[PATCH] xfs: fix system crash caused by null bp->b_pages

[Fengfei Xi <xi.fengfei@h3c.com>]

We have encountered the following problems several times:
    1、A raid slot or hardware problem causes block device loss.
    2、Continue to issue IO requests to the problematic block device.
    3、The system possibly crash after a few hours.

dmesg log as below:
[15205901.268313] blk_partition_remap: fail for partition 1
[15205901.319309] blk_partition_remap: fail for partition 1
[15205901.319341] blk_partition_remap: fail for partition 1
[15205901.319873] sysctl (3998546): drop_caches: 3
[15205901.371379] BUG: unable to handle kernel NULL pointer dereference at
[15205901.372602] IP: xfs_buf_offset+0x32/0x60 [xfs]
[15205901.373605] PGD 0 P4D 0
[15205901.374690] Oops: 0000 [#1] SMP
[15205901.375629] Modules linked in:
[15205901.382445] CPU: 6 PID: 18545 Comm: xfsaild/sdh1 Kdump: loaded Tainted: G
[15205901.384728] Hardware name:
[15205901.385830] task: ffff885216939e80 task.stack: ffffb28ba9b38000
[15205901.386974] RIP: 0010:xfs_buf_offset+0x32/0x60 [xfs]
[15205901.388044] RSP: 0018:ffffb28ba9b3bc68 EFLAGS: 00010246
[15205901.389021] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 000000000000000b
[15205901.390016] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88627bebf000
[15205901.391075] RBP: ffffb28ba9b3bc98 R08: ffff88627bebf000 R09: 00000001802a000d
[15205901.392031] R10: ffff88521f3a0240 R11: ffff88627bebf000 R12: ffff88521041e000
[15205901.392950] R13: 0000000000000020 R14: ffff88627bebf000 R15: 0000000000000000
[15205901.393858] FS:  0000000000000000(0000) GS:ffff88521f380000(0000) knlGS:0000000000000000
[15205901.394774] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[15205901.395756] CR2: 0000000000000000 CR3: 000000099bc09001 CR4: 00000000007606e0
[15205901.396904] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[15205901.397869] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[15205901.398836] PKRU: 55555554
[15205901.400111] Call Trace:
[15205901.401058]  ? xfs_inode_buf_verify+0x8e/0xf0 [xfs]
[15205901.402069]  ? xfs_buf_delwri_submit_buffers+0x16d/0x2b0 [xfs]
[15205901.403060]  xfs_inode_buf_write_verify+0x10/0x20 [xfs]
[15205901.404017]  _xfs_buf_ioapply+0x88/0x410 [xfs]
[15205901.404990]  ? xfs_buf_delwri_submit_buffers+0x16d/0x2b0 [xfs]
[15205901.405929]  xfs_buf_submit+0x63/0x200 [xfs]
[15205901.406801]  xfs_buf_delwri_submit_buffers+0x16d/0x2b0 [xfs]
[15205901.407675]  ? xfs_buf_delwri_submit_nowait+0x10/0x20 [xfs]
[15205901.408540]  ? xfs_inode_item_push+0xb7/0x190 [xfs]
[15205901.409395]  xfs_buf_delwri_submit_nowait+0x10/0x20 [xfs]
[15205901.410249]  xfsaild+0x29a/0x780 [xfs]
[15205901.411121]  kthread+0x109/0x140
[15205901.411981]  ? xfs_trans_ail_cursor_first+0x90/0x90 [xfs]
[15205901.412785]  ? kthread_park+0x60/0x60
[15205901.413578]  ret_from_fork+0x2a/0x40

The "obvious" cause is that the bp->b_pages was NULL in function
xfs_buf_offset. Analyzing vmcore, we found that b_pages=NULL but
b_page_count=16, so b_pages is set to NULL for some reason.

crash> struct xfs_buf ffff88627bebf000 | less
    ...
  b_pages = 0x0,
  b_page_array = {0x0, 0x0},
  b_maps = 0xffff88627bebf118,
  __b_map = {
    bm_bn = 512,
    bm_len = 128
  },
  b_map_count = 1,
  b_io_length = 128,
  b_pin_count = {
    counter = 0
  },
  b_io_remaining = {
    counter = 1
  },
  b_page_count = 16,
  b_offset = 0,
  b_error = 0,
    ...

To avoid system crash, we can add the check of 'bp->b_pages' to
xfs_inode_buf_verify(). If b_pages == NULL, we mark the buffer
as -EFSCORRUPTED and the IO will not dispatched.

Signed-off-by: Fengfei Xi <xi.fengfei@h3c.com>
Reviewed-by: Xianting Tian <tian.xianting@h3c.com>
---
 fs/xfs/libxfs/xfs_inode_buf.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/xfs/libxfs/xfs_inode_buf.c b/fs/xfs/libxfs/xfs_inode_buf.c
index c667c63f2..5a485c51f 100644
--- a/fs/xfs/libxfs/xfs_inode_buf.c
+++ b/fs/xfs/libxfs/xfs_inode_buf.c
@@ -45,6 +45,17 @@ xfs_inode_buf_verify(
 	int		i;
 	int		ni;
 
+	/*
+	 * Don't crash and mark buffer EFSCORRUPTED when b_pages is NULL
+	 */
+	if (!bp->b_pages) {
+		xfs_buf_ioerror(bp, -EFSCORRUPTED);
+		xfs_alert(mp,
+			"xfs_buf(%p) b_pages corruption detected at %pS\n",
+			bp, __return_address);
+		return;
+	}
+
 	/*
 	 * Validate the magic number and version of every inode in the buffer
 	 */
-- 
2.17.1