[dm-devel] [PATCH 5.20 1/4] block: add bio_rewind() API

[Kent Overstreet]

On Fri, Jun 24, 2022 at 10:12:52PM +0800, Ming Lei wrote:
> Commit 7759eb23fd98 ("block: remove bio_rewind_iter()") removes
> the similar API because the following reasons:
> 
>     ```
>     It is pointed that bio_rewind_iter() is one very bad API[1]:
> 
>     1) bio size may not be restored after rewinding
> 
>     2) it causes some bogus change, such as 5151842b9d8732 (block: reset
>     bi_iter.bi_done after splitting bio)
> 
>     3) rewinding really makes things complicated wrt. bio splitting
> 
>     4) unnecessary updating of .bi_done in fast path
> 
>     [1] https://marc.info/?t=153549924200005&r=1&w=2
> 
>     So this patch takes Kent's suggestion to restore one bio into its original
>     state via saving bio iterator(struct bvec_iter) in bio_integrity_prep(),
>     given now bio_rewind_iter() is only used by bio integrity code.
>     ```
> 
> However, it isn't easy to restore bio by saving 32 bytes bio->bi_iter, and saving
> it only can't restore crypto and integrity info.
> 
> Add bio_rewind() back for some use cases which may not be same with
> previous generic case:
> 
> 1) most of bio has fixed end sector since bio split is done from front of the bio,
> if driver just records how many sectors between current bio's start sector and
> the bio's end sector, the original position can be restored
> 
> 2) if one bio's end sector won't change, usually bio_trim() isn't called, user can
> restore original position by storing sectors from current ->bi_iter.bi_sector to
> bio's end sector; together by saving bio size, 8 bytes can restore to
> original bio.
> 
> 3) dm's requeue use case: when BLK_STS_DM_REQUEUE happens, dm core needs to
> restore to the original bio which represents current dm io to be requeued.
> By storing sectors to the bio's end sector and dm io's size,
> bio_rewind() can restore such original bio, then dm core code needn't to
> allocate one bio beforehand just for handling BLK_STS_DM_REQUEUE which
> is actually one unusual event.
> 
> 4) Not like original rewind API, this one needn't to add .bi_done, and no any
> effect on fast path

It seems like perhaps the real issue here is that we need a real bio_iter,
separate from bvec_iter, that also encapsulates iterating over integrity &
fscrypt.