Re: [RFC] mm: MADV_COLLAPSE semantics

[Yang Shi <shy828301@gmail.com>]

On Fri, May 27, 2022 at 2:46 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Thu 26-05-22 10:39:42, Yang Shi wrote:
> > On Thu, May 26, 2022 at 12:12 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Wed 25-05-22 10:32:44, Yang Shi wrote:
> > > > On Wed, May 25, 2022 at 1:24 AM Michal Hocko <mhocko@suse.com> wrote:
> > > > >
> > > > > On Mon 23-05-22 17:18:32, Zach O'Keefe wrote:
> > > > > [...]
> > > > > > Idea: MADV_COLLAPSE should respect VM_NOHUGEPAGE and "never" THP mode,
> > > > > > but otherwise would attempt to collapse.
> > > > >
> > > > > I do agree that {process_}madvise should fail on VM_NOHUGEPAGE. The
> > > > > process has explicitly noted that THP shouldn't be used on such a VMA
> > > > > and seeing THP could be observed as not complying with that contract.
> > > > >
> > > > > I am not so sure about the global "never" policy, though. The global
> > > > > policy controls _kernel_ driven THPs. As the request to collapse memory
> > > > > comes from the userspace I do not think it should be limited by the
> > > > > kernel policy. I also think it can be beneficial to implement userspace
> > > > > based THP policies and exclude any kernel interference and that could be
> > > > > achieved by global kernel "never" policy and implement the whole
> > > > > functionality by process_madvise.
> > > >
> > > > I'd prefer to respect "never" for now since it is typically used to
> > > > disable THP globally even though the mappings are madvised
> > > > (MADV_HUGEPAGE). IMHO I treat MADV_COLLAPSE as weaker MADV_HUGEPAGE
> > > > (take effect for non-madvised mappings but not flip VM_NOHUGEPAGE) +
> > > > best-effort synchronous THP collapse.
> > >
> > > MADV_HUGEPAGE is a way to tell the kernel what and how to do in future
> > > time by the kernel.  MADV_COLLAPSE is a way tell what the userspace want
> > > at the moment of the call. So I do not really think they are directly
> > > related in any way except they somehow control THP.
> > >
> > > The primary question here is whether we want to support usecases which
> > > want to completely rule out THP handling by the kernel and only rely on
> > > the userspace. If yes, I do not see other way than using never global
> > > policy and rely on MADV_COLLAPSE from the userspace. Or am I missing
> > > something?
> >
> > I'm not sure whether we want to reach that eventually.
>
> My experience tells me that sooner or later somebody comes with a
> usecase for that. We are are not sure that is just a sign somebody will
> have that idea. So either we have very good reasons to not allow that
> possibility now and ideally we also document that or we should simply
> assume it will happen.

Yeah, it is definitely possible and nothing prevents that from happening.

>
> > But isn't
> > "madvise" good enough? "madvise" also means to give the delegation to
> > the users IMHO. The users decide whether huge page is preferred or
> > not. The users could implement policies:
> >
> > No - MADV_NOHUGEPAGE
> > Yes - MADV_HUGEPAGE
> >
> > But the THP allocation is deferred to real access (page fault) or
> > khugepaged. So I treated MADV_COLLAPSE as weaker MAD_HUGEPAGE +
> > synchronous THP allocation.
>
> I really do not see any good reason to tightly couple kernel and user
> policies. Hints like MADV_{NO}HUGEPAGE are one thing and both kernel
> and userspace might decide to interpret them. But binding MADV_COLLAPSE
> to in kernel THP tunables just seems like pushing ourselves into the
> corner.

I don't mean we should tightly couple kernel and user policies. I
think it is about how "never" is treated. AFAICT, typically sys admins
tend to expect "never" as a global switch and they don't expect any
THP allocation should happen in "never" mode even though it is
requested by the users. Maybe they should not expect so in the first
place.

> --
> Michal Hocko
> SUSE Labs