[PATCH v3 7/7] bio: don't copy bvec for direct IO

Pavel Begunkov
2021-01-09T16:08:23Z

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>
---
Documentation/filesystems/porting.rst | 9 ++++
block/bio.c | 67 ++++++++++++---------------
include/linux/bio.h | 5 +-
3 files changed, 42 insertions(+), 39 deletions(-)

diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
index c722d94f29ea..1f8cf8e10b34 100644
--- a/Documentation/filesystems/porting.rst
+++ b/Documentation/filesystems/porting.rst
@@ -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.
diff --git a/block/bio.c b/block/bio.c
index 9f26984af643..6f031a04b59a 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -960,21 +960,17 @@ 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;
-
- if (WARN_ON_ONCE(iter->iov_offset > bv->bv_len))
- return -EINVAL;
-
- 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);
+ WARN_ON_ONCE(BVEC_POOL_IDX(bio) != 0);
+
+ 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;
+
+ iov_iter_advance(iter, iter->count);
return 0;
}

@@ -1088,12 +1084,12 @@ 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
@@ -1105,27 +1101,22 @@ 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;
-
- if (WARN_ON_ONCE(bio->bi_vcnt))
- return -EINVAL;
+ int ret = 0;

- 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);
diff --git a/include/linux/bio.h b/include/linux/bio.h
index d8f9077c43ef..1d30572a8c53 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -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);
}

--
2.24.0