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[80.187.83.189]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4255d8a6daasm34080730f8f.7.2025.10.09.02.19.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 09 Oct 2025 02:19:05 -0700 (PDT) Message-ID: <129379f6-18c7-4d10-8241-8c6c5596d6d5@redhat.com> Date: Thu, 9 Oct 2025 11:19:02 +0200 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: Zi Yan , Alexei Starovoitov , Johannes Weiner , 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 Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 43CDC180005 X-Stat-Signature: i8f9tcn1c6agun8o83bh9z9gxbto7fbu X-Rspam-User: X-HE-Tag: 1760001550-777848 X-HE-Meta: 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 aH8yLCH9 IlglhodzPjBd6Xqk8o7t036f6aOWf2B6QANw6REegVVTBjdNdqX0ZxAEkwAkdQtoIDepMQiuj11PM7BzIK/pJZq8SeYRtIZ1RKzfjAyxIrtwqTRO9Jtl5oK8R5FXsr7ECgOUDs58VnpbL6N6I9FaHNpTcdg7vur3qf3dWoeSSV7TK71nDTfKzczFNisANPDr1EFSKjOux8PZCVTs= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 08.10.25 15:11, Yafang Shao wrote: > On Wed, Oct 8, 2025 at 8:07 PM David Hildenbrand wrote: >> >> 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. > > Our standard is to run only one instance of a BPF program type > system-wide to avoid conflicts. For example, we can't have both > systemd and a container runtime running bpf-thp simultaneously. Right, it's a good question how to combine policies, or "who wins". > > Perhaps Alexei can enlighten us, though we'd need to read between his > characteristically brief lines. ;-) There might be some insights to be had in the bpf OOM discussion at https://lkml.kernel.org/r/CAEf4BzafXv-PstSAP6krers=S74ri1+zTB4Y2oT6f+33yznqsA@mail.gmail.com I didn't completely read through that, but that discussion also seems to be about interaction between cgroups and bpd programs. -- Cheers David / dhildenb