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Date:   Tue, 15 Dec 2020 00:20:25 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     linux-block@...r.kernel.org
Cc:     Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Ming Lei <ming.lei@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Jonathan Corbet <corbet@....net>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org, target-devel@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-doc@...r.kernel.org
Subject: [PATCH v1 6/6] block/iomap: 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@...radead.org>
Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 Documentation/filesystems/porting.rst |  9 ++++
 block/bio.c                           | 64 +++++++++++----------------
 include/linux/bio.h                   |  3 ++
 3 files changed, 38 insertions(+), 38 deletions(-)

diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
index 867036aa90b8..47a622879952 100644
--- a/Documentation/filesystems/porting.rst
+++ b/Documentation/filesystems/porting.rst
@@ -865,3 +865,12 @@ no matter what.  Everything is handled by the caller.
 
 clone_private_mount() returns a longterm mount now, so the proper destructor of
 its result is kern_unmount() or kern_unmount_array().
+
+---
+
+**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 3192358c411f..f8229be24562 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -960,25 +960,16 @@ 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;
-	struct page *page = bv->bv_page;
-	bool same_page = false;
-	unsigned int off, len;
-
-	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);
-	off = bv->bv_offset + iter->iov_offset;
-
-	if (!__bio_try_merge_page(bio, page, len, off, &same_page)) {
-		if (bio_full(bio, len))
-			return -EINVAL;
-		bio_add_page_noaccount(bio, page, len, off);
-	}
-	iov_iter_advance(iter, len);
+	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;
 }
 
@@ -1092,12 +1083,13 @@ 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. If we're adding kernel pages, it doesn't take extra page references
+ * and reuses the provided bvec, so the caller must ensure that the bvec isn't
+ * freed and page references remain to be taken until I/O has completed. If
+ * the I/O is completed asynchronously, the bvec must not be freed before
+ * ->ki_complete() has been called. 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
@@ -1109,27 +1101,23 @@ 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;
+	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;
+	}
 
 	do {
-		if (bio_op(bio) == REQ_OP_ZONE_APPEND) {
-			if (WARN_ON_ONCE(is_bvec))
-				return -EINVAL;
+		if (bio_op(bio) == REQ_OP_ZONE_APPEND)
 			ret = __bio_iov_append_get_pages(bio, iter);
-		} else {
-			if (is_bvec)
-				ret = __bio_iov_bvec_add_pages(bio, iter);
-			else
-				ret = __bio_iov_iter_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);
 	return bio->bi_vcnt ? 0 : ret;
 }
 EXPORT_SYMBOL_GPL(bio_iov_iter_get_pages);
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 2a9f3f0bbe0a..337f4280b639 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -444,6 +444,9 @@ static inline void bio_wouldblock_error(struct bio *bio)
 
 static inline int bio_iov_vecs_to_alloc(struct iov_iter *iter, int max_segs)
 {
+	/* reuse iter->bvec */
+	if (iov_iter_is_bvec(iter))
+		return 0;
 	return iov_iter_npages(iter, max_segs);
 }
 
-- 
2.24.0

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