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[2003:d8:2f37:2b00:948c:dd9f:29c8:73f4]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b8f6cd502sm15397475e9.1.2025.09.01.09.46.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 01 Sep 2025 09:46:51 -0700 (PDT) Message-ID: Date: Mon, 1 Sep 2025 18:46:44 +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 v10 00/13] khugepaged: mTHP support To: Baolin Wang , Dev Jain , Lorenzo Stoakes Cc: Nico Pache , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, ziy@nvidia.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> <95012dfc-d82d-4ae2-b4cd-1e8dcf15e44b@redhat.com> <2a141eef-46e2-46e1-9b0f-066ec537600d@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: 8bit On 29.08.25 03:55, Baolin Wang wrote: > > > On 2025/8/28 18:48, Dev Jain wrote: >> >> On 28/08/25 3:16 pm, Baolin Wang wrote: >>> (Sorry for chiming in late) >>> >>> On 2025/8/22 22:10, David Hildenbrand wrote: >>>>>> Once could also easily support the value 255 (HPAGE_PMD_NR / 2- 1), >>>>>> but not sure >>>>>> if we have to add that for now. >>>>> >>>>> Yeah not so sure about this, this is a 'just have to know' too, and >>>>> yes you >>>>> might add it to the docs, but people are going to be mightily >>>>> confused, esp if >>>>> it's a calculated value. >>>>> >>>>> I don't see any other way around having a separate tunable if we >>>>> don't just have >>>>> something VERY simple like on/off. >>>> >>>> Yeah, not advocating that we add support for other values than 0/511, >>>> really. >>>> >>>>> >>>>> Also the mentioned issue sounds like something that needs to be >>>>> fixed elsewhere >>>>> honestly in the algorithm used to figure out mTHP ranges (I may be >>>>> wrong - and >>>>> happy to stand corrected if this is somehow inherent, but reallly >>>>> feels that >>>>> way). >>>> >>>> I think the creep is unavoidable for certain values. >>>> >>>> If you have the first two pages of a PMD area populated, and you >>>> allow for at least half of the #PTEs to be non/zero, you'd collapse >>>> first a >>>> order-2 folio, then and order-3 ... until you reached PMD order. >>>> >>>> So for now we really should just support 0 / 511 to say "don't >>>> collapse if there are holes" vs. "always collapse if there is at >>>> least one pte used". >>> >>> If we only allow setting 0 or 511, as Nico mentioned before, "At 511, >>> no mTHP collapses would ever occur anyway, unless you have 2MB >>> disabled and other mTHP sizes enabled. Technically, at 511, only the >>> highest enabled order would ever be collapsed." >> I didn't understand this statement. At 511, mTHP collapses will occur if >> khugepaged cannot get a PMD folio. Our goal is to collapse to the >> highest order folio. > > Yes, I’m not saying that it’s incorrect behavior when set to 511. What I > mean is, as in the example I gave below, users may only want to allow a > large order collapse when the number of present PTEs reaches half of the > large folio, in order to avoid RSS bloat. How do these users control allocation at fault time where this parameter is completely ignored? -- Cheers David / dhildenb