vdev_disk: simplify alignment checks for linear ABDs

A linear ABD is a single contiguous range, so only its first page
can carry an offset and only its last one can be short. That leaves
the alignment of the buffer as the only thing to check, and no
reason to look up the pages.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Rob Norris <[email protected]>
Signed-off-by: Alexander Motin <[email protected]>
Closes #19102

module/os/linux/zfs/vdev_disk.c

@@ -982,8 +982,7 @@ vdev_disk_check_alignment_cb(struct page *page, size_t off, size_t len,
 }
 
 /*
- * Check if we can submit the pages in this ABD to the kernel as-is. Returns
- * the number of pages, or 0 if it can't be submitted like this.
+ * Check if we can submit the pages in this ABD to the kernel as-is.
  */
 static boolean_t
 vdev_disk_check_alignment(abd_t *abd, uint64_t size, struct block_device *bdev)
@@ -992,6 +991,17 @@ vdev_disk_check_alignment(abd_t *abd, uint64_t size, struct block_device *bdev)
 	    .blocksize = bdev_logical_block_size(bdev),
 	};
 
+	/*
+	 * A linear ABD is a single contiguous range, so only its first page
+	 * can carry an offset and only its last one can be short. That leaves
+	 * the alignment of the buffer as the only thing to check, and no
+	 * reason to look up the pages.
+	 */
+	if (abd_is_linear(abd)) {
+		ASSERT(IS_P2ALIGNED(size, s.blocksize));
+		return (IS_P2ALIGNED(abd_to_buf(abd), s.blocksize));
+	}
+
 	if (abd_iterate_page_func(abd, 0, size,
 	    vdev_disk_check_alignment_cb, &s))
 		return (B_FALSE);