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[80.187.83.110]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4255d8f0853sm29247216f8f.50.2025.10.08.01.28.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 08 Oct 2025 01:28:25 -0700 (PDT) Message-ID: <7723a2c7-3750-44f7-9eb5-4ef64b64fbb8@redhat.com> Date: Wed, 8 Oct 2025 10:28:22 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v9 mm-new 03/11] mm: thp: add support for BPF based THP order selection To: Yafang Shao Cc: Alexei Starovoitov , Andrew Morton , ziy@nvidia.com, baolin.wang@linux.alibaba.com, Lorenzo Stoakes , Liam Howlett , npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, Johannes Weiner , usamaarif642@gmail.com, gutierrez.asier@huawei-partners.com, Matthew Wilcox , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Amery Hung , David Rientjes , Jonathan Corbet , 21cnbao@gmail.com, Shakeel Butt , Tejun Heo , lance.yang@linux.dev, Randy Dunlap , bpf , linux-mm , "open list:DOCUMENTATION" , LKML References: <20250930055826.9810-1-laoar.shao@gmail.com> <20250930055826.9810-4-laoar.shao@gmail.com> <27e002e3-b39f-40f9-b095-52da0fbd0fc7@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 08.10.25 10:18, Yafang Shao wrote: > On Wed, Oct 8, 2025 at 4:08 PM David Hildenbrand wrote: >> >> On 03.10.25 04:18, Alexei Starovoitov wrote: >>> On Mon, Sep 29, 2025 at 10:59 PM Yafang Shao 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. > > To achieve per-process granularity, the struct-ops must be embedded > within the mm_struct as follows: > > +#ifdef CONFIG_BPF_MM > +struct bpf_mm_ops { > +#ifdef CONFIG_BPF_THP > + struct bpf_thp_ops bpf_thp; > +#endif > +}; > +#endif > + > /* > * Opaque type representing current mm_struct flag state. Must be accessed via > * mm_flags_xxx() helper functions. > @@ -1268,6 +1281,10 @@ struct mm_struct { > #ifdef CONFIG_MM_ID > mm_id_t mm_id; > #endif /* CONFIG_MM_ID */ > + > +#ifdef CONFIG_BPF_MM > + struct bpf_mm_ops bpf_mm; > +#endif > } __randomize_layout; > > We should be aware that this will involve extensive changes in mm/. That's what we do on linux-mm :) It would be great to use Alexei's feedback/experience to come up with something that is flexible for various use cases. So I think this is likely the right direction. It would be great to evaluate which scenarios we could unlock with this (global vs. per-process vs. per-cgroup) approach, and how extensive/involved the changes will be. If we need a slot in the bi-weekly mm alignment session to brainstorm, we can ask Dave R. for one in the upcoming weeks. -- Cheers David / dhildenb