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

[Christoph Hellwig]

On Mon, Aug 24, 2020 at 10:28:23AM -0400, Brian Foster wrote:
> Do I understand the current code (__bio_try_merge_page() ->
> page_is_mergeable()) correctly in that we're checking for physical page
> contiguity and not necessarily requiring a new bio_vec per physical
> page?


Yes.

> With regard to Dave's earlier point around seeing excessively sized bio
> chains.. If I set up a large memory box with high dirty mem ratios and
> do contiguous buffered overwrites over a 32GB range followed by fsync, I
> can see upwards of 1GB per bio and thus chains on the order of 32+ bios
> for the entire write. If I play games with how the buffered overwrite is
> submitted (i.e., in reverse) however, then I can occasionally reproduce
> a ~32GB chain of ~32k bios, which I think is what leads to problems in
> I/O completion on some systems. Granted, I don't reproduce soft lockup
> issues on my system with that behavior, so perhaps there's more to that
> particular issue.
> 
> Regardless, it seems reasonable to me to at least have a conservative
> limit on the length of an ioend bio chain. Would anybody object to
> iomap_ioend growing a chain counter and perhaps forcing into a new ioend
> if we chain something like more than 1k bios at once?

So what exactly is the problem of processing a long chain in the
workqueue vs multiple small chains?  Maybe we need a cond_resched()
here and there, but I don't see how we'd substantially change behavior.