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

[Brian Foster <bfoster@redhat.com>]

On Sat, Aug 22, 2020 at 02:13:12PM +0100, Christoph Hellwig wrote:
> On Sat, Aug 22, 2020 at 07:53:58AM +1000, Dave Chinner wrote:
> > but iomap only allows BIO_MAX_PAGES when creating the bio. And:
> > 
> > #define BIO_MAX_PAGES 256
> > 
> > So even on a 64k page machine, we should not be building a bio with
> > more than 16MB of data in it. So how are we getting 4GB of data into
> > it?
> 
> BIO_MAX_PAGES is the number of bio_vecs in the bio, it has no
> direct implication on the size, as each entry can fit up to UINT_MAX
> bytes.
> 

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?

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?

Brian