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

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Message-Id: <20190313090604.968100351b19338cacbfa3bc@linux-foundation.org>
Date:   Wed, 13 Mar 2019 09:06:04 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        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, 12 Mar 2019 20:10:19 -0400 Jerome Glisse <jglisse@...hat.com> wrote:

> > 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.

The patchset in -mm ("HMM updates for 5.1") has review from Ralph
Campbell @ nvidia.  Are there any other maintainers who we should have
feedback from?

>     - 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

Are there links to driver patchsets which we can add to the changelogs?

> 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.
>       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.

I agree that this should be more a philosophy than a set of hard rules.