md/md-bitmap: add 'behind_writes' and 'behind_wait' into struct md_bitmap_stats

There are no functional changes, avoid dereferencing bitmap directly to
prepare inventing a new bitmap.

Also fix following checkpatch warning by using wq_has_sleeper().

WARNING: waitqueue_active without comment
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-9-yukuai1@huaweicloud.comSigned-off-by: Song Liu <song@kernel.org>

drivers/md/md-bitmap.c

@@ -2115,6 +2115,8 @@ int md_bitmap_get_stats(struct bitmap *bitmap, struct md_bitmap_stats *stats)
 	stats->file_pages = storage->file_pages;
 	stats->file = storage->file;
 
+	stats->behind_writes = atomic_read(&bitmap->behind_writes);
+	stats->behind_wait = wq_has_sleeper(&bitmap->behind_wait);
 	stats->events_cleared = bitmap->events_cleared;
 	return 0;
 }

drivers/md/md-bitmap.h

@@ -236,6 +236,9 @@ struct bitmap {
 
 struct md_bitmap_stats {
 	u64		events_cleared;
+	int		behind_writes;
+	bool		behind_wait;
+
 	unsigned long	missing_pages;
 	unsigned long	file_pages;
 	unsigned long	sync_size;

drivers/md/raid1.c

@@ -1590,16 +1590,19 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
 			continue;
 
 		if (first_clone) {
+			unsigned long max_write_behind =
+				mddev->bitmap_info.max_write_behind;
+			struct md_bitmap_stats stats;
+			int err;
+
 			/* do behind I/O ?
 			 * Not if there are too many, or cannot
 			 * allocate memory, or a reader on WriteMostly
 			 * is waiting for behind writes to flush */
-			if (bitmap && write_behind &&
-			    (atomic_read(&bitmap->behind_writes)
-			     < mddev->bitmap_info.max_write_behind) &&
-			    !waitqueue_active(&bitmap->behind_wait)) {
+			err = md_bitmap_get_stats(bitmap, &stats);
+			if (!err && write_behind && !stats.behind_wait &&
+			    stats.behind_writes < max_write_behind)
 				alloc_behind_master_bio(r1_bio, bio);
-			}
 
 			md_bitmap_startwrite(bitmap, r1_bio->sector, r1_bio->sectors,
 					     test_bit(R1BIO_BehindIO, &r1_bio->state));