Re: [PATCH V11 03/19] block: introduce bio_for_each_bvec()

Christoph Hellwig
2018-11-21T17:12:21Z

On Wed, Nov 21, 2018 at 05:10:25PM +0100, Christoph Hellwig wrote:
> No - I think we can always use the code without any segment in
> bvec_iter_advance. Because bvec_iter_advance only operates on the
> iteractor, the generation of an actual single-page or multi-page
> bvec is left to the caller using the bvec_iter_bvec or segment_iter_bvec
> helpers. The only difference is how many bytes you can move the
> iterator forward in a single loop iteration - so if you pass in
> PAGE_SIZE as the max_seg_len you just will have to loop more often
> for a large enough bytes, but not actually do anything different.

FYI, this patch reverts the max_seg_len related changes back to where
we are in mainline, and as expected everything works fine for me:

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..138b4007b8f2 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")) {
@@ -112,18 +110,12 @@ 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));
+ unsigned iter_len = bvec_iter_len(bv, *iter);
+ unsigned len = min(bytes, iter_len);

- 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) {
iter->bi_bvec_done = 0;
@@ -157,13 +149,6 @@ 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 && \