Re: [dm-devel] [PATCH] dm integrity: reinitialize __bi_remaining when reusing bio

[Christoph Hellwig <hch@infradead.org>]

On Tue, Feb 25, 2020 at 08:54:07PM +0100, Daniel Glöckner wrote:
> bio_reset will reset too many fields. As you can see in the context of
> the diff, dm-integrity expects f.ex. the values modified by bio_advance
> to stay intact and the transfer should of course use the same disk and
> operation.
> 
> How about doing the atomic_set in bio_remaining_done (in block/bio.c)
> where the BIO_CHAIN flag is cleared once __bi_remaining hits zero?
> Or is requeuing a bio without bio_reset really a no-go? In that case a
> one-liner won't do...

That tends to add a overhead to the fast path for a rather exotic
case.  I'm having a bit of a hard time understanding the dm-integrity
code due to it's annoyingly obsfucated code, but it seems like it
tries to submit a bio again after it came out of a ->end_io handler.
That might have some other problems, but if we only want to paper
over the remaining count a isngle call to bio_inc_remaining might be all
you need.