[PATCH v2 5/8] block: Add support for small segments in blk_rq_map_user_iov()

[Bart Van Assche <bvanassche@acm.org>]

Before changing the return value of bio_add_hw_page() into a value in
the range [0, len], make blk_rq_map_user_iov() fall back to copying data
if mapping the data is not possible due to the segment limit.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Keith Busch <kbusch@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/blk-map.c | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/block/blk-map.c b/block/blk-map.c
index d2d6ee098514..f3f2ed9c6183 100644
--- a/block/blk-map.c
+++ b/block/blk-map.c
@@ -308,17 +308,26 @@ static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
 		else {
 			for (j = 0; j < npages; j++) {
 				struct page *page = pages[j];
-				unsigned int n = PAGE_SIZE - offs;
+				unsigned int n = PAGE_SIZE - offs, added;
 				bool same_page = false;
 
 				if (n > bytes)
 					n = bytes;
 
-				if (!bio_add_hw_page(rq->q, bio, page, n, offs,
-						     max_sectors, &same_page)) {
+				added = bio_add_hw_page(rq->q, bio, page, n,
+						offs, max_sectors, &same_page);
+				if (added == 0) {
 					if (same_page)
 						put_page(page);
 					break;
+				} else if (added != n) {
+					/*
+					 * The segment size is smaller than the
+					 * page size and an iov exceeds the
+					 * segment size. Give up.
+					 */
+					ret = -EREMOTEIO;
+					goto out_unmap;
 				}
 
 				bytes -= n;
@@ -672,10 +681,18 @@ int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
 
 	i = *iter;
 	do {
-		if (copy)
+		if (copy) {
 			ret = bio_copy_user_iov(rq, map_data, &i, gfp_mask);
-		else
+		} else {
 			ret = bio_map_user_iov(rq, &i, gfp_mask);
+			/*
+			 * Fall back to copying the data if bio_map_user_iov()
+			 * returns -EREMOTEIO.
+			 */
+			if (ret == -EREMOTEIO)
+				ret = bio_copy_user_iov(rq, map_data, &i,
+							gfp_mask);
+		}
 		if (ret)
 			goto unmap_rq;
 		if (!bio)