scatterlist: introduce sg_unmark_end

This is useful in places that recycle the same scatterlist multiple
times, and do not want to incur the cost of sg_init_table every
time in hot paths.
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

block/blk-integrity.c

@@ -110,7 +110,7 @@ int blk_rq_map_integrity_sg(struct request_queue *q, struct bio *bio,
 			if (!sg)
 				sg = sglist;
 			else {
-				sg->page_link &= ~0x02;
+				sg_unmark_end(sg);
 				sg = sg_next(sg);
 			}
 

block/blk-merge.c

@@ -143,7 +143,7 @@ __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
 			 * termination bit to avoid doing a full
 			 * sg_init_table() in drivers for each command.
 			 */
-			(*sg)->page_link &= ~0x02;
+			sg_unmark_end(*sg);
 			*sg = sg_next(*sg);
 		}
 

include/linux/scatterlist.h

@@ -171,6 +171,22 @@ static inline void sg_mark_end(struct scatterlist *sg)
 	sg->page_link &= ~0x01;
 }
 
+/**
+ * sg_unmark_end - Undo setting the end of the scatterlist
+ * @sg:		 SG entryScatterlist
+ *
+ * Description:
+ *   Removes the termination marker from the given entry of the scatterlist.
+ *
+ **/
+static inline void sg_unmark_end(struct scatterlist *sg)
+{
+#ifdef CONFIG_DEBUG_SG
+	BUG_ON(sg->sg_magic != SG_MAGIC);
+#endif
+	sg->page_link &= ~0x02;
+}
+
 /**
  * sg_phys - Return physical address of an sg entry
  * @sg:	     SG entry