Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

Elliot Berman
2024-06-21T16:55:12Z

On Fri, Jun 21, 2024 at 11:16:31AM +0100, Fuad Tabba wrote:
> Hi David,
>
> On Fri, Jun 21, 2024 at 10:10 AM David Hildenbrand <david@redhat.com> wrote:
> >
> > On 21.06.24 10:54, Fuad Tabba wrote:
> > > Hi David,
> > >
> > > On Fri, Jun 21, 2024 at 9:44 AM David Hildenbrand <david@redhat.com> wrote:
> > >>
> > >>>> Again from that thread, one of most important aspects guest_memfd is that VMAs
> > >>>> are not required. Stating the obvious, lack of VMAs makes it really hard to drive
> > >>>> swap, reclaim, migration, etc. from code that fundamentally operates on VMAs.
> > >>>>
> > >>>> : More broadly, no VMAs are required. The lack of stage-1 page tables are nice to
> > >>>> : have; the lack of VMAs means that guest_memfd isn't playing second fiddle, e.g.
> > >>>> : it's not subject to VMA protections, isn't restricted to host mapping size, etc.
> > >>>>
> > >>>> [1] https://lore.kernel.org/all/Zfmpby6i3PfBEcCV@google.com
> > >>>> [2] https://lore.kernel.org/all/Zg3xF7dTtx6hbmZj@google.com
> > >>>
> > >>> I wonder if it might be more productive to also discuss this in one of
> > >>> the PUCKs, ahead of LPC, in addition to trying to go over this in LPC.
> > >>
> > >> I don't know in which context you usually discuss that, but I could
> > >> propose that as a topic in the bi-weekly MM meeting.
> > >>
> > >> This would, of course, be focused on the bigger MM picture: how to mmap,
> > >> how how to support huge pages, interaction with page pinning, ... So
> > >> obviously more MM focused once we are in agreement that we want to
> > >> support shared memory in guest_memfd and how to make that work with core-mm.
> > >>
> > >> Discussing if we want shared memory in guest_memfd might be betetr
> > >> suited for a different, more CC/KVM specific meeting (likely the "PUCKs"
> > >> mentioned here?).
> > >
> > > Sorry, I should have given more context on what a PUCK* is :) It's a
> > > periodic (almost weekly) upstream call for KVM.
> > >
> > > [*] https://lore.kernel.org/all/20230512231026.799267-1-seanjc@google.com/
> > >
> > > But yes, having a discussion in one of the mm meetings ahead of LPC
> > > would also be great. When do these meetings usually take place, to try
> > > to coordinate across timezones.
> >
> > It's Wednesday, 9:00 - 10:00am PDT (GMT-7) every second week.
> >
> > If we're in agreement, we could (assuming there are no other planned
> > topics) either use the slot next week (June 26) or the following one
> > (July 10).
> >
> > Selfish as I am, I would prefer July 10, because I'll be on vacation
> > next week and there would be little time to prepare.
> >
> > @David R., heads up that this might become a topic ("shared and private
> > memory in guest_memfd: mmap, pinning and huge pages"), if people here
> > agree that this is a direction worth heading.
>
> Thanks for the invite! Tentatively July 10th works for me, but I'd
> like to talk to the others who might be interested (pKVM, Gunyah, and
> others) to see if that works for them. I'll get back to you shortly.
>

I'd like to join too, July 10th at that time works for me.

- Elliot