Re: [PATCH 09/10] mm/hmm: allow to mirror vma of a file on a DAX backed filesystem

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>,
	Ralph Campbell <rcampbell@nvidia.com>,
	John Hubbard <jhubbard@nvidia.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 09/10] mm/hmm: allow to mirror vma of a file on a DAX backed filesystem
Date: Tue, 12 Mar 2019 17:46:51 -0700	[thread overview]
Message-ID: <CAPcyv4huAHnWoLQHhVRC_U6c-1DG2joOktA-ZWa-TQ1=KaTQLA@mail.gmail.com> (raw)
In-Reply-To: <20190313001018.GA3312@redhat.com>

On Tue, Mar 12, 2019 at 5:10 PM Jerome Glisse <jglisse@redhat.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@intel.com> wrote:
> >
> > > On Tue, Mar 12, 2019 at 12:06 PM Jerome Glisse <jglisse@redhat.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@redhat.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?

>
> 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.