[PATCH V2 1/2] block: add bio_iov_iter_nvecs for figuring out nr_vecs

[Ming Lei <ming.lei@redhat.com>]

Pavel reported that iov_iter_npages is a bit heavy in case of bvec
iter.

Turns out it isn't necessary to iterate every page in the bvec iter,
and we call iov_iter_npages() just for figuring out how many bio
vecs need to be allocated. And we can simply map each vector in bvec iter
to bio's vec, so just return iter->nr_segs from bio_iov_iter_nvecs() for
bvec iter.

This patch is based on Mathew's post:

https://lore.kernel.org/linux-block/20201120123931.GN29991@casper.infradead.org/

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 fs/block_dev.c       |  4 ++--
 fs/iomap/direct-io.c |  4 ++--
 include/linux/bio.h  | 10 ++++++++++
 3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 9e56ee1f2652..fbcc900229af 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -417,7 +417,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
 		dio->size += bio->bi_iter.bi_size;
 		pos += bio->bi_iter.bi_size;
 
-		nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES);
+		nr_pages = bio_iov_iter_nvecs(iter, BIO_MAX_PAGES);
 		if (!nr_pages) {
 			bool polled = false;
 
@@ -482,7 +482,7 @@ blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 {
 	int nr_pages;
 
-	nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES + 1);
+	nr_pages = bio_iov_iter_nvecs(iter, BIO_MAX_PAGES + 1);
 	if (!nr_pages)
 		return 0;
 	if (is_sync_kiocb(iocb) && nr_pages <= BIO_MAX_PAGES)
diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index 933f234d5bec..0635469e811a 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -250,7 +250,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
 	orig_count = iov_iter_count(dio->submit.iter);
 	iov_iter_truncate(dio->submit.iter, length);
 
-	nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
+	nr_pages = bio_iov_iter_nvecs(dio->submit.iter, BIO_MAX_PAGES);
 	if (nr_pages <= 0) {
 		ret = nr_pages;
 		goto out;
@@ -308,7 +308,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
 		dio->size += n;
 		copied += n;
 
-		nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
+		nr_pages = bio_iov_iter_nvecs(dio->submit.iter, BIO_MAX_PAGES);
 		iomap_dio_submit_bio(dio, iomap, bio, pos);
 		pos += n;
 	} while (nr_pages);
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 1edda614f7ce..8a7f44006f65 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -10,6 +10,7 @@
 #include <linux/ioprio.h>
 /* struct bio, bio_vec and BIO_* flags are defined in blk_types.h */
 #include <linux/blk_types.h>
+#include <linux/uio.h>
 
 #define BIO_DEBUG
 
@@ -820,4 +821,13 @@ static inline void bio_set_polled(struct bio *bio, struct kiocb *kiocb)
 		bio->bi_opf |= REQ_NOWAIT;
 }
 
+static inline int bio_iov_iter_nvecs(const struct iov_iter *i, int maxvecs)
+{
+	if (!iov_iter_count(i))
+		return 0;
+	if (iov_iter_is_bvec(i))
+		return min_t(int, maxvecs, i->nr_segs);
+	return iov_iter_npages(i, maxvecs);
+}
+
 #endif /* __LINUX_BIO_H */
-- 
2.28.0