block: remove bio_rewind_iter()

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 5151842b (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.

Cc: Dmitry Monakhov <dmonakhov@openvz.org>
Cc: Hannes Reinecke <hare@suse.com>
Suggested-by: Kent Overstreet <kent.overstreet@gmail.com>
Acked-by: Kent Overstreet <kent.overstreet@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio-integrity.c

@@ -306,6 +306,8 @@ bool bio_integrity_prep(struct bio *bio)
 	if (bio_data_dir(bio) == WRITE) {
 		bio_integrity_process(bio, &bio->bi_iter,
 				      bi->profile->generate_fn);
+	} else {
+		bip->bio_iter = bio->bi_iter;
 	}
 	return true;
 
@@ -331,20 +333,14 @@ static void bio_integrity_verify_fn(struct work_struct *work)
 		container_of(work, struct bio_integrity_payload, bip_work);
 	struct bio *bio = bip->bip_bio;
 	struct blk_integrity *bi = blk_get_integrity(bio->bi_disk);
-	struct bvec_iter iter = bio->bi_iter;
 
 	/*
 	 * At the moment verify is called bio's iterator was advanced
 	 * during split and completion, we need to rewind iterator to
 	 * it's original position.
 	 */
-	if (bio_rewind_iter(bio, &iter, iter.bi_done)) {
-		bio->bi_status = bio_integrity_process(bio, &iter,
+	bio->bi_status = bio_integrity_process(bio, &bip->bio_iter,
 						bi->profile->verify_fn);
-	} else {
-		bio->bi_status = BLK_STS_IOERR;
-	}
-
 	bio_integrity_free(bio);
 	bio_endio(bio);
 }

block/bio.c

@@ -1807,7 +1807,6 @@ struct bio *bio_split(struct bio *bio, int sectors,
 		bio_integrity_trim(split);
 
 	bio_advance(bio, split->bi_iter.bi_size);
-	bio->bi_iter.bi_done = 0;
 
 	if (bio_flagged(bio, BIO_TRACE_COMPLETION))
 		bio_set_flag(split, BIO_TRACE_COMPLETION);

include/linux/bio.h

@@ -170,27 +170,11 @@ static inline void bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
 {
 	iter->bi_sector += bytes >> 9;
 
-	if (bio_no_advance_iter(bio)) {
+	if (bio_no_advance_iter(bio))
 		iter->bi_size -= bytes;
-		iter->bi_done += bytes;
-	} else {
+	else
 		bvec_iter_advance(bio->bi_io_vec, iter, bytes);
 		/* TODO: It is reasonable to complete bio with error here. */
-	}
-}
-
-static inline bool bio_rewind_iter(struct bio *bio, struct bvec_iter *iter,
-		unsigned int bytes)
-{
-	iter->bi_sector -= bytes >> 9;
-
-	if (bio_no_advance_iter(bio)) {
-		iter->bi_size += bytes;
-		iter->bi_done -= bytes;
-		return true;
-	}
-
-	return bvec_iter_rewind(bio->bi_io_vec, iter, bytes);
 }
 
 #define __bio_for_each_segment(bvl, bio, iter, start)			\
@@ -353,6 +337,8 @@ struct bio_integrity_payload {
 	unsigned short		bip_max_vcnt;	/* integrity bio_vec slots */
 	unsigned short		bip_flags;	/* control flags */
 
+	struct bvec_iter	bio_iter;	/* for rewinding parent bio */
+
 	struct work_struct	bip_work;	/* I/O completion */
 
 	struct bio_vec		*bip_vec;

include/linux/bvec.h

@@ -40,8 +40,6 @@ struct bvec_iter {
 
 	unsigned int		bi_idx;		/* current index into bvl_vec */
 
-	unsigned int            bi_done;	/* number of bytes completed */
-
 	unsigned int            bi_bvec_done;	/* number of bytes completed in
 						   current bvec */
 };
@@ -85,7 +83,6 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv,
 		bytes -= len;
 		iter->bi_size -= len;
 		iter->bi_bvec_done += len;
-		iter->bi_done += len;
 
 		if (iter->bi_bvec_done == __bvec_iter_bvec(bv, *iter)->bv_len) {
 			iter->bi_bvec_done = 0;