Re: [bug report][stable] xfstests:generic/538 failed on xfs

[gregkh <gregkh@linuxfoundation.org>]

On Wed, Jun 05, 2019 at 08:21:44PM +0800, Alvin Zheng wrote:
> Hi,
>   I was using kernel v4.19.48 and found that it cannot pass the generic/538 on xfs. The error output is as follows:

Has 4.19 ever been able to pass that test?  If not, I wouldn't worry
about it :)

> 
>   FSTYP         -- xfs (non-debug)
>   PLATFORM      -- Linux/x86_64 alinux2-6 4.19.48
>   MKFS_OPTIONS  -- -f -bsize=4096 /dev/vdc
>   MOUNT_OPTIONS -- /dev/vdc /mnt/testarea/scra
>   generic/538 0s ... - output mismatch (see /root/usr/local/src/xfstests/results//generic/538.out.bad)
>       --- tests/generic/538.out   2019-05-27 13:57:06.505666465 +0800
>       +++ /root/usr/local/src/xfstests/results//generic/538.out.bad       2019-06-05 16:43:14.702002326 +0800
>       @@ -1,2 +1,10 @@
>        QA output created by 538
>       +Data verification fails
>       +Find corruption
>       +00000000  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>       +*
>       +00000200  5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
>       +00002000
>       ...
>       (Run 'diff -u /root/usr/local/src/xfstests/tests/generic/538.out /root/usr/local/src/xfstests/results//generic/538.out.bad'  to see the entire diff)
>   Ran: generic/538
>   Failures: generic/538
>   Failed 1 of 1 tests
>   
> I also found that the latest kernel (v5.2.0-rc2) of upstream can pass the generic/538 test. Therefore, I bisected and found the first good commit is 3110fc79606. This commit adds the hardware queue into the sort function. Besides, the sort function returns a negative value when the offset and queue (software and hardware) of two I/O requests are same. I think the second part of the change make senses. The kernel should not change the relative position of two I/O requests when their offset and queue are same. So I made the following changes and merged it into the kernel 4.19.48. After the modification, we can pass the generic/538 test on xfs. The same case can be passed on ext4, since ext4 has corresponding fix 0db24122bd7f ("ext4: fix data corruption caused by overlapping unaligned and aligned IO"). Though I think xfs should be responsible for this issue, the block layer code below is also problematic. Any ideas?
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 4e563ee..a7309cd 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1610,7 +1610,7 @@ static int plug_ctx_cmp(void *priv, struct list_head *a, struct list_head *b)
> 
>         return !(rqa->mq_ctx < rqb->mq_ctx ||
>                  (rqa->mq_ctx == rqb->mq_ctx &&
> -                 blk_rq_pos(rqa) < blk_rq_pos(rqb)));
> +                 blk_rq_pos(rqa) <= blk_rq_pos(rqb)));
>  }
> 
>  void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)

I would not like to take a patch that is not upstream, but rather take
the original commit.

Can 3110fc79606f ("blk-mq: improve plug list sorting") on its own
resolve this issue for 4.19.y?

thanks,

greg k-h