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

[Christoph Hellwig]

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.

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?