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bio: don't copy bvec for direct IO 

The block layer spends quite a while in blkdev_direct_IO() to copy and
initialise bio's bvec. However, if we've already got a bvec in the input
iterator it might be reused in some cases, i.e. when new
ITER_BVEC_FLAG_FIXED flag is set. Simple tests show considerable
performance boost, and it also reduces memory footprint.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Pavel Begunkov 2021-01-09 16:03:03 +00:00 committed by Jens Axboe
parent 3e1a88ec96
commit c42bca92be
3 changed files with 40 additions and 37 deletions
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9
Documentation/filesystems/porting.rst
View File

@ -872,3 +872,12 @@ its result is kern_unmount() or kern_unmount_array().
zero-length bvec segments are disallowed, they must be filtered out before
passed on to an iterator.
---
**mandatory**
For bvec based itererators bio_iov_iter_get_pages() now doesn't copy bvecs but
uses the one provided. Anyone issuing kiocb-I/O should ensure that the bvec and
page references stay until I/O has completed, i.e. until ->ki_complete() has
been called or returned with non -EIOCBQUEUED code.

63
block/bio.c
View File

@ -942,21 +942,17 @@ void bio_release_pages(struct bio *bio, bool mark_dirty)
}
EXPORT_SYMBOL_GPL(bio_release_pages);
static int __bio_iov_bvec_add_pages(struct bio *bio, struct iov_iter *iter)
static int bio_iov_bvec_set(struct bio *bio, struct iov_iter *iter)
{
const struct bio_vec *bv = iter->bvec;
unsigned int len;
size_t size;
WARN_ON_ONCE(BVEC_POOL_IDX(bio) != 0);
if (WARN_ON_ONCE(iter->iov_offset > bv->bv_len))
return -EINVAL;
bio->bi_vcnt = iter->nr_segs;
bio->bi_max_vecs = iter->nr_segs;
bio->bi_io_vec = (struct bio_vec *)iter->bvec;
bio->bi_iter.bi_bvec_done = iter->iov_offset;
bio->bi_iter.bi_size = iter->count;
len = min_t(size_t, bv->bv_len - iter->iov_offset, iter->count);
size = bio_add_page(bio, bv->bv_page, len,
bv->bv_offset + iter->iov_offset);
if (unlikely(size != len))
return -EINVAL;
iov_iter_advance(iter, size);
iov_iter_advance(iter, iter->count);
return 0;
}
@ -1070,12 +1066,12 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
* This takes either an iterator pointing to user memory, or one pointing to
* kernel pages (BVEC iterator). If we're adding user pages, we pin them and
* map them into the kernel. On IO completion, the caller should put those
* pages. If we're adding kernel pages, and the caller told us it's safe to
* do so, we just have to add the pages to the bio directly. We don't grab an
* extra reference to those pages (the user should already have that), and we
* don't put the page on IO completion. The caller needs to check if the bio is
* flagged BIO_NO_PAGE_REF on IO completion. If it isn't, then pages should be
* released.
* pages. For bvec based iterators bio_iov_iter_get_pages() uses the provided
* bvecs rather than copying them. Hence anyone issuing kiocb based IO needs
* to ensure the bvecs and pages stay referenced until the submitted I/O is
* completed by a call to ->ki_complete() or returns with an error other than
* -EIOCBQUEUED. The caller needs to check if the bio is flagged BIO_NO_PAGE_REF
* on IO completion. If it isn't, then pages should be released.
*
* The function tries, but does not guarantee, to pin as many pages as
* fit into the bio, or are requested in @iter, whatever is smaller. If
@ -1087,27 +1083,22 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
*/
int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
{
const bool is_bvec = iov_iter_is_bvec(iter);
int ret;
int ret = 0;
if (WARN_ON_ONCE(bio->bi_vcnt))
return -EINVAL;
do {
if (bio_op(bio) == REQ_OP_ZONE_APPEND) {
if (WARN_ON_ONCE(is_bvec))
return -EINVAL;
ret = __bio_iov_append_get_pages(bio, iter);
} else {
if (is_bvec)
ret = __bio_iov_bvec_add_pages(bio, iter);
if (iov_iter_is_bvec(iter)) {
if (WARN_ON_ONCE(bio_op(bio) == REQ_OP_ZONE_APPEND))
return -EINVAL;
bio_iov_bvec_set(bio, iter);
bio_set_flag(bio, BIO_NO_PAGE_REF);
return 0;
} else {
do {
if (bio_op(bio) == REQ_OP_ZONE_APPEND)
ret = __bio_iov_append_get_pages(bio, iter);
else
ret = __bio_iov_iter_get_pages(bio, iter);
}
} while (!ret && iov_iter_count(iter) && !bio_full(bio, 0));
if (is_bvec)
bio_set_flag(bio, BIO_NO_PAGE_REF);
} while (!ret && iov_iter_count(iter) && !bio_full(bio, 0));
}
/* don't account direct I/O as memory stall */
bio_clear_flag(bio, BIO_WORKINGSET);

5
include/linux/bio.h
View File

@ -444,10 +444,13 @@ static inline void bio_wouldblock_error(struct bio *bio)
/*
* Calculate number of bvec segments that should be allocated to fit data
* pointed by @iter.
* pointed by @iter. If @iter is backed by bvec it's going to be reused
* instead of allocating a new one.
*/
static inline int bio_iov_vecs_to_alloc(struct iov_iter *iter, int max_segs)
{
if (iov_iter_is_bvec(iter))
return 0;
return iov_iter_npages(iter, max_segs);
}