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

[Jason Gunthorpe <jgg@ziepe.ca>]

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.

> 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..

Jason