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[2003:d8:2f03:5b00:f549:a879:b2d3:73ee]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-450d7fb8751sm12473845e9.29.2025.05.30.02.16.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 30 May 2025 02:16:52 -0700 (PDT) Message-ID: <6caefe0b-c909-4692-a006-7f8b9c0299a6@redhat.com> Date: Fri, 30 May 2025 11:16:51 +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 0/2] fix MADV_COLLAPSE issue if THP settings are disabled From: David Hildenbrand To: Ryan Roberts , Baolin Wang , akpm@linux-foundation.org, hughd@google.com Cc: lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, dev.jain@arm.com, ziy@nvidia.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org References: <05d60e72-3113-41f0-b81f-225397f06c81@arm.com> <9b1bac6c-fd9f-4dc1-8c94-c4da0cbb9e7f@arm.com> 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 30.05.25 11:10, David Hildenbrand wrote: > On 30.05.25 10:59, Ryan Roberts wrote: >> On 30/05/2025 09:44, David Hildenbrand wrote: >>> On 30.05.25 10:04, Ryan Roberts wrote: >>>> On 29/05/2025 09:23, Baolin Wang wrote: >>>>> As we discussed in the previous thread [1], the MADV_COLLAPSE will ignore >>>>> the system-wide anon/shmem THP sysfs settings, which means that even though >>>>> we have disabled the anon/shmem THP configuration, MADV_COLLAPSE will still >>>>> attempt to collapse into a anon/shmem THP. This violates the rule we have >>>>> agreed upon: never means never. This patch set will address this issue. >>>> >>>> This is a drive-by comment from me without having the previous context, but... >>>> >>>> Surely MADV_COLLAPSE *should* ignore the THP sysfs settings? It's a deliberate >>>> user-initiated, synchonous request to use huge pages for a range of memory. >>>> There is nothing *transparent* about it, it just happens to be implemented using >>>> the same logic that THP uses. >>>> >>>> I always thought this was a deliberate design decision. >>> >>> If the admin said "never", then why should a user be able to overwrite that? >> >> Well my interpretation would be that the admin is saying never *transparently* >> give anyone any hugepages; on balance it does more harm than good for my >> workloads. The toggle is called transparent_hugepage/enabled, after all. > > I'd say it's "enabling transparent huge pages" not "transparently > enabling huge pages". After all, these things are ... transparent huge > pages. > > But yeah, it's confusing. > >> >> Whereas MADV_COLLAPSE is deliberately applied to a specific region at an >> opportune moment in time, presumably because the user knows that the region >> *will* benefit and because that point in the execution is not sensitive to latency. > > Not sure if MADV_HUGEPAGE is really *that* different. > >> >> I see them as logically separate. >> >>> >>> The design decision I recall is that if VM_NOHUGEPAGE is set, we'll ignore that. >>> Because that was set by the app itself (MADV_NOHUEPAGE). >> >> Hmm, ok. My instinct would have been the opposite; MADV_NOHUGEPAGE means "I >> don't want the risk of latency spikes and memory bloat that THP can cause". Not >> "ignore my explicit requests to MADV_COLLAPSE". >> >> But if that descision was already taken and that's the current behavior then I >> agree we have an inconsistency with respect to the sysfs control. >> >> Perhaps we should be guided by real world usage - AIUI there is a cloud that >> disables THP at system level today (Google?). > The use case I am aware of for disabling it for debugging purposes. > Saved us quite some headake in the past at customer sites for > troubleshooting + workarounds ... > > > Let's take a look at the man page: > > MADV_COLLAPSE is independent of any sysfs (see sysfs(5)) setting > under /sys/kernel/mm/transparent_hugepage, both in terms of determining > THP eligibility, and allocation semantics. > > I recall we discussed that it should ignore the max_ptes_none/swap/shared. > > But "any" setting would include "enable" ... It kind-of contradicts the linked Documentation/admin-guide/mm/transhuge.rst, where we have this *beautiful* comment "Transparent Hugepage Support for anonymous memory can be entirely disable (mostly for debugging purposes". I mean, "entirely" is also pretty clear to me. I would assume that the man page of MADV_COLLAPSE should have talked about ignoring *khugepaged* toggles (max_ptes_none ...), at least that's what I recall from the discussions back then. -- Cheers, David / dhildenb