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[2003:d8:2f45:5500:8267:647f:4209:dedd]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a1f57dde27sm4664654f8f.17.2025.05.09.15.42.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 09 May 2025 15:43:01 -0700 (PDT) Message-ID: Date: Sat, 10 May 2025 00:42:59 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/1] prctl: allow overriding system THP policy to always To: Johannes Weiner , Yafang Shao Cc: Usama Arif , Zi Yan , 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> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: 7qgbdzerirdteyfosp7yxz1zibeho6bc X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 20C8C80004 X-Rspam-User: X-HE-Tag: 1746830584-725883 X-HE-Meta: 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 pELo7Fk0 Fsqyod36LP6yiChXl7uespEJsE5i86LJ5nhBu0Xq+6NUH4w5UaK/a7vfkG1yTZphhKZKHdj1/N2nbx8v538qf/0AVgEruminzc7H0CEn7o583RdLom+Guo0g1UrTrp5EmmljMfiI5qJRgJX0ALBaKlYJ5Z7EGAIZep2XTZq7EiwxRlc7iTfXlzq1eqms0eIRAXcPHojtBwgMQBIr3IyFnnIw5YNlBrwvgVS3Kct42SOqQCbF5io1obqz80z5wbQp4Y5jPxbXamw14tEHWl6Xu396r9Gna1cvbNHJoSqsIrjJvU0/uejA9NB1Pu5PcEdpgsJ+cJ3b2SADV9wi4lTWk/l0Uodh8Axn86ryoslVCV1LqvgTKjk3GE5KcsuRqZXMleg+o3UFuw9ZaetVckknS1FHLUVwaJqH0yzq5jA0KvN/EaiP+k9xuswczEc6kqs/BtcjrNUO+/tQt2fJLHy1tROp+C1n1X6/fufZz 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: >>>> - 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? -- Cheers, David / dhildenb