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 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/dm-devel