Per Qu Wenruo in case we have a very large disk, e.g. 8TiB device,
mostly empty although we will do the split according to our super block
locations, the last super block ends at 256G, we can submit a huge
discard for the range [256G, 8T), causing a super large delay.

We now split the space left to discard based on BTRFS_MAX_DISCARD_CHUNK_SIZE
in preparation of introduction of cancellation signals handling.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=219180
Link: https://bugzilla.suse.com/show_bug.cgi?id=1229737
Signed-off-by: Luca Stefani <luca.stefani.ge1@gmail.com>
---
 fs/btrfs/extent-tree.c | 19 +++++++++++++++----
 fs/btrfs/volumes.h     |  1 +
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index a5966324607d..ad70548d1f72 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -1300,13 +1300,24 @@ static int btrfs_issue_discard(struct block_device *bdev, u64 start, u64 len,
 		bytes_left = end - start;
 	}
 
-	if (bytes_left) {
+	while (bytes_left) {
+		u64 bytes_to_discard = min(BTRFS_MAX_DISCARD_CHUNK_SIZE, bytes_left);
+
 		ret = blkdev_issue_discard(bdev, start >> SECTOR_SHIFT,
-					   bytes_left >> SECTOR_SHIFT,
+					   bytes_to_discard >> SECTOR_SHIFT,
 					   GFP_NOFS);
-		if (!ret)
-			*discarded_bytes += bytes_left;
+
+		if (ret) {
+			if (ret != -EOPNOTSUPP)
+				break;
+			continue;
+		}
+
+		start += bytes_to_discard;
+		bytes_left -= bytes_to_discard;
+		*discarded_bytes += bytes_to_discard;
 	}
+
 	return ret;
 }
 
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 03d2d60afe0c..c15062a435bd 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -29,6 +29,7 @@ struct btrfs_trans_handle;
 struct btrfs_zoned_device_info;
 
 #define BTRFS_MAX_DATA_CHUNK_SIZE	(10ULL * SZ_1G)
+#define BTRFS_MAX_DISCARD_CHUNK_SIZE	(1ULL * SZ_1G)
 
 extern struct mutex uuid_mutex;
 
-- 
2.46.0