From 5fbc7c59fd22c5a6531b40b0759624b680a95e52 Mon Sep 17 00:00:00 2001
From: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Date: Wed, 11 Jun 2014 10:55:22 +0800
Subject: [PATCH] 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>
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