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[2003:d8:2f26:ba00:803:6ec5:9918:6fd]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b50dd02ebsm10113115e9.9.2025.08.21.13.43.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 21 Aug 2025 13:43:37 -0700 (PDT) Message-ID: <95012dfc-d82d-4ae2-b4cd-1e8dcf15e44b@redhat.com> Date: Thu, 21 Aug 2025 22:43:35 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 00/13] khugepaged: mTHP support To: Lorenzo Stoakes , Nico Pache Cc: Dev Jain , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, corbet@lwn.net, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, akpm@linux-foundation.org, baohua@kernel.org, willy@infradead.org, peterx@redhat.com, wangkefeng.wang@huawei.com, usamaarif642@gmail.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, kirill.shutemov@linux.intel.com, aarcange@redhat.com, raquini@redhat.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, tiwai@suse.de, will@kernel.org, dave.hansen@linux.intel.com, jack@suse.cz, cl@gentwo.org, jglisse@google.com, surenb@google.com, zokeefe@google.com, hannes@cmpxchg.org, rientjes@google.com, mhocko@suse.com, rdunlap@infradead.org, hughd@google.com References: <20250819134205.622806-1-npache@redhat.com> <38b37195-28c8-4471-bd06-951083118efd@arm.com> <0d9c6088-536b-4d7a-8f75-9be5f0faa86f@lucifer.local> <5bea5efa-2efc-4c01-8aa1-a8711482153c@lucifer.local> 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 > > The one thing we absolutely cannot have is a default that causes this > 'creeping' behaviour. This feels like shipping something that is broken and > alluding to it in the documentation. > > I spoke to David off-list and he gave some insight into this and perhaps > some reasonable means of avoiding an additional tunable. > > I don't want to rehash what he said as I think it's more productive for him > to reply when he has time but broadly I think how we handle this needs > careful consideration. > > To me it's clear that some sense of ratio is just immediately very very > confusing, but then again this interface is already confusing, as with much > of THP. > > Anyway I'll let David respond here so we don't loop around before he has a > chance to add his input. > > Cheers, Lorenzo > [Resending because Thunderbird decided to use the wrong smtp server] I've been summoned. As raised in the past, I would initially only support specific values here like 0 : Never collapse with any pte_none/zeropage 511 (HPAGE_PMD_NR - 1) / default : Always collapse, ignoring pte_none/zeropage Once could also easily support the value 255 (HPAGE_PMD_NR / 2- 1), but not sure if we have to add that for now. Because, as raised in the past, I'm afraid nobody on this earth has a clue how to set this parameter to values different to 0 (don't waste memory with khugepaged) and 511 (page fault behavior). If any other value is set, essentially pr_warn("Unsupported 'max_ptes_none' value for mTHP collapse"); for now and just disable it. -- Cheers David / dhildenb