[PATCH v2 15/51] btrfs: comment on direct access bvec table

@@ -541,6 +541,10 @@ blk_status_t 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);

@@ -8055,6 +8055,12 @@ static void btrfs_retry_endio_nocsum(struct bio *bio)
 	if (bio->bi_status)
 		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);
 	io_tree = &BTRFS_I(inode)->io_tree;
 	failure_tree = &BTRFS_I(inode)->io_failure_tree;

@@ -8146,6 +8152,12 @@ static void btrfs_retry_endio(struct bio *bio)
 
 	uptodate = 1;
 
+	/*
+	 * 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);
 	ASSERT(bio->bi_io_vec->bv_len == btrfs_inode_sectorsize(done->inode));