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[2003:d8:2f1e:c400:a7bb:d0cb:2506:c714]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a1f58f28f7sm8464596f8f.41.2025.05.11.01.15.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 11 May 2025 01:15:55 -0700 (PDT) Message-ID: <913bdc9b-a3c2-401c-99d0-18799850db9e@redhat.com> Date: Sun, 11 May 2025 10:15:53 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/1] prctl: allow overriding system THP policy to always To: Zi Yan Cc: Johannes Weiner , Yafang Shao , Usama Arif , Andrew Morton , linux-mm@kvack.org, shakeel.butt@linux.dev, riel@surriel.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <96eccc48-b632-40b7-9797-1b0780ea59cd@gmail.com> <8E3EC5A4-4387-4839-926F-3655188C20F4@nvidia.com> <279d29ad-cbd6-4a0e-b904-0a19326334d1@gmail.com> <20250509051328.GF323143@cmpxchg.org> <41e60fa0-2943-4b3f-ba92-9f02838c881b@redhat.com> <20250509164654.GA608090@cmpxchg.org> <8A18FB29-CC41-456F-A80E-807984691F0F@nvidia.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <8A18FB29-CC41-456F-A80E-807984691F0F@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10.05.25 01:34, Zi Yan wrote: > On 9 May 2025, at 18:42, David Hildenbrand wrote: > >>>>>> - madvise >>>>>> The sysadmin gently encourages the use of THP, but it is only >>>>>> enabled when explicitly requested by the application. >>> >>> And this "user mode" or "manual mode", where applications self-manage >>> which parts of userspace they want to enroll. >>> >>> Both madvise() and unprivileged prctl() should work here as well, >>> IMO. There is no policy or security difference between them, it's just >>> about granularity and usability. >>> >>>>>> - never >>>>>> The sysadmin discourages the use of THP, and "its use is only permitted >>>>>> with explicit approval" . >>> >>> This one I don't quite agree with, and IMO conflicts with what David >>> is saying as well. >> >> Yeah ... "never" does not mean "sometimes" in my reality :) >> >>> >>>>> "never" so far means "no thps, no exceptions". We've had serious THP >>>>> issues in the past, where our workaround until we sorted out the issue >>>>> for affected customers was to force-disable THPs on that system during boot. >>>> >>>> Right, that reflects the current behavior. What we aim to enhance is >>>> by adding the requirement that "its use is only permitted with >>>> explicit approval." >>> >>> I think you're conflating a safety issue with a security issue. >>> >>> David is saying there can be cases where the kernel is broken, and >>> "never" is a production escape hatch to disable the feature until a >>> kernel upgrade for the fix is possible. In such a case, it doesn't >>> make sense to override this decision based on any sort of workload >>> policy, privileged or not. >>> >>> The way I understand you is that you want enrollment (and/or >>> self-management) only for blessed applications. Because you don't >>> generally trust workloads in the wild enough to switch the global >>> default away from "never", given the semantics of always/madvise. >> >> Assuming "never" means "never" and "always" means "always" ( crazy, right? :) ), could be make use of "madvise" mode, which essentially means "VM_HUGEPAGE" takes control? >> >> We'd need >> >> a) A way to enable THP for a process. Changing the default/vma settings to VM_HUGEPAGE as discussed using a prctl could work. >> >> b) A way to ignore VM_HUGEPAGE for processes. Maybe the existing prctl to force-disable THPs could work? > > This means process level control overrides VMA level control, which > overrides global control, right? > > Intuitively, it should be that VMA level control overrides process level > control, which overrides global control, namely finer granularly control > precedes coarse one. But some apps might not use VMA level control > (e.g., madvise) carefully, we want to override that. Maybe ignoring VMA > level control is what we want? Let's take a step back: Current behavior is 1) If anybody (global / process / VM) says "never" (never/PR_SET_THP_DISABLE/VM_NOHUGEPAGE), the behavior is "never". 2) In "madvise" mode, only "VM_HUGEPAGE" gets THP unless PR_SET_THP_DISABLE is set. So per-process overrides per-VMA. 3) In "always" mode, everything gets THP unless per-VMA (VM_NOHUGEPAGE) or per-process (PR_SET_THP_DISABLE) disables it. Interestingly, PR_SET_THP_DISABLE used to mimic exactly what I proposed for the other direction (change default of VM_HUGEPAGE), except that it wouldn't modify already existing mappings. Worth looking at 1860033237d4. Not sure if that commit was the right call, but it's the semantics we have today. That commit notes: "It should be noted, that the new implementation makes PR_SET_THP_DISABLE master override to any per-VMA setting, which was not the case previously." Whatever we do, we have to be careful to not create more mess or inconsistency. Especially, if anybody sets VM_NOHUGEPAGE or PR_SET_THP_DISABLE, we must not use THPs, ever. -- Cheers, David / dhildenb