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Date:   Thu, 22 Nov 2018 11:30:33 +0100
From:   Christoph Hellwig <hch@....de>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Theodore Ts'o <tytso@....edu>,
        Omar Sandoval <osandov@...com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Dave Chinner <dchinner@...hat.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Shaohua Li <shli@...nel.org>,
        linux-raid@...r.kernel.org, David Sterba <dsterba@...e.com>,
        linux-btrfs@...r.kernel.org,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org, Gao Xiang <gaoxiang25@...wei.com>,
        linux-ext4@...r.kernel.org, Coly Li <colyli@...e.de>,
        linux-bcache@...r.kernel.org, Boaz Harrosh <ooo@...ctrozaur.com>,
        Bob Peterson <rpeterso@...hat.com>, cluster-devel@...hat.com
Subject: Re: [PATCH V11 03/19] block: introduce bio_for_each_bvec()

Btw, this patch instead of the plain rever might make it a little
more clear what is going on by skipping the confusing helper altogher
and operating on the raw bvec array:


diff --git a/include/linux/bio.h b/include/linux/bio.h
index e5b975fa0558..926550ce2d21 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -137,24 +137,18 @@ static inline bool bio_full(struct bio *bio)
 	for (i = 0, iter_all.idx = 0; iter_all.idx < (bio)->bi_vcnt; iter_all.idx++)	\
 		bvec_for_each_segment(bvl, &((bio)->bi_io_vec[iter_all.idx]), i, iter_all)
 
-static inline void __bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
-				      unsigned bytes, unsigned max_seg_len)
+static inline void bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
+				    unsigned bytes)
 {
 	iter->bi_sector += bytes >> 9;
 
 	if (bio_no_advance_iter(bio))
 		iter->bi_size -= bytes;
 	else
-		__bvec_iter_advance(bio->bi_io_vec, iter, bytes, max_seg_len);
+		bvec_iter_advance(bio->bi_io_vec, iter, bytes);
 		/* TODO: It is reasonable to complete bio with error here. */
 }
 
-static inline void bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
-				    unsigned bytes)
-{
-	__bio_advance_iter(bio, iter, bytes, PAGE_SIZE);
-}
-
 #define __bio_for_each_segment(bvl, bio, iter, start)			\
 	for (iter = (start);						\
 	     (iter).bi_size &&						\
@@ -168,7 +162,7 @@ static inline void bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
 	for (iter = (start);						\
 	     (iter).bi_size &&						\
 		((bvl = bio_iter_mp_iovec((bio), (iter))), 1);	\
-	     __bio_advance_iter((bio), &(iter), (bvl).bv_len, BVEC_MAX_LEN))
+	     bio_advance_iter((bio), &(iter), (bvl).bv_len))
 
 /* returns one real segment(multi-page bvec) each time */
 #define bio_for_each_bvec(bvl, bio, iter)			\
diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index cab36d838ed0..7d0f9bdb6f05 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -25,8 +25,6 @@
 #include <linux/errno.h>
 #include <linux/mm.h>
 
-#define BVEC_MAX_LEN  ((unsigned int)-1)
-
 /*
  * was unsigned short, but we might as well be ready for > 64kB I/O pages
  */
@@ -102,8 +100,8 @@ struct bvec_iter_all {
 	.bv_offset	= segment_iter_offset((bvec), (iter)),	\
 })
 
-static inline bool __bvec_iter_advance(const struct bio_vec *bv,
-		struct bvec_iter *iter, unsigned bytes, unsigned max_seg_len)
+static inline bool bvec_iter_advance(const struct bio_vec *bv,
+		struct bvec_iter *iter, unsigned bytes)
 {
 	if (WARN_ONCE(bytes > iter->bi_size,
 		     "Attempted to advance past end of bvec iter\n")) {
@@ -112,20 +110,15 @@ static inline bool __bvec_iter_advance(const struct bio_vec *bv,
 	}
 
 	while (bytes) {
-		unsigned segment_len = segment_iter_len(bv, *iter);
-
-		if (max_seg_len < BVEC_MAX_LEN)
-			segment_len = min_t(unsigned, segment_len,
-					    max_seg_len -
-					    bvec_iter_offset(bv, *iter));
+		const struct bio_vec *cur = bv + iter->bi_idx;
+		unsigned len = min3(bytes, iter->bi_size,
+				    cur->bv_len - iter->bi_bvec_done);
 
-		segment_len = min(bytes, segment_len);
-
-		bytes -= segment_len;
-		iter->bi_size -= segment_len;
-		iter->bi_bvec_done += segment_len;
+		bytes -= len;
+		iter->bi_size -= len;
+		iter->bi_bvec_done += len;
 
-		if (iter->bi_bvec_done == __bvec_iter_bvec(bv, *iter)->bv_len) {
+		if (iter->bi_bvec_done == cur->bv_len) {
 			iter->bi_bvec_done = 0;
 			iter->bi_idx++;
 		}
@@ -157,13 +150,6 @@ static inline bool bvec_iter_rewind(const struct bio_vec *bv,
 	return true;
 }
 
-static inline bool bvec_iter_advance(const struct bio_vec *bv,
-				     struct bvec_iter *iter,
-				     unsigned bytes)
-{
-	return __bvec_iter_advance(bv, iter, bytes, PAGE_SIZE);
-}
-
 #define for_each_bvec(bvl, bio_vec, iter, start)			\
 	for (iter = (start);						\
 	     (iter).bi_size &&						\

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