linux-kernel - Re: [PATCH 1/2] virtio-blk: fix hw_queue stopped on arbitrary error

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Date:   Wed, 19 Feb 2020 16:42:18 +0100
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Ming Lei <tom.leiming@...il.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Cornelia Huck <cohuck@...hat.com>,
        Ram Pai <linuxram@...ibm.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Viktor Mihajlovski <mihajlov@...ux.ibm.com>
Subject: Re: [PATCH 1/2] virtio-blk: fix hw_queue stopped on arbitrary error

On Wed, 19 Feb 2020 09:46:56 +0800
Ming Lei <tom.leiming@...il.com> wrote:

> On Tue, Feb 18, 2020 at 8:35 PM Halil Pasic <pasic@...ux.ibm.com> wrote:
> >
> > On Tue, 18 Feb 2020 10:21:18 +0800
> > Ming Lei <tom.leiming@...il.com> wrote:
> >
> > > On Thu, Feb 13, 2020 at 8:38 PM Halil Pasic <pasic@...ux.ibm.com> wrote:
> > > >
> > > > Since nobody else is going to restart our hw_queue for us, the
> > > > blk_mq_start_stopped_hw_queues() is in virtblk_done() is not sufficient
> > > > necessarily sufficient to ensure that the queue will get started again.
> > > > In case of global resource outage (-ENOMEM because mapping failure,
> > > > because of swiotlb full) our virtqueue may be empty and we can get
> > > > stuck with a stopped hw_queue.
> > > >
> > > > Let us not stop the queue on arbitrary errors, but only on -EONSPC which
> > > > indicates a full virtqueue, where the hw_queue is guaranteed to get
> > > > started by virtblk_done() before when it makes sense to carry on
> > > > submitting requests. Let us also remove a stale comment.
> > >
> > > The generic solution may be to stop queue only when there is any
> > > in-flight request
> > > not completed.
> > >
> >
> > I think this is a pretty close to that. The queue is stopped only on
> > ENOSPC, which means virtqueue is full.
> >
> > > Checking -ENOMEM may not be enough, given -EIO can be returned from
> > > virtqueue_add()
> > > too in case of dma map failure.
> >
> > I'm not checking on -ENOMEM. So the queue would not be stopped on EIO.
> > Maybe I'm misunderstanding something In any case, please have another
> > look at the diff, and if your concerns persist please help me understand.
> 
> Looks I misread the patch, and this patch is fine:
> 
> Reviewed-by: Ming Lei <ming.lei@...hat.com>

Thank you very much!

Regards,
Halil

> 
> 
> Thanks,
> Ming Lei