Re: [RFC] mm: MADV_COLLAPSE semantics

["Zach O'Keefe" <zokeefe@google.com>]

On Tue, May 24, 2022 at 6:26 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Mon, May 23, 2022 at 05:18:32PM -0700, Zach O'Keefe wrote:
> > (*) If we could verify that "never" THP mode was used _only_ for
> > debugging, then I'd actually opt to ignore "never" in MADV_COLLAPSE.
>
> Some real time users may have used thp=never to make sure there's no
> pgtable uncertainty in all cases (and pages will always be mlocked for the
> RT apps, so pre-faulted).
>

Thanks for the great example here!

> Debattably it's the same as TRANSPARENT_HUGEPAGE=n but the user might want
> to use the same kernel with other purpose where thp could still be wanted?
> I've no solid clue.  It's just that as long as we have the knob taking
> "never" as an option then people may be using it, I'm afraid.
>
> "no" is indeed stronger than "yes" in many cases, at least for thp it's
> always like that: thp=never will guarantee no thp globally, while
> thp=always will only provide thp when it's still possible.  The same to
> MADV_[NO]HUGEPAGE but just for vmas.  From that POV I think your current
> plan looks reasonable on respecting "no"s more than "yes"s for both layers.
>

This makes sense to me. Best to be safe / follow existing "strong no"
convention.

Again, thanks for taking the time to read and provide feedback - very
much appreciated.

Best,
Zach

> Thanks,
>
> --
> Peter Xu
>