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: <CAADnVQ+S590wKn0rdaDAHk=txQenXn6KyfhSZ3ks6vJA3nKrNg@mail.gmail.com>
Date: Tue, 7 Oct 2025 20:25:28 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Yafang Shao <laoar.shao@...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 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. bpf-thp didn't have any
production exposure. Everything that you said above is wishful thinking.
In actual production every programmable component needs to be
scoped in some way. One can argue that scheduling is a global
property too, yet sched-ext only works on a specific scheduling class.
All bpf program types are scoped except tracing, since kprobe/fentry
are global by definition, and even than multiple tracing programs
can be attached to the same kprobe.

> execution. In other words, it is functionally a variant of fmod_ret.

hid-bpf initially went with fmod_ret approach, deleted the whole thing
and redesigned it with _scoped_ struct-ops.

> If we allow multiple attachments and they return different values, how
> do we resolve the conflict?
>
> If one program returns order-9 and another returns order-1, which
> value should be chosen? Neither 1, 9, nor a combination (1 & 9) is
> appropriate.

No. If you cannot figure out how to stack multiple programs
it means that the api you picked is broken.

> A single global program is a natural and logical extension of the
> existing global /sys/kernel/mm/transparent_hugepage/ interface. It is
> a good fit for BPF-THP and avoids unnecessary complexity.

The Nack to single global prog is not negotiable.