Re: [BUG 4.4.26] bio->bi_bdev == NULL in raid6 return_io()

From: Konstantin Khlebnikov
Date: Mon Nov 21 2016 - 10:32:56 EST

On 21.11.2016 04:23, NeilBrown wrote:
On Sun, Nov 20 2016, Konstantin Khlebnikov wrote:

On 07.11.2016 23:34, Konstantin Khlebnikov wrote:
On Mon, Nov 7, 2016 at 10:46 PM, Shaohua Li <shli@xxxxxxxxxx> wrote:
On Sat, Nov 05, 2016 at 01:48:45PM +0300, Konstantin Khlebnikov wrote:
return_io() resolves request_queue even if trace point isn't active:

static inline struct request_queue *bdev_get_queue(struct block_device *bdev)
return bdev->bd_disk->queue; /* this is never NULL */

static void return_io(struct bio_list *return_bi)
struct bio *bi;
while ((bi = bio_list_pop(return_bi)) != NULL) {
bi->bi_iter.bi_size = 0;
bi, 0);

I can't see how this could happen. What kind of tests/environment are these running?

That was a random piece of production somewhere.
Cording to time all crashes happened soon after reboot.
There're several raids, probably some of them were still under resync.

For now we have only few machines with this kernel. But I'm sure that
I'll get much more soon =)

I've added this debug patch for catching overflow of active stripes in bio

--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -164,6 +164,7 @@ static inline void raid5_inc_bi_active_stripes(struct bio *bio)
atomic_t *segments = (atomic_t *)&bio->bi_phys_segments;
+ BUG_ON(!(atomic_read(segments) & 0xffff));

And got this. Counter in %edx = 0x00010000

So, looks like one bio (discard?) can cover more than 65535 stripes

65535 stripes - 256M. I guess that is possible. Christoph has
suggested that now would be a good time to stop using bi_phys_segments
like this.

Is it possible to fix this by limiting max_hw_sectors and
max_hw_discard_sectors for raid queue?

This should be much easier to backport into stable kernels.

I've found that setup also have dm/lvm on the top of md raid so
hat might be more complicated problem.
Because I cannot see how bio could be big enough to overflow that counter.
That was raid6 with 10 disks and 256k chunk. max_hw_discard_sectors and
max_hw_sectors cannot be bigger than UINT_MAX. Thus in this case bio
cannot cover more than 16384 data chunks, 20480 chunks including checksums.
Please fix me if I'm wrong.

I have some patches which should fix this. I'll post them shortly. I'd
appreciate it if you would test and confirm that they work (and don't
break anything else)

Ok, I'll try to check that patchset.