[PATCH v3 1/2] ublk: use copy_{to,from}_iter() for user copy

[Caleb Sander Mateos <csander@purestorage.com>]

ublk_copy_user_pages()/ublk_copy_io_pages() currently uses
iov_iter_get_pages2() to extract the pages from the iov_iter and
memcpy()s between the bvec_iter and the iov_iter's pages one at a time.
Switch to using copy_to_iter()/copy_from_iter() instead. This avoids the
user page reference count increments and decrements and needing to split
the memcpy() at user page boundaries. It also simplifies the code
considerably.
Ming reports a 40% throughput improvement when issuing I/O to the
selftests null ublk server with zero-copy disabled.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/ublk_drv.c | 62 +++++++++-------------------------------
 1 file changed, 14 insertions(+), 48 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 97cc4bc0a6ce..40eee3e15a4c 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -911,58 +911,47 @@ static const struct block_device_operations ub_fops = {
 	.open =		ublk_open,
 	.free_disk =	ublk_free_disk,
 	.report_zones =	ublk_report_zones,
 };
 
-#define UBLK_MAX_PIN_PAGES	32
-
 struct ublk_io_iter {
-	struct page *pages[UBLK_MAX_PIN_PAGES];
 	struct bio *bio;
 	struct bvec_iter iter;
 };
 
-/* return how many pages are copied */
-static void ublk_copy_io_pages(struct ublk_io_iter *data,
-		size_t total, size_t pg_off, int dir)
+/* return how many bytes are copied */
+static size_t ublk_copy_io_pages(struct ublk_io_iter *data,
+		struct iov_iter *uiter, int dir)
 {
-	unsigned done = 0;
-	unsigned pg_idx = 0;
+	size_t done = 0;
 
-	while (done < total) {
+	for (;;) {
 		struct bio_vec bv = bio_iter_iovec(data->bio, data->iter);
-		unsigned int bytes = min3(bv.bv_len, (unsigned)total - done,
-				(unsigned)(PAGE_SIZE - pg_off));
 		void *bv_buf = bvec_kmap_local(&bv);
-		void *pg_buf = kmap_local_page(data->pages[pg_idx]);
+		size_t copied;
 
 		if (dir == ITER_DEST)
-			memcpy(pg_buf + pg_off, bv_buf, bytes);
+			copied = copy_to_iter(bv_buf, bv.bv_len, uiter);
 		else
-			memcpy(bv_buf, pg_buf + pg_off, bytes);
+			copied = copy_from_iter(bv_buf, bv.bv_len, uiter);
 
-		kunmap_local(pg_buf);
 		kunmap_local(bv_buf);
 
-		/* advance page array */
-		pg_off += bytes;
-		if (pg_off == PAGE_SIZE) {
-			pg_idx += 1;
-			pg_off = 0;
-		}
-
-		done += bytes;
+		done += copied;
+		if (copied < bv.bv_len)
+			break;
 
 		/* advance bio */
-		bio_advance_iter_single(data->bio, &data->iter, bytes);
+		bio_advance_iter_single(data->bio, &data->iter, copied);
 		if (!data->iter.bi_size) {
 			data->bio = data->bio->bi_next;
 			if (data->bio == NULL)
 				break;
 			data->iter = data->bio->bi_iter;
 		}
 	}
+	return done;
 }
 
 static bool ublk_advance_io_iter(const struct request *req,
 		struct ublk_io_iter *iter, unsigned int offset)
 {
@@ -986,38 +975,15 @@ static bool ublk_advance_io_iter(const struct request *req,
  */
 static size_t ublk_copy_user_pages(const struct request *req,
 		unsigned offset, struct iov_iter *uiter, int dir)
 {
 	struct ublk_io_iter iter;
-	size_t done = 0;
 
 	if (!ublk_advance_io_iter(req, &iter, offset))
 		return 0;
 
-	while (iov_iter_count(uiter) && iter.bio) {
-		unsigned nr_pages;
-		ssize_t len;
-		size_t off;
-		int i;
-
-		len = iov_iter_get_pages2(uiter, iter.pages,
-				iov_iter_count(uiter),
-				UBLK_MAX_PIN_PAGES, &off);
-		if (len <= 0)
-			return done;
-
-		ublk_copy_io_pages(&iter, len, off, dir);
-		nr_pages = DIV_ROUND_UP(len + off, PAGE_SIZE);
-		for (i = 0; i < nr_pages; i++) {
-			if (dir == ITER_DEST)
-				set_page_dirty(iter.pages[i]);
-			put_page(iter.pages[i]);
-		}
-		done += len;
-	}
-
-	return done;
+	return ublk_copy_io_pages(&iter, uiter, dir);
 }
 
 static inline bool ublk_need_map_req(const struct request *req)
 {
 	return ublk_rq_has_data(req) && req_op(req) == REQ_OP_WRITE;
-- 
2.45.2