raid5-cache: new helper: r5_reserve_log_entry

Factor out code to reserve log space.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Shaohua Li <shli@fb.com>
Signed-off-by: NeilBrown <neilb@suse.com>

drivers/md/raid5-cache.c

@@ -271,6 +271,23 @@ static struct bio *r5l_bio_alloc(struct r5l_log *log, struct r5l_io_unit *io)
 	return bio;
 }
 
+static void r5_reserve_log_entry(struct r5l_log *log, struct r5l_io_unit *io)
+{
+	log->log_start = r5l_ring_add(log, log->log_start, BLOCK_SECTORS);
+
+	/*
+	 * If we filled up the log device start from the beginning again,
+	 * which will require a new bio.
+	 *
+	 * Note: for this to work properly the log size needs to me a multiple
+	 * of BLOCK_SECTORS.
+	 */
+	if (log->log_start == 0)
+		io->current_bio = NULL;
+
+	io->log_end = log->log_start;
+}
+
 static struct r5l_io_unit *r5l_new_meta(struct r5l_log *log)
 {
 	struct r5l_io_unit *io;
@@ -299,11 +316,7 @@ static struct r5l_io_unit *r5l_new_meta(struct r5l_log *log)
 	bio_add_page(io->current_bio, io->meta_page, PAGE_SIZE, 0);
 
 	log->seq++;
-	log->log_start = r5l_ring_add(log, log->log_start, BLOCK_SECTORS);
-	io->log_end = log->log_start;
-	/* current bio hit disk end */
-	if (log->log_start == 0)
-		io->current_bio = NULL;
+	r5_reserve_log_entry(log, io);
 
 	spin_lock_irq(&log->io_list_lock);
 	list_add_tail(&io->log_sibling, &log->running_ios);
@@ -357,13 +370,8 @@ static void r5l_append_payload_page(struct r5l_log *log, struct page *page)
 		io->current_bio = NULL;
 		goto alloc_bio;
 	}
-	log->log_start = r5l_ring_add(log, log->log_start,
-				      BLOCK_SECTORS);
-	/* current bio hit disk end */
-	if (log->log_start == 0)
-		io->current_bio = NULL;
 
-	io->log_end = log->log_start;
+	r5_reserve_log_entry(log, io);
 }
 
 static void r5l_log_stripe(struct r5l_log *log, struct stripe_head *sh,