[PATCH v2] Btrfs: fix crash on endio of reading corrupted block

[Liu Bo <bo.li.liu@oracle.com>]

The crash is

------------[ cut here ]------------
kernel BUG at fs/btrfs/extent_io.c:2124!
invalid opcode: 0000 [#1] SMP
...
CPU: 3 PID: 88 Comm: kworker/u8:7 Not tainted 3.17.0-0.rc1.git0.1.fc22.x86_64 #1
Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
Workqueue: btrfs-endio normal_work_helper [btrfs]
task: ffff8800d7152700 ti: ffff8800d729c000 task.ti: ffff8800d729c000
RIP: 0010:[<ffffffffa02d6055>]  [<ffffffffa02d6055>] end_bio_extent_readpage+0xb45/0xcd0 [btrfs]
Call Trace:
  [<ffffffff810c3ef8>] ? __enqueue_entity+0x78/0x80
  [<ffffffff810ca969>] ? enqueue_entity+0x2e9/0x990
  [<ffffffff813464ab>] bio_endio+0x6b/0xa0
  [<ffffffff813464f2>] bio_endio_nodec+0x12/0x20
  [<ffffffffa02ab217>] end_workqueue_fn+0x37/0x40 [btrfs]
  [<ffffffffa02e4b5d>] normal_work_helper+0xbd/0x280 [btrfs]
  [<ffffffff810ac4fe>] process_one_work+0x17e/0x430
  [<ffffffff810ace8b>] worker_thread+0x6b/0x4a0
  [<ffffffff810ace20>] ? rescuer_thread+0x2a0/0x2a0
  [<ffffffff810b1fca>] kthread+0xea/0x100
  [<ffffffff810b1ee0>] ? kthread_create_on_node+0x1a0/0x1a0
  [<ffffffff8173dd7c>] ret_from_fork+0x7c/0xb0
  [<ffffffff810b1ee0>] ? kthread_create_on_node+0x1a0/0x1a0

This is in fact a regression introduced by commit
facc8a2247340a9735fe8cc123c5da2102f5ef1b(Btrfs: don't cache the csum value into
the extent state tree).

It is because we forgot to increase @offset properly in reading corrupted block,
so that the @offset remains unchanged, and it leads to checksum errors while
reading left blocks queued up in the same bio, and then btrfs tries to
iterate copies for those blocks in order to get good data, and hits the
BUG_ON() which we set to avoid finding good copies for blocks without problems.

Reported-by: Chris Murphy <lists@colorremedies.com>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
---
v2:
   - Improve the commit log to be clear, suggested by Eric.
v3:
   - Show the commit that introduces this bug, I forgot to add this in the v2
     version.

 fs/btrfs/extent_io.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 3af4966..be41e4d 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2602,6 +2602,7 @@ static void end_bio_extent_readpage(struct bio *bio, int err)
 					test_bit(BIO_UPTODATE, &bio->bi_flags);
 				if (err)
 					uptodate = 0;
+				offset += len;
 				continue;
 			}
 		}
-- 
1.8.1.4