[PATCH V2] iomap_dio_rw: Allocate AIO completion queue before submitting dio

[Chandan Rajendra <chandan@linux.vnet.ibm.com>]

Executing xfs/104 test in a loop on Linux-v4.13 kernel on a ppc64
machine can cause the following NULL pointer dereference,

Unable to handle kernel paging request for data at address 0x00000100
Faulting instruction address: 0xc0000000000fece8
Oops: Kernel access of bad area, sig: 11 [#1]
SMP NR_CPUS=2048
DEBUG_PAGEALLOC
NUMA
pSeries
Modules linked in:
CPU: 2 PID: 0 Comm: swapper/2 Not tainted 4.13.0 #1
task: c00000063849c200 task.stack: c0000006384f0000
NIP: c0000000000fece8 LR: c0000000000ff2cc CTR: c000000000383380
REGS: c00000063ffdb400 TRAP: 0300   Not tainted  (4.13.0)
MSR: 8000000000009032 <SF,EE,ME,IR,DR,RI>
  CR: 28002222  XER: 20000000
CFAR: c0000000000227a4 DAR: 0000000000000100 DSISR: 40000000 SOFTE: 0
GPR00: c0000000000ff2cc c00000063ffdb680 c000000001252e00 0000000000000800
GPR04: 0000000000000000 c0000006387b8730 c000000633d5db70 0000000fffffffe1
GPR08: c0000003aa89ed68 0000000000000000 0000000000000000 c000000633d5db70
GPR12: 0000000028002422 c00000000fd80a00 c0000006384f3f90 c00000063ffd8000
GPR16: 0000000000000000 c0000006387b8730 c0000000012475e0 c0000000012475e0
GPR20: 0000000000000002 0000000000000000 0000000000000001 0000000000000002
GPR24: 0000000000000010 c00000063e155800 0000000000000800 0000000000000800
GPR28: 0000000000000000 c000000633d5db00 0000000000000000 0000000000000000
NIP [c0000000000fece8] .__queue_work+0x68/0x600
LR [c0000000000ff2cc] .queue_work_on+0x4c/0x80
Call Trace:
[c00000063ffdb680] [c00000063ffdb720] 0xc00000063ffdb720 (unreliable)
[c00000063ffdb770] [c0000000000ff2cc] .queue_work_on+0x4c/0x80
[c00000063ffdb7f0] [c00000000038343c] .iomap_dio_bio_end_io+0xbc/0x1f0
[c00000063ffdb880] [c0000000006cdc18] .bio_endio+0x118/0x1f0
[c00000063ffdb910] [c0000000006d9e30] .blk_update_request+0xd0/0x470
[c00000063ffdb9b0] [c0000000006e8284] .blk_mq_end_request+0x24/0xc0
[c00000063ffdba30] [c00000000082ba50] .lo_complete_rq+0x40/0xe0
[c00000063ffdbab0] [c0000000006e5e48] .__blk_mq_complete_request_remote+0x28/0x40
[c00000063ffdbb20] [c00000000018e674] .flush_smp_call_function_queue+0xc4/0x1e0
[c00000063ffdbbb0] [c00000000003f94c] .smp_ipi_demux_relaxed+0x8c/0x100
[c00000063ffdbc40] [c000000000081054] .icp_hv_ipi_action+0x54/0xa0
[c00000063ffdbcc0] [c0000000001505a4] .__handle_irq_event_percpu+0x84/0x2c0
[c00000063ffdbd90] [c000000000150808] .handle_irq_event_percpu+0x28/0x80
[c00000063ffdbe20] [c000000000156858] .handle_percpu_irq+0x78/0xc0
[c00000063ffdbea0] [c00000000014ee70] .generic_handle_irq+0x40/0x70
[c00000063ffdbf10] [c000000000015a88] .__do_irq+0x88/0x200
[c00000063ffdbf90] [c000000000027a70] .call_do_irq+0x14/0x24
[c0000006384f3810] [c000000000015c84] .do_IRQ+0x84/0x130

This occurs due to the following sequence of events,

1. Allocate dio for Direct I/O write.
2. Invoke iomap_apply() until iov_iter_count() bytes have been submitted.
   - Assume that we have submitted atleast one bio. Hence iomap_dio->ref value
     will be >= 2.
   - If during the second iteration, iomap_apply() ends up returning -ENOSPC, we would
     break out of the loop and since the 'ret' value is a negative number we
     end up not allocating memory for super_block->s_dio_done_wq.
3. Meanwhile, iomap_dio_bio_end_io() is invoked for bios that have been
   submitted and here the code ends up dereferencing the NULL pointer stored
   at super_block->s_dio_done_wq.

This commit fixes the bug by allocating memory for
super_block->s_dio_done_wq before iomap_apply() is invoked.

Reported-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
---

Changelog:
v1->v2:
1. Fix indentation for the continuation of the if conditional.

 fs/iomap.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/fs/iomap.c b/fs/iomap.c
index 269b24a..d4f526a 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -993,6 +993,13 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 	WARN_ON_ONCE(ret);
 	ret = 0;
 
+	if (iov_iter_rw(iter) == WRITE && !is_sync_kiocb(iocb) &&
+	    !inode->i_sb->s_dio_done_wq) {
+		ret = sb_init_dio_done_wq(inode->i_sb);
+		if (ret < 0)
+			goto out_free_dio;
+	}
+
 	inode_dio_begin(inode);
 
 	blk_start_plug(&plug);
@@ -1015,13 +1022,6 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 	if (ret < 0)
 		iomap_dio_set_error(dio, ret);
 
-	if (ret >= 0 && iov_iter_rw(iter) == WRITE && !is_sync_kiocb(iocb) &&
-			!inode->i_sb->s_dio_done_wq) {
-		ret = sb_init_dio_done_wq(inode->i_sb);
-		if (ret < 0)
-			iomap_dio_set_error(dio, ret);
-	}
-
 	if (!atomic_dec_and_test(&dio->ref)) {
 		if (!is_sync_kiocb(iocb))
 			return -EIOCBQUEUED;
-- 
2.9.5