Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM

[Dan Williams]

On Tue, Nov 12, 2019 at 3:08 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
>
> On 11/12/19 2:43 PM, Dan Williams wrote:
> ...
> > Ah, sorry. This was the first time I had looked at this series and
> > jumped in without reading the background.
> >
> > Your patch as is looks ok, I assume you've removed the FOLL_LONGTERM
> > warning in get_user_pages_remote in another patch?
> >
>
> Actually, I haven't gone quite that far. Actually this patch is the last
> change to that function. Therefore, at the end of this patchset,
> get_user_pages_remote() ends up with this check in it which
> is a less-restrictive version of the warning:
>
>         /*
>          * Current FOLL_LONGTERM behavior is incompatible with
>          * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
>          * vmas. However, this only comes up if locked is set, and there are
>          * callers that do request FOLL_LONGTERM, but do not set locked. So,
>          * allow what we can.
>          */
>         if (gup_flags & FOLL_LONGTERM) {
>                 if (WARN_ON_ONCE(locked))
>                         return -EINVAL;
>         }
>
> Is that OK, or did you want to go further (possibly in a follow-up
> patchset, as I'm hoping to get this one in soon)?

That looks ok. Something to maybe push down into the core in a future
cleanup, but not something that needs to be done now.

> ...
> >>> I think check_vma_flags() should do the ((FOLL_LONGTERM | FOLL_GET) &&
> >>> vma_is_fsdax()) check and that would also remove the need for
> >>> __gup_longterm_locked.
> >>>
> >>
> >> Good idea, but there is still the call to check_and_migrate_cma_pages(),
> >> inside __gup_longterm_locked().  So it's a little more involved and
> >> we can't trivially delete __gup_longterm_locked() yet, right?
> >
> > [ add Aneesh ]
> >
> > Yes, you're right. I had overlooked that had snuck in there. That to
> > me similarly needs to be pushed down into the core with its own FOLL
> > flag, or it needs to be an explicit fixup that each caller does after
> > get_user_pages. The fact that migration silently happens as a side
> > effect of gup is too magical for my taste.
> >
>
> Yes. It's an intrusive side effect that is surprising, and not in a
> "happy surprise" way. :) .   Fixing up the CMA pages by splitting that
> functionality into separate function calls sounds like an improvement
> worth exploring.

Right, future work.