[PATCH v1 53/54] btrfs: comment on direct access bvec table

@@ -601,6 +601,10 @@ int btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
 
 	/* we need the actual starting offset of this extent in the file */
 	read_lock(&em_tree->lock);
+	/*
+	 * It is still safe to retrieve the 1st page of the bio
+	 * in this way after supporting multipage bvec.
+	 */
 	em = lookup_extent_mapping(em_tree,
 				   page_offset(bio->bi_io_vec->bv_page),
 				   PAGE_SIZE);

@@ -7914,6 +7914,11 @@ static int dio_read_error(struct inode *inode, struct bio *failed_bio,
 		return -EIO;
 	}
 
+	/*
+	 * In this case it is still correct to retrieve length
+	 * of the 1st bvec of the bio in this way after supporting
+	 * multipage bvec.
+	 */
 	if ((bio_segments_all(failed_bio) > 1)
 		|| (failed_bio->bi_io_vec->bv_len
 			> btrfs_inode_sectorsize(inode)))

@@ -7960,6 +7965,12 @@ static void btrfs_retry_endio_nocsum(struct bio *bio)
 	if (bio->bi_error)
 		goto end;
 
+	/*
+	 * WARNING:
+	 *
+	 * With multipage bvec, the following way of direct access to
+	 * bvec table is only safe if the bio includes single page.
+	 */
 	ASSERT(bio->bi_vcnt == 1);
 	inode = bio->bi_io_vec->bv_page->mapping->host;
 	ASSERT(bio->bi_io_vec->bv_len == btrfs_inode_sectorsize(inode));

@@ -8045,6 +8056,12 @@ static void btrfs_retry_endio(struct bio *bio)
 
 	start = done->start;
 
+	/*
+	 * WARNING:
+	 *
+	 * With multipage bvec, the following way of direct access to
+	 * bvec table is only safe if the bio includes single page.
+	 */
 	ASSERT(bio->bi_vcnt == 1);
 	inode = bio->bi_io_vec->bv_page->mapping->host;
 	ASSERT(bio->bi_io_vec->bv_len == btrfs_inode_sectorsize(inode));