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Date:   Wed, 6 Mar 2019 08:06:06 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jerome Glisse <jglisse@...hat.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 Wed, Mar 6, 2019 at 8:03 AM Jerome Glisse <jglisse@...hat.com> wrote:
>
> On Wed, Mar 06, 2019 at 07:57:30AM -0800, Dan Williams wrote:
> > On Wed, Mar 6, 2019 at 7:51 AM Jerome Glisse <jglisse@...hat.com> wrote:
> > >
> > > On Tue, Mar 05, 2019 at 08:20:10PM -0800, Dan Williams wrote:
> > > > On Tue, Mar 5, 2019 at 2:16 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> > > > >
> > > > > On Wed, 30 Jan 2019 21:44:46 -0800 Dan Williams <dan.j.williams@...el.com> wrote:
> > > > >
> > > > > > >
> > > > > > > > Another way to help allay these worries is commit to no new exports
> > > > > > > > without in-tree users. In general, that should go without saying for
> > > > > > > > any core changes for new or future hardware.
> > > > > > >
> > > > > > > I always intend to have an upstream user the issue is that the device
> > > > > > > driver tree and the mm tree move a different pace and there is always
> > > > > > > a chicken and egg problem. I do not think Andrew wants to have to
> > > > > > > merge driver patches through its tree, nor Linus want to have to merge
> > > > > > > drivers and mm trees in specific order. So it is easier to introduce
> > > > > > > mm change in one release and driver change in the next. This is what
> > > > > > > i am doing with ODP. Adding things necessary in 5.1 and working with
> > > > > > > Mellanox to have the ODP HMM patch fully tested and ready to go in
> > > > > > > 5.2 (the patch is available today and Mellanox have begin testing it
> > > > > > > AFAIK). So this is the guideline i will be following. Post mm bits
> > > > > > > with driver patches, push to merge mm bits one release and have the
> > > > > > > driver bits in the next. I do hope this sound fine to everyone.
> > > > > >
> > > > > > The track record to date has not been "merge HMM patch in one release
> > > > > > and merge the driver updates the next". If that is the plan going
> > > > > > forward that's great, and I do appreciate that this set came with
> > > > > > driver changes, and maintain hope the existing exports don't go
> > > > > > user-less for too much longer.
> > > > >
> > > > > Decision time.  Jerome, how are things looking for getting these driver
> > > > > changes merged in the next cycle?
> > > > >
> > > > > Dan, what's your overall take on this series for a 5.1-rc1 merge?
> > > >
> > > > My hesitation would be drastically reduced if there was a plan to
> > > > avoid dangling unconsumed symbols and functionality. Specifically one
> > > > or more of the following suggestions:
> > > >
> > > > * EXPORT_SYMBOL_GPL on all exports to avoid a growing liability
> > > > surface for out-of-tree consumers to come grumble at us when we
> > > > continue to refactor the kernel as we are wont to do.
> > > >
> > > > * A commitment to consume newly exported symbols in the same merge
> > > > window, or the following merge window. When that goal is missed revert
> > > > the functionality until such time that it can be consumed, or
> > > > otherwise abandoned.
> > > >
> > > > * No new symbol exports and functionality while existing symbols go unconsumed.
> > > >
> > > > These are the minimum requirements I would expect my work, or any
> > > > core-mm work for that matter, to be held to, I see no reason why HMM
> > > > could not meet the same.
> > >
> > > nouveau use all of this and other driver patchset have been posted to
> > > also use this API.
> > >
> > > > On this specific patch I would ask that the changelog incorporate the
> > > > motivation that was teased out of our follow-on discussion, not "There
> > > > is no reason not to support that case." which isn't a justification.
> > >
> > > mlx5 wants to use HMM without DAX support it would regress mlx5. Other
> > > driver like nouveau also want to access DAX filesystem. So yes there is
> > > no reason not to support DAX filesystem. Why do you not want DAX with
> > > mirroring ? You want to cripple HMM ? Why ?
> >
> > There is a misunderstanding... my request for this patch was to update
> > the changelog to describe the merits of DAX mirroring to replace the
> > "There is no reason not to support that case." Otherwise someone
> > reading this changelog in a year will wonder what the motivation was.
>
> So what about:
>
> HMM mirroring allow device to mirror process address onto device
> there is no reason for that mirroring to not work if the virtual
> address are the result of an mmap of a file on DAX enabled file-
> system.

Looks like an improvement to me.

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