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From: Lance Yang <ioworker0@gmail.com>
Date: Sat, 27 Jan 2024 16:06:39 +0800
Message-ID: <CAK1f24=tfxgUpdPix1n-4fre70UVFhLbzEqEj7SthUMRJtT59g@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] mm/madvise: add MADV_F_COLLAPSE_LIGHT to process_madvise()
To: "Zach O'Keefe" <zokeefe@google.com>
Cc: Michal Hocko <mhocko@suse.com>, akpm@linux-foundation.org, david@redhat.com, 
	songmuchun@bytedance.com, shy828301@gmail.com, peterx@redhat.com, 
	mknyszek@google.com, minchan@kernel.org, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
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How about MADV_F_COLLAPSE_NODEFRAG?

On Sat, Jan 27, 2024 at 7:27=E2=80=AFAM Zach O'Keefe <zokeefe@google.com> w=
rote:
>
> On Mon, Jan 22, 2024 at 6:35=E2=80=AFAM Lance Yang <ioworker0@gmail.com> =
wrote:
> >
> > Hey Zach,
> >
> > What do you think about the semantic?
>
> Hey Lance,
>
> Sorry for the late reply.
>
> I can see both sides of the argument; though I would argue that
> "non-blocking" is equally as vague in this context. E.g. we'll "block" on
> acquiring a number of different locks along the collapse path.
>
> If you really want to talk about not entering direct reclaim /
> compaction, then keeping with the sys/kernel/vm/thp notion of "defrag"
> would be better, IMO. I don't feel that strongly about it though.
>
> But I see you've provided some more use cases in another mail, so let
> me pick up my thoughts over there.
>
> Best,
> Zach
>
>
>
> > Thanks,
> > Lance
> >
> > On Mon, Jan 22, 2024 at 10:14=E2=80=AFPM Lance Yang <ioworker0@gmail.co=
m> wrote:
> > >
> > > On Mon, Jan 22, 2024 at 9:50=E2=80=AFPM Michal Hocko <mhocko@suse.com=
> wrote:
> > > >
> > > > On Sat 20-01-24 10:09:32, Lance Yang wrote:
> > > > [...]
> > > > > Hey Michal,
> > > > >
> > > > > Thanks for your suggestion!
> > > > >
> > > > > It seems that the implementation should try but not too hard alig=
ns well
> > > > > with my desired behavior.
> > > >
> > > > The problem I have with this semantic is that it is really hard to
> > > > define and then stick with. Our implementation might change over ti=
me
> > > > and what somebody considers good ATM might turn int "trying harder =
than
> > > > I wanted" later on.
> > > >
> > > > > Non-blocking in general is also a great idea.
> > > > > Perhaps in the future, we can add a MADV_F_COLLAPSE_NOBLOCK
> > > > > flag for scenarios where latency is extremely critical.
> > > >
> > > > Non blocking semantic is much easier to define and maintain. The ac=
tual
> > > > allocation/compaction implementation might change as well over time=
 but
> > > > the userspace at least knows that the request will not block waitin=
g for
> > > > any required resources.
> > >
> > > I appreciate your insights!
> > >
> > > It makes sense that a non-blocking semantic is easier to define and m=
aintain,
> > > providing userspace with the certainty that requests won=E2=80=99t be=
 blocked.
> > >
> > > Thanks,
> > > Lance
> > >
> > > >
> > > > --
> > > > Michal Hocko
> > > > SUSE Labs