diff options
| -rw-r--r-- | Documentation/block/biovecs.txt | 111 | ||||
| -rw-r--r-- | drivers/block/drbd/drbd_main.c | 4 | ||||
| -rw-r--r-- | drivers/block/nbd.c | 2 | ||||
| -rw-r--r-- | fs/bio.c | 27 | ||||
| -rw-r--r-- | include/linux/bio.h | 81 | ||||
| -rw-r--r-- | include/linux/blk_types.h | 3 | ||||
| -rw-r--r-- | include/linux/blkdev.h | 4 |
7 files changed, 194 insertions, 38 deletions
diff --git a/Documentation/block/biovecs.txt b/Documentation/block/biovecs.txt new file mode 100644 index 0000000..74a32ad --- /dev/null +++ b/Documentation/block/biovecs.txt @@ -0,0 +1,111 @@ + +Immutable biovecs and biovec iterators: +======================================= + +Kent Overstreet <kmo@daterainc.com> + +As of 3.13, biovecs should never be modified after a bio has been submitted. +Instead, we have a new struct bvec_iter which represents a range of a biovec - +the iterator will be modified as the bio is completed, not the biovec. + +More specifically, old code that needed to partially complete a bio would +update bi_sector and bi_size, and advance bi_idx to the next biovec. If it +ended up partway through a biovec, it would increment bv_offset and decrement +bv_len by the number of bytes completed in that biovec. + +In the new scheme of things, everything that must be mutated in order to +partially complete a bio is segregated into struct bvec_iter: bi_sector, +bi_size and bi_idx have been moved there; and instead of modifying bv_offset +and bv_len, struct bvec_iter has bi_bvec_done, which represents the number of +bytes completed in the current bvec. + +There are a bunch of new helper macros for hiding the gory details - in +particular, presenting the illusion of partially completed biovecs so that +normal code doesn't have to deal with bi_bvec_done. + + * Driver code should no longer refer to biovecs directly; we now have + bio_iovec() and bio_iovec_iter() macros that return literal struct biovecs, + constructed from the raw biovecs but taking into account bi_bvec_done and + bi_size. + + bio_for_each_segment() has been updated to take a bvec_iter argument + instead of an integer (that corresponded to bi_idx); for a lot of code the + conversion just required changing the types of the arguments to + bio_for_each_segment(). + + * Advancing a bvec_iter is done with bio_advance_iter(); bio_advance() is a + wrapper around bio_advance_iter() that operates on bio->bi_iter, and also + advances the bio integrity's iter if present. + + There is a lower level advance function - bvec_iter_advance() - which takes + a pointer to a biovec, not a bio; this is used by the bio integrity code. + +What's all this get us? +======================= + +Having a real iterator, and making biovecs immutable, has a number of +advantages: + + * Before, iterating over bios was very awkward when you weren't processing + exactly one bvec at a time - for example, bio_copy_data() in fs/bio.c, + which copies the contents of one bio into another. Because the biovecs + wouldn't necessarily be the same size, the old code was tricky convoluted - + it had to walk two different bios at the same time, keeping both bi_idx and + and offset into the current biovec for each. + + The new code is much more straightforward - have a look. This sort of + pattern comes up in a lot of places; a lot of drivers were essentially open + coding bvec iterators before, and having common implementation considerably + simplifies a lot of code. + + * Before, any code that might need to use the biovec after the bio had been + completed (perhaps to copy the data somewhere else, or perhaps to resubmit + it somewhere else if there was an error) had to save the entire bvec array + - again, this was being done in a fair number of places. + + * Biovecs can be shared between multiple bios - a bvec iter can represent an + arbitrary range of an existing biovec, both starting and ending midway + through biovecs. This is what enables efficient splitting of arbitrary + bios. Note that this means we _only_ use bi_size to determine when we've + reached the end of a bio, not bi_vcnt - and the bio_iovec() macro takes + bi_size into account when constructing biovecs. + + * Splitting bios is now much simpler. The old bio_split() didn't even work on + bios with more than a single bvec! Now, we can efficiently