Re: [PATCH 2/5] block: make bio_inc_remaining() interface accessible again

[Christoph Hellwig <hch@lst.de>]

On Thu, May 05, 2016 at 11:54:22AM -0400, Mike Snitzer wrote:
> Commit 326e1dbb57 ("block: remove management of bi_remaining when
> restoring original bi_end_io") made bio_inc_remaining() private to bio.c
> because the only use-case that made sense was confined to the
> bio_chain() interface.
> 
> Since that time DM thinp went on to use bio_chain() in its relatively
> complex implementation of async discard support.  That implementation,
> even when converted over to use the new async __blkdev_issue_discard()
> interface, depends on deferred completion of the original discard bio --
> which is most appropriately implemented using bio_inc_remaining().

Can you explain that code flow to me?  I still fail to why exactly
dm-thinp (and only dm-thinp) needs this.  Maybe start by pointing me
to the additional bio_endio that pairs with the bio_inc_remaining
call.

> All said, bio_inc_remaining() should really only be used in conjunction
> with bio_chain().  It isn't intended for generic bio reference counting.

It's obviously used outside bio_chain in dm-thinp, so this sentence
doesn't make too much sense to me.