siginificantly, which tends to help with performance and is often
> also done by scripts shipped by the distributions.
>
> > This issue was first observed while running a fio run on a system with
> > huge memory. But then here is an easy way we figured out to trigger the
> > issue almost everytime with loop device on my VM setup. I have provided
> > all the details on this below.
>
> Can you wire this up for xfstests?

Sure, will do that.
I do see generic/460 does play with such vm dirty_ratio params which
this test would also require to manipulate to trigger this issue.


Thanks for the suggestion!
-ritesh