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Date:   Sat, 20 Jan 2018 01:20:44 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Mike Snitzer <snitzer@redhat.com>,
        Bart Van Assche <Bart.VanAssche@wdc.com>,
        "dm-devel@redhat.com" <dm-devel@redhat.com>,
        "hch@infradead.org" <hch@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "osandov@fb.com" <osandov@fb.com>
Subject: Re: [RFC PATCH] blk-mq: fixup RESTART when queue becomes idle
Message-ID: <20180119172043.GG14827@ming.t460p>
References: <20180119160518.GC14827@ming.t460p>
 <4a5c049f-0fab-bbaf-bfe2-eb5bca73f2c8@kernel.dk>
 <20180119162618.GD14827@ming.t460p>
 <1f072086-533e-4b75-d0e3-9e621b2120d8@kernel.dk>
 <20180119163736.GE14827@ming.t460p>
 <26833249-cadf-ba9c-1128-0bcb70ceb9e1@kernel.dk>
 <20180119164722.GA29449@redhat.com>
 <a03b21d2-b367-62e8-d389-27ecadb38b32@kernel.dk>
 <20180119170519.GF14827@ming.t460p>
 <499af8b8-747a-ed0d-9b4c-e56e6e815a16@kernel.dk>
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On Fri, Jan 19, 2018 at 10:09:11AM -0700, Jens Axboe wrote:
> On 1/19/18 10:05 AM, Ming Lei wrote:
> > On Fri, Jan 19, 2018 at 09:52:32AM -0700, Jens Axboe wrote:
> >> On 1/19/18 9:47 AM, Mike Snitzer wrote:
> >>> On Fri, Jan 19 2018 at 11:41am -0500,
> >>> Jens Axboe <axboe@kernel.dk> wrote:
> >>>
> >>>> On 1/19/18 9:37 AM, Ming Lei wrote:
> >>>>> On Fri, Jan 19, 2018 at 09:27:46AM -0700, Jens Axboe wrote:
> >>>>>> On 1/19/18 9:26 AM, Ming Lei wrote:
> >>>>>>> On Fri, Jan 19, 2018 at 09:19:24AM -0700, Jens Axboe wrote:
> >>>>>>
> >>>>>> There are no pending requests for this case, nothing to restart the
> >>>>>> queue. When you fail that blk_get_request(), you are idle, nothing
> >>>>>> is pending.
> >>>>>
> >>>>> I think we needn't worry about that, once a device is attached to
> >>>>> dm-rq, it can't be mounted any more, and usually user don't use the device
> >>>>> directly and by dm-mpath at the same time.
> >>>>
> >>>> Even if it doesn't happen for a normal dm setup, it is a case that
> >>>> needs to be handled. The request allocation is just one example of
> >>>> a wider scope resource that can be unavailable. If the driver returns
> >>>> NO_DEV_RESOURCE (or whatever name), it will be a possibility that
> >>>> the device itself is currently idle.
> >>>
> >>> How would a driver's resources be exhausted yet the device is idle (so
> >>> as not to be able to benefit from RESTART)?
> >>
> >> I've outlined a number of these examples already. Another case might be:
> >>
> >> 1) Device is idle
> >> 2) Device gets request
> >> 3) Device attempts to DMA map
> >> 4) DMA map fails because the IOMMU is out of space (nic is using it all)
> >> 5) Device returns STS_RESOURCE
> >> 6) Queue is marked as needing a restart
> >>
> >> All's well, except there is no IO on this device that will notice the
> >> restart bit and retry the operation.
> >>
> >> Replace IOMMU failure with any other resource that the driver might need
> >> for an IO, which isn't tied to a device specific resource.
> >> blk_get_request() on dm is an example, as is any allocation failure
> >> occurring in the queue IO path for the driver.
> > 
> > Yeah, if we decide to take the approach of introducing NO_DEV_RESOURCE, all
> > the current STS_RESOURCE for non-device resource allocation(kmalloc, dma
> > map, get_request, ...) should be converted to NO_DEV_RESOURCE.

Or simply introduce BLK_STS_DEV_RESOURCE and convert out of real device
resource into it, this way may be much easy to do.

-- 
Ming