[PATCH v1 2/2] mm: remove extra ZONE_DEVICE struct page refcount

[Dan Williams]

On Wed, Oct 20, 2021 at 10:09 AM Joao Martins <joao.m.martins at oracle.com> wrote:
>
> On 10/19/21 20:21, Dan Williams wrote:
> > On Tue, Oct 19, 2021 at 9:02 AM Jason Gunthorpe <jgg at ziepe.ca> wrote:
> >>
> >> On Tue, Oct 19, 2021 at 04:13:34PM +0100, Joao Martins wrote:
> >>> On 10/19/21 00:06, Jason Gunthorpe wrote:
> >>>> On Mon, Oct 18, 2021 at 12:37:30PM -0700, Dan Williams wrote:
> >>>>
> >>>>>> device-dax uses PUD, along with TTM, they are the only places. I'm not
> >>>>>> sure TTM is a real place though.
> >>>>>
> >>>>> I was setting device-dax aside because it can use Joao's changes to
> >>>>> get compound-page support.
> >>>>
> >>>> Ideally, but that ideas in that patch series have been floating around
> >>>> for a long time now..
> >>>>
> >>> The current status of the series misses a Rb on patches 6,7,10,12-14.
> >>> Well, patch 8 too should now drop its tag, considering the latest
> >>> discussion.
> >>>
> >>> If it helps moving things forward I could split my series further into:
> >>>
> >>> 1) the compound page introduction (patches 1-7) of my aforementioned series
> >>> 2) vmemmap deduplication for memory gains (patches 9-14)
> >>> 3) gup improvements (patch 8 and gup-slow improvements)
> >>
> >> I would split it, yes..
> >>
> >> I think we can see a general consensus that making compound_head/etc
> >> work consistently with how THP uses it will provide value and
> >> opportunity for optimization going forward.
> >>
>
> I'll go do that. Meanwhile, I'll wait a couple days for Dan to review the
> dax subsystem patches (6 & 7), or otherwise send them over.
>
> >>> Whats the benefit between preventing longterm at start
> >>> versus only after mounting the filesystem? Or is the intended future purpose
> >>> to pass more context into an holder potential future callback e.g. nack longterm
> >>> pins on a page basis?
> >>
> >> I understood Dan's remark that the device-dax path allows
> >> FOLL_LONGTERM and the FSDAX path does not ?
> >>
> >> Which, IIRC, today is signaled basd on vma properties and in all cases
> >> fast-gup is denied.
> >
> > Yeah, I forgot that 7af75561e171 eliminated any possibility of
> > longterm-gup-fast for device-dax, let's not disturb that status quo.
> >
> I am slightly confused by this comment -- the status quo is what we are
> questioning here -- And we talked about changing that in the past too
> (thread below), that longterm-gup-fast was an oversight that that commit
> was only applicable to fsdax. [Maybe this is just my english confusion]

No, you have it correct. However that "regression" has gone unnoticed,
so unless there is data showing that gup-fast on device-dax is
critical for longterm page pinning workflows I'm ok for longterm to
continue to trigger gup-slow.