[PATCH v2 6/7] bio: add a helper calculating nr segments to alloc

[Pavel Begunkov]

Add a helper function calculating the number of bvec segments we need to
allocate to construct a bio. It doesn't change anything functionally,
but will be used to not duplicate special cases in the future.

Reviewed-by: Christoph Hellwig <h...@lst.de>
Signed-off-by: Pavel Begunkov <asml.sile...@gmail.com>
---
 fs/block_dev.c       |  7 ++++---
 fs/iomap/direct-io.c |  9 ++++-----
 include/linux/bio.h  | 10 ++++++++++
 3 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 3e5b02f6606c..034e6c8cfcab 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -416,7 +416,7 @@ __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_vecs_to_alloc(iter, BIO_MAX_PAGES);
                if (!nr_pages) {
                        bool polled = false;
 
@@ -481,9 +481,10 @@ blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 {
        int nr_pages;
 
-       nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES + 1);
-       if (!nr_pages)
+       if (!iov_iter_count(iter))
                return 0;
+
+       nr_pages = bio_iov_vecs_to_alloc(iter, BIO_MAX_PAGES + 1);
        if (is_sync_kiocb(iocb) && nr_pages <= BIO_MAX_PAGES)
                return __blkdev_direct_IO_simple(iocb, iter, nr_pages);
 
diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index 933f234d5bec..ea1e8f696076 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -250,11 +250,8 @@ 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);
-       if (nr_pages <= 0) {
-               ret = nr_pages;
+       if (!iov_iter_count(dio->submit.iter))
                goto out;
-       }
 
        if (need_zeroout) {
                /* zero out from the start of the block to the write offset */
@@ -263,6 +260,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t 
length,
                        iomap_dio_zero(dio, iomap, pos - pad, pad);
        }
 
+       nr_pages = bio_iov_vecs_to_alloc(dio->submit.iter, BIO_MAX_PAGES);
        do {
                size_t n;
                if (dio->error) {
@@ -308,7 +306,8 @@ 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_vecs_to_alloc(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..d8f9077c43ef 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
 
@@ -441,6 +442,15 @@ static inline void bio_wouldblock_error(struct bio *bio)
        bio_endio(bio);
 }
 
+/*
+ * Calculate number of bvec segments that should be allocated to fit data
+ * pointed by @iter.
+ */
+static inline int bio_iov_vecs_to_alloc(struct iov_iter *iter, int max_segs)
+{
+       return iov_iter_npages(iter, max_segs);
+}
+
 struct request_queue;
 
 extern int submit_bio_wait(struct bio *bio);
-- 
2.24.0