split arbitrary + size bios - because the new bio can share the old bio's biovec. + + Care must be taken to ensure the biovec isn't freed while the split bio is + still using it, in case the original bio completes first, though. Using + bio_chain() when splitting bios helps with this. + + * Submitting partially completed bios is now perfectly fine - this comes up + occasionally in stacking block drivers and various code (e.g. md and + bcache) had some ugly workarounds for this. + + It used to be the case that submitting a partially completed bio would work + fine to _most_ devices, but since accessing the raw bvec array was the + norm, not all drivers would respect bi_idx and those would break. Now, + since all drivers _must_ go through the bvec iterator - and have been + audited to make sure they are - submitting partially completed bios is + perfectly fine. + +Other implications: +=================== + + * Almost all usage of bi_idx is now incorrect and has been removed; instead, + where previously you would have used bi_idx you'd now use a bvec_iter, + probably passing it to one of the helper macros. + + I.e. instead of using bio_iovec_idx() (or bio->bi_iovec[bio->bi_idx]), you + now use bio_iter_iovec(), which takes a bvec_iter and returns a + literal struct bio_vec - constructed on the fly from the raw biovec but + taking into account bi_bvec_done (and bi_size). + + * bi_vcnt can't be trusted or relied upon by driver code - i.e. anything that + doesn't actually own the bio. The reason is twofold: firstly, it's not + actually needed for iterating over the bio anymore - we only use bi_size. + Secondly, when cloning a bio and reusing (a portion of) the original bio's + biovec, in order to calculate bi_vcnt for the new bio we'd have to iterate + over all the biovecs in the new bio - which is silly as it's not needed. + + So, don't use bi_vcnt anymore. diff --git a/drivers/block/drbd/drbd_main.c b/drivers/block/drbd/drbd_main.c index f4e5440..929468e 100644 --- a/drivers/block/drbd/drbd_main.c +++ b/drivers/block/drbd/drbd_main.c @@ -1546,7 +1546,7 @@ static int _drbd_send_bio(struct drbd_conf *mdev, struct bio *bio) err = _drbd_no_send_page(mdev, bvec.bv_page, bvec.bv_offset, bvec.bv_len, - bio_iter_last(bio, iter) + bio_iter_last(bvec, iter) ? 0 : MSG_MORE); if (err) return err; @@ -1565,7 +1565,7 @@ static int _drbd_send_zc_bio(struct drbd_conf *mdev, struct bio *bio) err = _drbd_send_page(mdev, bvec.bv_page, bvec.bv_offset, bvec.bv_len, - bio_iter_last(bio, iter) ? 0 : MSG_MORE); + bio_iter_last(bvec, iter) ? 0 : MSG_MORE); if (err) return err; } diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c index aa362f4..55298db 100644 --- a/drivers/block/nbd.c +++ b/drivers/block/nbd.c @@ -278,7 +278,7 @@ static int nbd_send_req(struct nbd_device *nbd, struct request *req) */ rq_for_each_segment(bvec, req, iter) { flags = 0; - if (!rq_iter_last(req, iter)) + if (!rq_iter_last(bvec, iter)) flags = MSG_MORE; dprintk(DBG_TX, "%s: request %p: sending %d bytes data\n", nbd->disk->disk_name, req, bvec.bv_len); @@ -532,13 +532,11 @@ void __bio_clone(struct bio *bio, struct bio *bio_src) * most users will be overriding ->bi_bdev with a new target, * so we don't set nor calculate new physical/hw segment counts here */ - bio->bi_iter.bi_sector = bio_src->bi_iter.bi_sector; bio->bi_bdev = bio_src->bi_bdev; bio->bi_flags |= 1 << BIO_CLONED; bio->bi_rw = bio_src->bi_rw; bio->bi_vcnt = bio_src->bi_vcnt; - bio->bi_iter.bi_size = bio_src->bi_iter.bi_size; - bio->bi_iter.bi_idx = bio_src->bi_iter.bi_idx; + bio->bi_iter = bio_src->bi_iter; } EXPORT_SYMBOL(__bio_clone); @@ -808,28 +806,7 @@ void bio_advance(struct bio *bio, unsigned bytes) if (bio_integrity(bio)) bio_integrity_advance(bio, bytes); - bio->bi_iter.bi_sector += bytes >> 9; - bio->bi_iter.bi_size -= bytes; - - if (bio->bi_rw & BIO_NO_ADVANCE_ITER_MASK) - return; - - while (bytes) { - if (unlikely(bio->bi_iter.bi_idx >= bio->bi_vcnt)) { - WARN_ONCE(1, "bio idx %d >= vcnt %d\n", - bio->bi_iter.bi_idx, bio->bi_vcnt); - break; - } - - if (bytes >= bio_iovec(bio).bv_len) { - bytes -= bio_iovec(bio).bv_len; - bio->bi_iter.bi_idx++; - } else { - bio_iovec(bio).bv_len -= bytes; - bio_iovec(bio).bv_offset += bytes; - bytes = 0; - } - } + bio_advance_iter(bio, &bio->bi_iter, bytes); } EXPORT_SYMBOL(bio_advance); diff --git a/include/linux/bio.h b/include/linux/bio.h index c16adb5..04e592e 100644 --- a/include/linux/bio.h +++ b/include/linux/bio.h @@ -64,11 +64,38 @@ #define bio_iovec_idx(bio, idx) (&((bio)->bi_io_vec[(idx)])) #define __bio_iovec(bio) bio_iovec_idx((bio), (bio)->bi_iter.bi_idx) -#define bio_iter_iovec(bio, iter) ((bio)->bi_io_vec[(iter).bi_idx]) +#define __bvec_iter_bvec(bvec, iter) (&(bvec)[(iter).bi_idx]) -#define bio_page(bio) (bio_iovec((bio)).bv_page) -#define bio_offset(bio) (bio_iovec((bio)).bv_offset) -#define bio_iovec(bio) (*__bio_iovec(bio)) +#define bvec_iter_page(bvec, iter) \ + (__bvec_iter_bvec((bvec), (iter))->bv_page) + +#define bvec_iter_len(bvec, iter) \ + min((iter).bi_size, \ + __bvec_iter_bvec((bvec), (iter))->bv_len - (iter).bi_bvec_done) + +#define bvec_iter_offset(bvec, iter) \ + (__bvec_iter_bvec((bvec), (iter))->bv_offset + (iter).bi_bvec_done) + +#define bvec_iter_bvec(bvec, iter) \ +((struct bio_vec) { \ + .bv_page = bvec_iter_page((bvec), (iter)), \ + .bv_len = bvec_iter_len((bvec), (iter)), \ + .bv_offset = bvec_iter_offset((bvec), (iter)), \ +}) + +#define bio_iter_iovec(bio, iter) \ + bvec_iter_bvec((bio)->bi_io_vec, (iter)) + +#define bio_iter_page(bio, iter) \ + bvec_iter_page((bio)->bi_io_vec, (iter)) +#define bio_iter_len(bio, iter) \ + bvec_iter_len((bio)->bi_io_vec, (iter)) +#define bio_iter_offset(bio, iter) \ + bvec_iter_offset((bio)->bi_io_vec, (iter)) + +#define bio_page(bio) bio_iter_page((bio), (bio)->bi_iter) +#define bio_offset(bio) bio_iter_offset((bio), (bio)->bi_iter) +#define bio_iovec(bio) bio_iter_iovec((bio), (bio)->bi_iter) #define bio_segments(bio) ((bio)->bi_vcnt - (bio)->bi_iter.bi_idx) #define bio_sectors(bio) ((bio)->bi_iter.bi_size >> 9) @@ -145,16 +172,54 @@ static inline void *bio_data(struct bio *bio) bvl = bio_iovec_idx((bio), (i)), i < (bio)->bi_vcnt; \ i++) +static inline void bvec_iter_advance(struct bio_vec *bv, struct bvec_iter *iter, + unsigned bytes) +{ + WARN_ONCE(bytes > iter->bi_size, + "Attempted to advance past end of bvec iter\n"); + + while (bytes) { + unsigned len = min(bytes, bvec_iter_len(bv, *iter)); + + 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; + iter->bi_idx++; + } + } +} + +#define for_each_bvec(bvl, bio_vec, iter, start) \ + for ((iter) = start; \ + (bvl) = bvec_iter_bvec((bio_vec), (iter)), \ + (iter).bi_size; \ + bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len)) + + +static inline void bio_advance_iter(struct bio *bio, struct bvec_iter *iter, + unsigned bytes) +{ + iter->bi_sector += bytes >> 9; + + if (bio->bi_rw & BIO_NO_ADVANCE_ITER_MASK) + iter->bi_size -= bytes; + else + bvec_iter_advance(bio->bi_io_vec, iter, bytes); +} + #define __bio_for_each_segment(bvl, bio, iter, start) \ for (iter = (start); \ - bvl = bio_iter_iovec((bio), (iter)), \ - (iter).bi_idx < (bio)->bi_vcnt; \ - (iter).bi_idx++) + (iter).bi_size && \ + ((bvl = bio_iter_iovec((bio), (iter))), 1); \ + bio_advance_iter((bio), &(iter), (bvl).bv_len)) #define bio_for_each_segment(bvl, bio, iter) \ __bio_for_each_segment(bvl, bio, iter, (bio)->bi_iter) -#define bio_iter_last(bio, iter) ((iter).bi_idx == (bio)->bi_vcnt - 1) +#define bio_iter_last(bvec, iter) ((iter).bi_size == (bvec).bv_len) /* * get a reference to a bio, so it won't disappear. the intended use is diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h index 29b5b84..d369f8f 100644 --- a/include/linux/blk_types.h +++ b/include/linux/blk_types.h @@ -34,6 +34,9 @@ struct bvec_iter { unsigned int bi_size; /* residual I/O count */ unsigned int bi_idx; /* current index into bvl_vec */ + + unsigned int bi_bvec_done; /* number of bytes completed in + current bvec */ }; /* diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index 337b92a..02cb6f0 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -750,9 +750,9 @@ struct req_iterator { __rq_for_each_bio(_iter.bio, _rq) \ bio_for_each_segment(bvl, _iter.bio, _iter.iter) -#define rq_iter_last(rq, _iter) \ +#define rq_iter_last(bvec, _iter) \ (_iter.bio->bi_next == NULL && \ - bio_iter_last(_iter.bio, _iter.iter)) + bio_iter_last(bvec, _iter.iter)) #ifndef ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE # error "You should define ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE for your platform" |
