Re: [PATCH V2] raid1: ensure write behind bio has less than BIO_MAX_VECS sectors

On Wed, Aug 18, 2021 at 03:37:38PM +0800, Guoqing Jiang wrote:

 	for (i = 0;  i < disks; i++) {
 		struct md_rdev *rdev = rcu_dereference(conf->mirrors[i].rdev);
+
+		if (test_bit(WriteMostly, &mirror->rdev->flags))
+			write_behind = true;

How does this condition relate to the ones used for actually calling
alloc_behind_master_bio?  It looks related, but as someone not familiar
with the code I can't really verify if this is correct, so a comment
explaining it might be useful.

+	/*
+	 * When using a bitmap, we may call alloc_behind_master_bio below.
+	 * alloc_behind_master_bio allocates a copy of the data payload a page
+	 * at a time and thus needs a new bio that can fit the whole payload
+	 * this bio in page sized chunks.
+	 */
+	if (write_behind && bitmap)
+		max_sectors = min_t(int, max_sectors, BIO_MAX_VECS * PAGE_SECTORS);

Overly long line here.