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

[Dan Williams]

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.
>
> > 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.

> > Maybe we can start by at least not add any flags and just prevent
> > FOLL_LONGTERM on fsdax -- which I guess was the original purpose of
> > commit 7af75561e171 ("mm/gup: add FOLL_LONGTERM capability to GUP fast").
> > This patch (which I can formally send) has a sketch of that (below scissors mark):
> >
> > https://lore.kernel.org/linux-mm/6a18179e-65f7-367d-89a9-d5162f10fef0@oracle.com/
>
> Yes, basically, whatever test we want for 'deny fast gup foll
> longterm' is fine.
>
> Personally I'd like to see us move toward a set of flag specifying
> each special behavior and not a collection of types that imply special
> behaviors.
>
> Eg we have at least:
>  - Block gup fast on foll_longterm
>  - Capture the refcount ==1 and use the pgmap free hook
>    (confusingly called page_is_devmap_managed())
>  - Always use a swap entry
>  - page->index/mapping are used in the usual file based way?
>
> Probably more things..

Yes, agree with the principle of reducing type-implied special casing.