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

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Message-ID: <20190319191849.GA4310@redhat.com>
Date:   Tue, 19 Mar 2019 15:18:49 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Christian König <christian.koenig@....com>,
        Ralph Campbell <rcampbell@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Alex Deucher <alexander.deucher@....com>
Subject: Re: [PATCH 00/10] HMM updates for 5.1

On Tue, Mar 19, 2019 at 12:13:40PM -0700, Dan Williams wrote:
> On Tue, Mar 19, 2019 at 12:05 PM Jerome Glisse <jglisse@...hat.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@...hat.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@...hat.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@...hat.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.

hmm_range_dma_map() is a superset of gup_range_dma_map() because on
top of gup_range_dma_map() the hmm version deals with mmu notifier.

But everything that is not mmu notifier related can be share through
gup_range_dma_map() so plan is to end up with:
    hmm_range_dma_map(hmm_struct) {
        hmm_mmu_notifier_specific_prep_step();
        gup_range_dma_map(hmm_struct->common_base_struct);
        hmm_mmu_notifier_specific_post_step();
    }

ie share as much as possible. Does that not make sense ? To get
there i will need to do non trivial addition to GUP and so i went
first to get HMM bits working and then work on common gup API.

Cheers,
Jérôme