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[80.187.83.110]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46fa9d4caa4sm35056875e9.12.2025.10.08.05.07.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 08 Oct 2025 05:07:13 -0700 (PDT) Message-ID: Date: Wed, 8 Oct 2025 14:07:09 +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: Zi Yan , Yafang Shao , Alexei Starovoitov , Johannes Weiner Cc: Andrew Morton , baolin.wang@linux.alibaba.com, Lorenzo Stoakes , Liam Howlett , npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, 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> <7723a2c7-3750-44f7-9eb5-4ef64b64fbb8@redhat.com> <96AE1C18-3833-4EB8-9145-202517331DF5@nvidia.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 13:27, Zi Yan wrote: > On 8 Oct 2025, at 5:04, Yafang Shao wrote: > >> On Wed, Oct 8, 2025 at 4:28 PM David Hildenbrand wrote: >>> >>> 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. >> >> I'm still not entirely convinced that allowing individual processes or >> cgroups to run independent progs is a valid use case. However, since >> we have a consensus that this is the right direction, I will proceed >> with this approach. >> >>> >>> 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. >> >> 1. Global Approach >> - Pros: >> Simple; >> Can manage different THP policies for different cgroups or processes. >> - Cons: >> Does not allow individual processes to run their own BPF programs. >> >> 2. Per-Process Approach >> - Pros: >> Enables each process to run its own BPF program. >> - Cons: >> Introduces significant complexity, as it requires handling the >> BPF program's lifecycle (creation, destruction, inheritance) within >> every mm_struct. >> >> 3. Per-Cgroup Approach >> - Pros: >> Allows individual cgroups to run their own BPF programs. >> Less complex than the per-process model, as it can leverage the >> existing cgroup operations structure. >> - Cons: >> Creates a dependency on the cgroup subsystem. >> might not be easy to control at the per-process level. > > Another issue is that how and who to deal with hierarchical cgroup, where one > cgroup is a parent of another. Should bpf program to do that or mm code > to do that? I remember hierarchical cgroup is the main reason THP control > at cgroup level is rejected. If we do per-cgroup bpf control, wouldn't we > get the same rejection from cgroup folks? Valid point. I do wonder if that problem was already encountered elsewhere with bpf and if there is already a solution. Focusing on processes instead of cgroups might be easier initially. -- Cheers David / dhildenb