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: <7176597b-006f-40ad-9421-860d80d7e696@redhat.com>
Date: Mon, 13 Oct 2025 14:41:59 +0200
From: David Hildenbrand <david@...hat.com>
To: Yafang Shao <laoar.shao@...il.com>, Tejun Heo <tj@...nel.org>,
 Michal Hocko <mhocko@...e.com>, Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Zi Yan <ziy@...dia.com>, Alexei Starovoitov
 <alexei.starovoitov@...il.com>, Johannes Weiner <hannes@...xchg.org>,
 Andrew Morton <akpm@...ux-foundation.org>, 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, 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>,
 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

>> I came to the same conclusion. At least it's a valid start.
>>
>> Maybe we would later want a global fallback BPF-THP prog if none was
>> enabled for a specific MM.
> 
> good idea. We can fallback to the global model when attaching pid 1.
> 
>>
>> But I would expect to start with a per MM way of doing it, it gives you
>> way more flexibility in the long run.
> 
> THP, such as shmem and file-backed THP, are shareable across multiple
> processes and cgroups. If we allow different BPF-THP policies to be
> applied to these shared resources, it could lead to policy
> inconsistencies.

Sure, but nothing new about that (e.g., VM_HUGEPAGE, VM_NOHUGEPAGE, 
PR_GET_THP_DISABLE).

I'd expect that we focus on anon THP as the first step either way.

Skimming over this series, anon memory seems to be the main focus.

> This would ultimately recreate a long-standing issue
> in memcg, which still lacks a robust solution for this problem [0].
> 
> This suggests that applying SCOPED policies to SHAREABLE memory may be
> fundamentally flawed ;-)

Yeah, shared memory is usually more tricky: see mempolicy handling for 
shmem. There, the policy is much rather glued to a file than to a process.

-- 
Cheers

David / dhildenb