Re: [PATCH 00/10] HMM updates for 5.1

From: Dan Williams <dan.j.williams@intel.com>
To: Jerome Glisse <jglisse@redhat.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Linux MM" <linux-mm@kvack.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Felix Kuehling" <Felix.Kuehling@amd.com>,
	"Christian König" <christian.koenig@amd.com>,
	"Ralph Campbell" <rcampbell@nvidia.com>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Jason Gunthorpe" <jgg@mellanox.com>,
	"Alex Deucher" <alexander.deucher@amd.com>
Subject: Re: [PATCH 00/10] HMM updates for 5.1
Date: Tue, 19 Mar 2019 12:13:40 -0700	[thread overview]
Message-ID: <CAPcyv4hg5Y_NC1iu56zcznYkCRnwg+_7bGFr==7=AC6ii=O=Ng@mail.gmail.com> (raw)
In-Reply-To: <20190319190528.GA4012@redhat.com>

On Tue, Mar 19, 2019 at 12:05 PM Jerome Glisse <jglisse@redhat.com> wrote:
>
> On Tue, Mar 19, 2019 at 11:42:00AM -0700, Dan Williams wrote:
> > On Tue, Mar 19, 2019 at 10:45 AM Jerome Glisse <jglisse@redhat.com> wrote:
> > >
> > > On Tue, Mar 19, 2019 at 10:33:57AM -0700, Dan Williams wrote:
> > > > On Tue, Mar 19, 2019 at 10:19 AM Jerome Glisse <jglisse@redhat.com> wrote:
> > > > >
> > > > > On Tue, Mar 19, 2019 at 10:12:49AM -0700, Andrew Morton wrote:
> > > > > > On Tue, 19 Mar 2019 12:58:02 -0400 Jerome Glisse <jglisse@redhat.com> wrote:
> > > > [..]
> > > > > > Also, the discussion regarding [07/10] is substantial and is ongoing so
> > > > > > please let's push along wth that.
> > > > >
> > > > > I can move it as last patch in the serie but it is needed for ODP RDMA
> > > > > convertion too. Otherwise i will just move that code into the ODP RDMA
> > > > > code and will have to move it again into HMM code once i am done with
> > > > > the nouveau changes and in the meantime i expect other driver will want
> > > > > to use this 2 helpers too.
> > > >
> > > > I still hold out hope that we can find a way to have productive
> > > > discussions about the implementation of this infrastructure.
> > > > Threatening to move the code elsewhere to bypass the feedback is not
> > > > productive.
> > >
> > > I am not threatening anything that code is in ODP _today_ with that
> > > patchset i was factering it out so that i could also use it in nouveau.
> > > nouveau is built in such way that right now i can not use it directly.
> > > But i wanted to factor out now in hope that i can get the nouveau
> > > changes in 5.2 and then convert nouveau in 5.3.
> > >
> > > So when i said that code will be in ODP it just means that instead of
> > > removing it from ODP i will keep it there and it will just delay more
> > > code sharing for everyone.
> >
> > The point I'm trying to make is that the code sharing for everyone is
> > moving the implementation closer to canonical kernel code and use
> > existing infrastructure. For example, I look at 'struct hmm_range' and
> > see nothing hmm specific in it. I think we can make that generic and
> > not build up more apis and data structures in the "hmm" namespace.
>
> Right now i am trying to unify driver for device that have can support
> the mmu notifier approach through HMM. Unify to a superset of driver
> that can not abide by mmu notifier is on my todo list like i said but
> it comes after. I do not want to make the big jump in just one go. So
> i doing thing under HMM and thus in HMM namespace, but once i tackle
> the larger set i will move to generic namespace what make sense.
>
> This exact approach did happen several time already in the kernel. In
> the GPU sub-system we did it several time. First do something for couple
> devices that are very similar then grow to a bigger set of devices and
> generalise along the way.
>
> So i do not see what is the problem of me repeating that same pattern
> here again. Do something for a smaller set before tackling it on for
> a bigger set.

All of that is fine, but when I asked about the ultimate trajectory
that replaces hmm_range_dma_map() with an updated / HMM-aware GUP
implementation, the response was that hmm_range_dma_map() is here to
stay. The issue is not with forking off a small side effort, it's the
plan to absorb that capability into a common implementation across
non-HMM drivers where possible.