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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 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 Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 611A540006 X-Rspam-User: X-Stat-Signature: tbzfdtib9pigp1swump1rwuhzk5wuota X-HE-Tag: 1747381522-105062 X-HE-Meta: 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 zlJFiFAK 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 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 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