linux-kernel - Re: [PATCH v2 1/1] mm/madvise: add MADV_F_COLLAPSE_LIGHT to process

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Date: Fri, 26 Jan 2024 15:26:32 -0800
From: "Zach O'Keefe" <zokeefe@...gle.com>
To: Lance Yang <ioworker0@...il.com>
Cc: Michal Hocko <mhocko@...e.com>, akpm@...ux-foundation.org, david@...hat.com, 
	songmuchun@...edance.com, shy828301@...il.com, peterx@...hat.com, 
	mknyszek@...gle.com, minchan@...nel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v2 1/1] mm/madvise: add MADV_F_COLLAPSE_LIGHT to process_madvise()

On Mon, Jan 22, 2024 at 6:35 AM Lance Yang <ioworker0@...il.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 PM Lance Yang <ioworker0@...il.com> wrote:
> >
> > On Mon, Jan 22, 2024 at 9:50 PM Michal Hocko <mhocko@...e.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 aligns 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 time
> > > 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 actual
> > > allocation/compaction implementation might change as well over time but
> > > the userspace at least knows that the request will not block waiting for
> > > any required resources.
> >
> > I appreciate your insights!
> >
> > It makes sense that a non-blocking semantic is easier to define and maintain,
> > providing userspace with the certainty that requests won’t be blocked.
> >
> > Thanks,
> > Lance
> >
> > >
> > > --
> > > Michal Hocko
> > > SUSE Labs