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[2003:d8:2f03:5b00:f549:a879:b2d3:73ee]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-450d7fc281csm14756015e9.40.2025.05.30.03.45.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 30 May 2025 03:45:43 -0700 (PDT) Message-ID: <290b753a-47be-4c3a-b775-f2c10a0bc536@redhat.com> Date: Fri, 30 May 2025 12:45:42 +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 To: Baolin Wang , Ryan Roberts , 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> <6caefe0b-c909-4692-a006-7f8b9c0299a6@redhat.com> <19faab84-dd8e-4dfd-bf91-80bcb4a34fe8@linux.alibaba.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 30.05.25 11:52, Baolin Wang wrote: > > > On 2025/5/30 17:16, David Hildenbrand wrote: >> 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). > > IIUC, MADV_COLLAPSE does not ignore the VM_NOHUGEPAGE setting, if we set > VM_NOHUGEPAGE, then MADV_COLLAPSE will not be allowed to collapse a THP. > See: > __thp_vma_allowable_orders() ---> vma_thp_disabled() Interesting, maybe I misremember things. Maybe because process_madvise() could try MADV_COLLAPSE on a different process. And if that process as VM_NOHUGEPAGE set, it could be problematic. -- Cheers, David / dhildenb