linux-kernel - Re: [PATCH v9 mm-new 03/11] mm: thp: add support for BPF based THP order selection

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Message-ID: <CALOAHbCOQ_S-b+8pMxwGsjoc9QnAdij=gjkPEm0--cR7iCRQ3w@mail.gmail.com>
Date: Wed, 8 Oct 2025 14:02:34 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, ziy@...dia.com, 
	baolin.wang@...ux.alibaba.com, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Liam Howlett <Liam.Howlett@...cle.com>, npache@...hat.com, ryan.roberts@....com, 
	dev.jain@....com, Johannes Weiner <hannes@...xchg.org>, usamaarif642@...il.com, 
	gutierrez.asier@...wei-partners.com, Matthew Wilcox <willy@...radead.org>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Amery Hung <ameryhung@...il.com>, 
	David Rientjes <rientjes@...gle.com>, Jonathan Corbet <corbet@....net>, 21cnbao@...il.com, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Tejun Heo <tj@...nel.org>, lance.yang@...ux.dev, 
	Randy Dunlap <rdunlap@...radead.org>, bpf <bpf@...r.kernel.org>, 
	linux-mm <linux-mm@...ck.org>, "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 mm-new 03/11] mm: thp: add support for BPF based THP
 order selection

On Wed, Oct 8, 2025 at 12:39 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Oct 7, 2025 at 9:25 PM Yafang Shao <laoar.shao@...il.com> wrote:
> >
> > On Wed, Oct 8, 2025 at 12:10 PM Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> > >
> > > On Tue, Oct 7, 2025 at 8:51 PM Yafang Shao <laoar.shao@...il.com> wrote:
> > > >
> > > > On Wed, Oct 8, 2025 at 11:25 AM Alexei Starovoitov
> > > > <alexei.starovoitov@...il.com> wrote:
> > > > >
> > > > > On Tue, Oct 7, 2025 at 1:47 AM Yafang Shao <laoar.shao@...il.com> wrote:
> > > > > > has shown that multiple attachments often introduce conflicts. This is
> > > > > > precisely why system administrators prefer to manage BPF programs with
> > > > > > a single manager—to avoid undefined behaviors from competing programs.
> > > > >
> > > > > I don't believe this a single bit.
> > > >
> > > > You should spend some time seeing how users are actually applying BPF
> > > > in practice. Some information for you :
> > > >
> > > > https://github.com/bpfman/bpfman
> > > > https://github.com/DataDog/ebpf-manager
> > > > https://github.com/ccfos/huatuo
> > >
> > > By seeing the above you learned the wrong lesson.
> > > These orchestrators and many others were created because
> > > we made mistakes in the kernel by not scoping the progs enough.
> > > XDP is a prime example. It allows one program per netdev.
> > > This was a massive mistake which we're still trying to fix.
> >
> > Since we don't use XDP in production, I can't comment on it. However,
> > for our multi-attachable cgroup BPF programs, a key issue arises: if a
> > program has permission to attach to one cgroup, it can attach to any
> > cgroup. While scoping enables attachment to individual cgroups, it
> > does not enforce isolation. This means we must still check for
> > conflicts between programs, which begs the question: what is the
> > functional purpose of this scoping mechanism?
>
> cgroup mprog was added to remove the need for an orchestrator.

However, this approach would still require a userspace manager to
coordinate the mprog attachments and prevent conflicts between
different programs, no ?

>
> > My position is that the only valid scope for bpf-thp is at the level
> > of specific THP modes like madvise and always. This patch correctly
> > implements that precise design.
>
> I'm done with this thread.
>
> Nacked-by: Alexei Starovoitov <ast@...nel.org>

Given its experimental status, I believe any scoping mechanism would
be premature and over-engineered. Even integrating it into the
mm_struct introduces unnecessary complexity at this stage.

-- 
Regards
Yafang