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[87.161.171.222]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a6e8051153sm4063968f8f.7.2025.06.25.03.07.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 25 Jun 2025 03:07:13 -0700 (PDT) Message-ID: <5de8ca8e-f293-4bfc-b49d-6e2a2e049f2b@redhat.com> Date: Wed, 25 Jun 2025 12:07:12 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/2] fix MADV_COLLAPSE issue if THP settings are disabled To: Baolin Wang , Lorenzo Stoakes Cc: Hugh Dickins , akpm@linux-foundation.org, ziy@nvidia.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, zokeefe@google.com, shy828301@gmail.com, usamaarif642@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <75c02dbf-4189-958d-515e-fa80bb2187fc@google.com> <008ec97f-3b33-4b47-a112-9cef8c1d7f58@redhat.com> <01d679f2-fd64-472c-b9dc-ebe87268ce82@lucifer.local> <23b8ad10-cd1f-45df-a25c-78d01c8af44f@redhat.com> <37fc3553-0d5a-4bdc-b473-cd740d47598e@linux.alibaba.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: A624D100010 X-Rspamd-Server: rspam10 X-Stat-Signature: p3xg3156mwphmyxsf1rfzwfpdaxmi4ya X-HE-Tag: 1750846038-58191 X-HE-Meta: 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 1abYjn8Q Q6fb1bFGSbcNMWQURxuJSqG3DmBRgTFgUXp8/YFR8NQmxJH3f6QohTAx6WWlqSYpkHVlQ+vtXeIWb5AjI2TSkCejg1J83vX6PDyHGijgpUu7yioQ4H19fGIJc+V2+MSWoDF+zVz0z7dzxghB4mM8qK57VLIPXF/xZPaRPK0KZvjQPOWEDhXP3qWe998hJCSeh2AG5oBOhY8SZ237D451PvFDHbvNpEYtaDs6oFT4ewrdOuvRsRXC+f41nh2xFToZ5ntmTLpgK0/CyZdH5EJVjwanpxARnzGpRr6Y4sv6TdF0Gm4NJVlkzgUFn/6sSoR7hMUX5IEFk48PYqSECrMVom3/PeM4zUDtnStAR9zq3VMRZzas2lEwXQEXqU+0lugy7qxVQZAIvpOtPTdB/0dOqanltfU4WHKUjRMyYFlzSKHKTEE6ftYjde+k4FChzL2LYYTEUr7uWdJOEOKhtokji5LO2XQ== 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 25.06.25 12:02, Baolin Wang wrote: > > > On 2025/6/25 17:31, Lorenzo Stoakes wrote: >> On Wed, Jun 25, 2025 at 04:52:03PM +0800, Baolin Wang wrote: >>> >>> >>> On 2025/6/25 16:37, Lorenzo Stoakes wrote: >>>> Yeah maybe the best way is to just have another tunable for this? >>>> >>>> /sys/kernel/mm/transparent_hugepage/disable_collapse perhaps? >>>> >>>> What do you think Hugh, Baolin? >>> >>> I think it's not necessary to find a way to disable madvise_collapse. >>> Essentially, it's a conflict between the semantics of madvise_collapse and >>> the '/sys/kernel/mm/transparent_hugepage/enabled' interface. We should reach >>> a consensus on the semantics first: >>> >>> Semantic 1: madv_collapse() should ignore any THP system settings, meaning >>> we need to update the 'never' semantics in >>> '/sys/kernel/mm/transparent_hugepage/enabled', which would only disable page >>> fault and khugepaged, not including madvise_collapse. If we agree on this, >>> then the 'never' for per-sized mTHP would have the same semantics, i.e., >>> when I set 64K mTHP to 'always' and 2M mTHP to 'never', madvise_collapse >>> would still allow the collapse of 2M THP. We should document this clearly in >>> case users still want 64K mTHP from madvise_collapse. >> >> Right yeah, I mean this is in effect how things are now. So the task is >> documentation. >> >>> >>> >>> Semantic 2: madv_collapse() needs to respect THP system settings, which is >>> what my patch does. Never means never, and we would need to update the >>> documentation of madv_collapse() to make it clearer. >> >> Yes, and indeed this is the choice. >> >> I think, as David said, it comes down to whether we have a legit use case that >> truly relies on this. >> >>>> (One side note on PMD-sized MADV_COLLAPSE - this is basically completely >>>> useless for 64 KB page size arm64 systems where PMD's are 512 MB :) >>>> >>>> Thoughts Baolin? >>> >>> We should not collapse 512MB THP on 64K pagesize kernel. So seems >>> madv_collapse() can not work on 64K pagesize kernel. >> >> Well I don't think anything would prevent this now right? So MADV_COLLAPSE is >> pretty problematic on 64K pagesize kernels in general. > > Yes, I don't mean it will prevent madvise_collapse(), just as you said > that it could be problematic (it's horrible to try to collapse 512MB). Well, assume you have a VM at that is 2 GiB and could use 4 THPs. It's stupid, but there might be some selected use cases where it's not completely stupid. -- Cheers, David / dhildenb