Date:   Tue, 12 Mar 2019 21:00:57 -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>,
        Ralph Campbell <rcampbell@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 09/10] mm/hmm: allow to mirror vma of a file on a DAX
 backed filesystem

On Tue, Mar 12, 2019 at 05:46:51PM -0700, Dan Williams wrote:
> On Tue, Mar 12, 2019 at 5:10 PM Jerome Glisse <jglisse@...hat.com> wrote:
> >
> > On Tue, Mar 12, 2019 at 02:52:14PM -0700, Andrew Morton wrote:
> > > On Tue, 12 Mar 2019 12:30:52 -0700 Dan Williams <dan.j.williams@...el.com> wrote:
> > >
> > > > On Tue, Mar 12, 2019 at 12:06 PM Jerome Glisse <jglisse@...hat.com> wrote:
> > > > > On Tue, Mar 12, 2019 at 09:06:12AM -0700, Dan Williams wrote:
> > > > > > On Tue, Mar 12, 2019 at 8:26 AM Jerome Glisse <jglisse@...hat.com> wrote:
> > > > [..]
> > > > > > > Spirit of the rule is better than blind application of rule.
> > > > > >
> > > > > > Again, I fail to see why HMM is suddenly unable to make forward
> > > > > > progress when the infrastructure that came before it was merged with
> > > > > > consumers in the same development cycle.
> > > > > >
> > > > > > A gate to upstream merge is about the only lever a reviewer has to
> > > > > > push for change, and these requests to uncouple the consumer only
> > > > > > serve to weaken that review tool in my mind.
> > > > >
> > > > > Well let just agree to disagree and leave it at that and stop
> > > > > wasting each other time
> > > >
> > > > I'm fine to continue this discussion if you are. Please be specific
> > > > about where we disagree and what aspect of the proposed rules about
> > > > merge staging are either acceptable, painful-but-doable, or
> > > > show-stoppers. Do you agree that HMM is doing something novel with
> > > > merge staging, am I off base there?
> > >
> > > You're correct.  We chose to go this way because the HMM code is so
> > > large and all-over-the-place that developing it in a standalone tree
> > > seemed impractical - better to feed it into mainline piecewise.
> > >
> > > This decision very much assumed that HMM users would definitely be
> > > merged, and that it would happen soon.  I was skeptical for a long time
> > > and was eventually persuaded by quite a few conversations with various
> > > architecture and driver maintainers indicating that these HMM users
> > > would be forthcoming.
> > >
> > > In retrospect, the arrival of HMM clients took quite a lot longer than
> > > was anticipated and I'm not sure that all of the anticipated usage
> > > sites will actually be using it.  I wish I'd kept records of
> > > who-said-what, but I didn't and the info is now all rather dissipated.
> > >
> > > So the plan didn't really work out as hoped.  Lesson learned, I would
> > > now very much prefer that new HMM feature work's changelogs include
> > > links to the driver patchsets which will be using those features and
> > > acks and review input from the developers of those driver patchsets.
> >
> > This is what i am doing now and this patchset falls into that. I did
> > post the ODP and nouveau bits to use the 2 new functions (dma map and
> > unmap). I expect to merge both ODP and nouveau bits for that during
> > the next merge window.
> >
> > Also with 5.1 everything that is upstream is use by nouveau at least.
> > They are posted patches to use HMM for AMD, Intel, Radeon, ODP, PPC.
> > Some are going through several revisions so i do not know exactly when
> > each will make it upstream but i keep working on all this.
> >
> > So the guideline we agree on:
> >     - no new infrastructure without user
> >     - device driver maintainer for which new infrastructure is done
> >       must either sign off or review of explicitly say that they want
> >       the feature I do not expect all driver maintainer will have
> >       the bandwidth to do proper review of the mm part of the infra-
> >       structure and it would not be fair to ask that from them. They
> >       can still provide feedback on the API expose to the device
> >       driver.
> >     - driver bits must be posted at the same time as the new infra-
> >       structure even if they target the next release cycle to avoid
> >       inter-tree dependency
> >     - driver bits must be merge as soon as possible
> 
> What about EXPORT_SYMBOL_GPL?

I explained why i do not see value in changing export, but i will not
oppose that change either.


> > Thing we do not agree on:
> >     - If driver bits miss for any reason the +1 target directly
> >       revert the new infra-structure. I think it should not be black
> >       and white and the reasons why the driver bit missed the merge
> >       window should be taken into account. If the feature is still
> >       wanted and the driver bits missed the window for simple reasons
> >       then it means that we push everything by 2 release ie the
> >       revert is done in +1 then we reupload the infra-structure in
> >       +2 and finaly repush the driver bit in +3 so we loose 1 cycle.
> 
> I think that pain is reasonable.
> 
> >       Hence why i would rather that the revert would only happen if
> >       it is clear that the infrastructure is not ready or can not
> >       be use in timely (over couple kernel release) fashion by any
> >       drivers.
> 
> This seems too generous to me, but in the interest of moving this
> discussion forward let's cross that bridge if/when it happens.
> Hopefully the threat of this debate recurring means consumers put in
> the due diligence to get things merged at infrastructure + 1 time.

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