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[2003:d8:2f47:4700:e6f9:f453:9ece:7602]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a35ca8d185sm1956924f8f.98.2025.05.16.00.45.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 16 May 2025 00:45:18 -0700 (PDT) Message-ID: <8f0a22c2-3176-4942-994d-58d940901ecf@redhat.com> Date: Fri, 16 May 2025 09:45:17 +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 1/6] prctl: introduce PR_THP_POLICY_DEFAULT_HUGE for the process To: Lorenzo Stoakes Cc: Usama Arif , Andrew Morton , linux-mm@kvack.org, hannes@cmpxchg.org, shakeel.butt@linux.dev, riel@surriel.com, ziy@nvidia.com, laoar.shao@gmail.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kernel-team@meta.com References: <20250515133519.2779639-1-usamaarif642@gmail.com> <20250515133519.2779639-2-usamaarif642@gmail.com> <02ead03b-339b-45c8-b252-d31a66501c39@lucifer.local> <3a2a329d-2592-4e31-a763-d87dcd925966@redhat.com> <8ea288f2-5196-41f9-bd65-e29f22bb29e8@lucifer.local> <5f77366d-e100-46bb-ac85-aa4b216eb2cf@redhat.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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 15.05.25 22:35, Lorenzo Stoakes wrote: > On Thu, May 15, 2025 at 09:12:13PM +0200, David Hildenbrand wrote: >> On 15.05.25 20:08, Lorenzo Stoakes wrote: >>> On Thu, May 15, 2025 at 06:11:55PM +0200, David Hildenbrand wrote: >>>>>>> So if you're not overriding VM_NOHUGEPAGE, the whole point of this exercise >>>>>>> is to override global 'never'? >>>>>>> >>>>>> >>>>>> Again, I am not overriding never. >>>>>> >>>>>> hugepage_global_always and hugepage_global_enabled will evaluate to false >>>>>> and you will not get a hugepage. >>>>> >>>>> Yeah, again ack, but I kind of hate that we set VM_HUGEPAGE everywhere even >>>>> if the policy is never. >>>> >>>> I think it should behave just as if someone does manually an madvise(). So >>>> whatever we do here during an madvise, we should try to do the same thing >>>> here. >>> >>> Ack I agree with this. >>> >>> It actually simplifies things a LOT to view it this way - we're saying 'by >>> default apply madvise(...) to new VMAs'. >>> >>> Hm I wonder if we could have a more generic version of this... >>> >>> Note though that we're not _quite_ doing this. >>> >>> So in hugepage_madvise(): >>> >>> int hugepage_madvise(struct vm_area_struct *vma, >>> unsigned long *vm_flags, int advice) >>> { >>> ... >>> >>> switch (advice) { >>> case MADV_HUGEPAGE: >>> *vm_flags &= ~VM_NOHUGEPAGE; >>> *vm_flags |= VM_HUGEPAGE; >>> >>> ... >>> >>> break; >>> >>> ... >>> } >>> >>> ... >>> } >>> >>> So here we're actually clearing VM_NOHUGEPAGE and overriding it, but in the >>> proposed code we're not. >> >> Yeah, I think I suggested that, but probably we should just do exactly what >> madvise() does. > > Yes, agreed. > > Usama - do you have any issue with us switching to how madvise() does it? > >> >>> >>> So we're back into confusing territory again :) >>> >>> I wonder if we could... >>> >>> 1. Add an MADV_xxx that mimics the desired behaviour here. >>> >>> 2. Add a generic 'madvise() by default' thing at a process level? >>> >>> Is this crazy? >> >> I think that's what I had in mind, just a bit twisted. >> >> What could work is >> >> 1) prctl to set the default >> >> 2) madvise() to adjust all existing VMAs >> >> >> We might have to teach 2) to ignore non-compatible VMAs / holes. Maybe not, >> worth an investigation. > > Yeah, I think it'd _probably_ be ok except on s390 (which can fail, and so > we'd have to be able to say - skip on error, carry on). > > We'll just get an -ENOMEM at the end for the gaps (god how I hate > that). Otherwise I don't think MADV_HUGEPAGE actually is really that > restrictive. > > That would simplify :) > > But I still so hate using prctl()... this might be one of those cases where > we simply figure out we have no other choice. > > But when you put it as simply as this maybe it's not so bad. With the > flags2 gone by fixing this stupid 32-bit limit it's less awful. > > Perhaps worth seeing what an improved RFC of this series looks like with > all the various bits fixed to give an idea. Yes. > > But you do then wonder if we could make this _generic_ for _any_ madvise(), > and how _that_ would look. > > But perhaps that's insane because many VMAs would simply not be suited to > having certain madvise flags set hmm. Same thinking. I think this is rather special. In a perfect world not even the madvise(*HUGEPAGE) would exist. But here we are ... 14 years (wow!) after commit 0af4e98b6b095c74588af04872f83d333c958c32 Author: Andrea Arcangeli Date: Thu Jan 13 15:46:55 2011 -0800 thp: madvise(MADV_HUGEPAGE) (I'm surprised you don't complain about madvise(). IMHO, prctl() is even a better interface than catch-all madvise(); a syscall where an advise might not be an advise. I saw some funny rants about MADV_DONTNEED on reddit at some point ... :) mctrl() would have been clearer, at least for me :D ) -- Cheers, David / dhildenb