Re: [PATCH v2 00/12] mm: userspace hugepage collapse

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

On Sat, Apr 16, 2022 at 12:26 PM Peter Xu <peterx@redhat.com> wrote:
>
> Hi, Zach,
>
> On Fri, Apr 15, 2022 at 01:04:04PM -0700, Zach O'Keefe wrote:
> > On Fri, Apr 15, 2022 at 6:39 AM Peter Xu <peterx@redhat.com> wrote:
> > >
> > > On Thu, Apr 14, 2022 at 05:52:43PM -0700, Zach O'Keefe wrote:
> > > > Hey Peter,
> > > >
> > > > Thanks for taking the time to review!
> > > >
> > > > On Thu, Apr 14, 2022 at 5:04 PM Peter Xu <peterx@redhat.com> wrote:
> > > > >
> > > > > Hi, Zach,
> > > > >
> > > > > On Thu, Apr 14, 2022 at 11:06:00AM -0700, Zach O'Keefe wrote:
> > > > > > process_madvise(2)
> > > > > >
> > > > > >       Performs a synchronous collapse of the native pages
> > > > > >       mapped by the list of iovecs into transparent hugepages.
> > > > > >
> > > > > >       Allocation semantics are the same as khugepaged, and depend on
> > > > > >       (1) the active sysfs settings
> > > > > >       /sys/kernel/mm/transparent_hugepage/enabled and
> > > > > >       /sys/kernel/mm/transparent_hugepage/khugepaged/defrag, and (2)
> > > > > >       the VMA flags of the memory range being collapsed.
> > > > > >
> > > > > >       Collapse eligibility criteria differs from khugepaged in that
> > > > > >       the sysfs files
> > > > > >       /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_[none|swap|shared]
> > > > > >       are ignored.
> > > > >
> > > > > The userspace khugepaged idea definitely makes sense to me, though I'm
> > > > > curious how the line is drown on the different behaviors here by explicitly
> > > > > ignoring the max_ptes_* entries.
> > > > >
> > > > > Let's assume the initiative is to duplicate a more data-aware khugepaged in
> > > > > the userspace, then IMHO it makes more sense to start with all the policies
> > > > > that applies to khugepaged already, including max_pte_*.
> > > > >
> > > > > I can understand the willingness to provide even stronger semantics here
> > > > > than khugepaged since the userspace could have very clear knowledge of how
> > > > > to provision the memories (better than a kernel scanner).  It's just that
> > > > > IMHO it could be slightly confusing if the new interface only partially
> > > > > apply the khugepaged rules.
> > > > >
> > > > > No strong opinion here.  It could already been a trade-off after the
> > > > > discussion from the RFC with Michal which I read..  Just curious about how
> > > > > you made that design decision so feel free to read it as a pure question.
> > > > >
> > > >
> > > > Understand your point here. The allocation and max_pte_* semantics are
> > > > split between khugepaged-like and fault-like, respectively - which
> > > > could be confusing. Originally, I proposed a MADV_F_COLLAPSE_LIMITS
> > > > flag to control the former's behavior, but agreed to keep things
> > > > simple to start, and expand the interface if/when necessary. I opted
> > > > to ignore max_ptes_* as the default since I envisioned that early
> > > > adopters would "just want it to work". One such example would be
> > > > backing executable text by hugepages on program load when many pages
> > > > haven't been demand-paged in yet.
> > > >
> > > > What do you think?
> > >
> > > I'm just slightly worried that'll make the default MADV_COLLAPSE semantics
> > > blurred.
> > >
> > > To me, a clean default definition for MADV_COLLAPSE would be nice, as "do
> > > khugepaged on this range, and with current thread context".  IMHO any
> > > feature bits then can be supplementing special needs, and I'll take the thp
> > > backing executable example to be one of the (good?) reason we'd need an
> > > extra flag for ignoring the max_ptes_* knobs.
> > >
> > > So personally if I were you maybe I'll start with the simple scheme of that
> > > (even if it won't immediately service a thing) but then add either the
> > > defrag or ignore_max_ptes_* as feature bits later on, with clear use case
> > > descriptions about why we need each of the feature flags.  IMHO numbers
> > > would be even more helpful when there's specific use cases on the show.
> > >
> > > Or, perhaps you think all potential MADV_COLLAPSE users should literally
> > > skip max_ptes_* limitations always?
> > >
> >
> > Thanks for your time and valuable feedback here, Peter. I had a response typed
> > up, but after a few iterations became increasingly unsatisfied with my
> > own response.
> >
> > I think this feature should be able to stand on its own without
> > consideration of a userspace khugepaged, as we have existing concrete
> > examples where it would be useful. In these cases, and I assume almost
> > all other use-cases outside userspace khugepaged, max_ptes_* should be
> > ignored as the fundamental assumption of MADV_COLLAPSE is that the
> > user knows better, and IMHO, khugepaged heuristics shouldn't tell
> > users they are wrong.
>
> Valid point.  And actually right after I replied I thought similarly on
> whether we need to connect the two interfaces at all..
>
> It's just that it's very easy to go think like that after reading the cover
> letter since that's exactly what it is comparing to. :)
>

Yes, this is my fault :) After others have had a chance to review /
align, I'll include the immediate use cases
in the v3 cover letter as well, rather than deep in individual patch
messages.

> There's definitely a difference view on user/kernel level of things, then
> it sounds reasonable to me if we add a new interface it by default has a
> stronger semantics otherwise we may not bother if with MADV_HUGEPAGE's
> existance.
>

Yes, good point.

> So maybe max_ptes_* won't even make sense for MADV_COLLAPSE in most cases
> as you said.  And that's a real pure question I asked above, and I feel
> like your answer is actually "yes" we should always ignore the max_ptes_*
> fields until there's a proof that it'll be helpful.
>
> >
> > But this, as you mention, unsatisfactorily blurs the semantics of
> > MADV_COLLAPSE: "act like khugepaged here, but not here".
> >
> > As such, WDYT about the reverse-side of the coin of what you proposed:
> > to not couple the default behavior of MADV_COLLAPSE with khugepaged at
> > all? I.e. Not tie the allocation semantics to
> > /sys/kernel/mm/transparent_hugepage/khugepaged/defrag. We can add
> > flags as necessary when/if a reimplementation of khugepaged in
> > userspace proves fruitful.
>
> Let's see whether others have thoughts, but what you proposed here makes
> sense to me.
>

Great! Sounds good to me. Thank you again for your time, questions,
and feedback!

Best,

Zach

> Thanks,
>
> --
> Peter Xu
>