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: <27e002e3-b39f-40f9-b095-52da0fbd0fc7@redhat.com>
Date: Wed, 8 Oct 2025 10:08:33 +0200
From: David Hildenbrand <david@...hat.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
 Yafang Shao <laoar.shao@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 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 03.10.25 04:18, Alexei Starovoitov wrote:
> On Mon, Sep 29, 2025 at 10:59 PM Yafang Shao <laoar.shao@...il.com> wrote:
>>
>> +unsigned long bpf_hook_thp_get_orders(struct vm_area_struct *vma,
>> +                                     enum tva_type type,
>> +                                     unsigned long orders)
>> +{
>> +       thp_order_fn_t *bpf_hook_thp_get_order;
>> +       int bpf_order;
>> +
>> +       /* No BPF program is attached */
>> +       if (!test_bit(TRANSPARENT_HUGEPAGE_BPF_ATTACHED,
>> +                     &transparent_hugepage_flags))
>> +               return orders;
>> +
>> +       rcu_read_lock();
>> +       bpf_hook_thp_get_order = rcu_dereference(bpf_thp.thp_get_order);
>> +       if (WARN_ON_ONCE(!bpf_hook_thp_get_order))
>> +               goto out;
>> +
>> +       bpf_order = bpf_hook_thp_get_order(vma, type, orders);
>> +       orders &= BIT(bpf_order);
>> +
>> +out:
>> +       rcu_read_unlock();
>> +       return orders;
>> +}
> 
> I thought I explained it earlier.
> Nack to a single global prog approach.

I agree. We should have the option to either specify a policy globally, 
or more refined for cgroups/processes.

It's an interesting question if a program would ever want to ship its 
own policy: I can see use cases for that.

So I agree that we should make it more flexible right from the start.

-- 
Cheers

David / dhildenb