Re: [PATCH] block/io_uring: resubmit when result is -EAGAIN

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Cc'ing the maintainers for you. See
https://wiki.qemu.org/Contribute/SubmitAPatch#CC_the_relevant_maintainer

$ ./scripts/get_maintainer.pl -f block/io_uring.c
Aarushi Mehta <mehta.aaru20@gmail.com> (maintainer:Linux io_uring)
Julia Suvorova <jusual@redhat.com> (maintainer:Linux io_uring)
Stefan Hajnoczi <stefanha@redhat.com> (maintainer:Linux io_uring)
Kevin Wolf <kwolf@redhat.com> (supporter:Block layer core)
Max Reitz <mreitz@redhat.com> (supporter:Block layer core)
qemu-block@nongnu.org (open list:Linux io_uring)
qemu-devel@nongnu.org (open list:All patches CC here)

Also Cc'ing Stefano for commit b4e44c9944e ("io_uring: retry
io_uring_submit() if it fails with errno=EINTR").
(Stefano, you might want to add yourself a R: tag in MAINTAINERS).

On 7/28/21 12:35 PM, Fabian Ebner wrote:
> Quoting from [0]:
> 
>  Some setups, like SCSI, can throw spurious -EAGAIN off the softirq
>  completion path. Normally we expect this to happen inline as part
>  of submission, but apparently SCSI has a weird corner case where it
>  can happen as part of normal completions.
> 
> Host kernels without patch [0] can panic when this happens [1], and
> resubmitting makes the panic more likely. On the other hand, for
> kernels with patch [0], resubmitting ensures that a block job is not
> aborted just because of such spurious errors.
> 
> [0]: https://lore.kernel.org/io-uring/20210727165811.284510-3-axboe@kernel.dk/T/#u
> 
> [1]:
>   #9 [ffffb732000c8b70] asm_exc_page_fault at ffffffffa4800ade
>  #10 [ffffb732000c8bf8] io_prep_async_work at ffffffffa3d89c16
>  #11 [ffffb732000c8c50] io_rw_reissue at ffffffffa3d8b2e1
>  #12 [ffffb732000c8c78] io_complete_rw at ffffffffa3d8baa8
>  #13 [ffffb732000c8c98] blkdev_bio_end_io at ffffffffa3d62a80
>  #14 [ffffb732000c8cc8] bio_endio at ffffffffa3f4e800
>  #15 [ffffb732000c8ce8] dec_pending at ffffffffa432f854
>  #16 [ffffb732000c8d30] clone_endio at ffffffffa433170c
>  #17 [ffffb732000c8d70] bio_endio at ffffffffa3f4e800
>  #18 [ffffb732000c8d90] blk_update_request at ffffffffa3f53a37
>  #19 [ffffb732000c8dd0] scsi_end_request at ffffffffa4233a5c
>  #20 [ffffb732000c8e08] scsi_io_completion at ffffffffa423432c
>  #21 [ffffb732000c8e58] scsi_finish_command at ffffffffa422c527
>  #22 [ffffb732000c8e88] scsi_softirq_done at ffffffffa42341e4
> 
> Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
> ---
> 
> I'm new to this code and io_uring, so I don't know what all the
> implications are, but retrying upon EAGAIN does not sound like
> a bad thing to my inexperienced ears.
> 
> Some more context, leading up to this patch:
> 
> We had some users reporting issues after we switched to using io_uring
> by default. Namely, kernel panics [2] for some, and failing block jobs
> [3] (with a custom backup mechanism we implemented on top of QEMU's
> block layer) for others.
> 
> I had luck and managed to reprouce the issue, and it was a failed
> block job about half of the time and a kernel panic the other half.
> When using a host kernel with [0], it's a failed block job all the
> time, and this patch attempts to fix that, by resubmitting instead
> of bubbling up the error.
> 
> [2]: https://forum.proxmox.com/threads/kernel-panic-whole-server-crashes-about-every-day.91803/post-404382
> [3]: https://forum.proxmox.com/threads/backup-job-failed-with-err-11-on-2-of-6-vms.92568/
> 
>  block/io_uring.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/io_uring.c b/block/io_uring.c
> index 00a3ee9fb8..77d162cb24 100644
> --- a/block/io_uring.c
> +++ b/block/io_uring.c
> @@ -165,7 +165,7 @@ static void luring_process_completions(LuringState *s)
>          total_bytes = ret + luringcb->total_read;
>  
>          if (ret < 0) {
> -            if (ret == -EINTR) {
> +            if (ret == -EINTR || ret == -EAGAIN) {
>                  luring_resubmit(s, luringcb);
>                  continue;
>              }
>