Re: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec

[Matthew Wilcox]

On Fri, Nov 20, 2020 at 08:14:29AM +0000, Christoph Hellwig wrote:
> On Fri, Nov 20, 2020 at 02:54:57AM +0000, Matthew Wilcox wrote:
> > On Fri, Nov 20, 2020 at 02:25:08AM +0000, Pavel Begunkov wrote:
> > > On 20/11/2020 02:22, Ming Lei wrote:
> > > > iov_iter_npages(bvec) still can be improved a bit by the following way:
> > > 
> > > Yep, was doing exactly that, +a couple of other places that are in my way.
> > 
> > Are you optimising the right thing here?  Assuming you're looking at
> > the one in do_blockdev_direct_IO(), wouldn't we be better off figuring
> > out how to copy the bvecs directly from the iov_iter into the bio
> > rather than calling dio_bio_add_page() for each page?
> 
> Which is most effectively done by stopping to to use *blockdev_direct_IO
> and switching to iomap instead :)

But iomap still calls iov_iter_npages().  So maybe we need something like
this ...

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index 933f234d5bec..1c5a802a45d9 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_npages(dio->submit.iter);
 	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_npages(dio->submit.iter);
 		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 c6d765382926..86cc74f84b30 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
 
@@ -447,6 +448,16 @@ bool __bio_try_merge_page(struct bio *bio, struct page *page,
 void __bio_add_page(struct bio *bio, struct page *page,
 		unsigned int len, unsigned int off);
 int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter);
+
+static inline int bio_iov_iter_npages(const struct iov_iter *i)
+{
+	if (!iov_iter_count(i))
+		return 0;
+	if (iov_iter_is_bvec(i))
+		return 1;
+	return iov_iter_npages(i, BIO_MAX_PAGES);
+}
+
 void bio_release_pages(struct bio *bio, bool mark_dirty);
 extern void bio_set_pages_dirty(struct bio *bio);
 extern void bio_check_pages_dirty(struct bio *bio);