Btrfs: fix unfinished readahead thread for raid5/6 degraded mounting

Steps to reproduce:

 # mkfs.btrfs -f /dev/sd[b-f] -m raid5 -d raid5
 # mkfs.ext4 /dev/sdc --->corrupt one of btrfs device
 # mount /dev/sdb /mnt -o degraded
 # btrfs scrub start -BRd /mnt

This is because readahead would skip missing device, this is not true
for RAID5/6, because REQ_GET_READ_MIRRORS return 1 for RAID5/6 block
mapping. If expected data locates in missing device, readahead thread
would not call __readahead_hook() which makes event @rc->elems=0
wait forever.

Fix this problem by checking return value of btrfs_map_block(),we
can only skip missing device safely if there are several mirrors.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

diff --git a/fs/btrfs/reada.c b/fs/btrfs/reada.c
index 30947f9..09230cf 100644 (file)
--- a/fs/btrfs/reada.c
+++ b/fs/btrfs/reada.c
@@ -428,8 +428,13 @@ static struct reada_extent *reada_find_extent(struct btrfs_root *root,
                        continue;
                }
                if (!dev->bdev) {
-                       /* cannot read ahead on missing device */
-                       continue;
+                       /*
+                        * cannot read ahead on missing device, but for RAID5/6,
+                        * REQ_GET_READ_MIRRORS return 1. So don't skip missing
+                        * device for such case.
+                        */
+                       if (nzones > 1)
+                               continue;
                }
                if (dev_replace_is_ongoing &&
                    dev == fs_info->dev_replace.tgtdev) {