Re: [PATCH 5.20 1/4] block: add bio_rewind() API

[Ming Lei <ming.lei@redhat.com>]

On Sun, Jun 26, 2022 at 04:14:58PM -0400, Kent Overstreet wrote:
> 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. 

Not mention bio_iter, bvec_iter has been 32 bytes, which is too big to
hold in per-io data structure. With this patch, 8bytes is enough
to rewind one bio if the end sector is fixed.


Thanks,
Ming