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[2003:d8:2f22:1000:d72d:fd5f:4118:c70b]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38bf3275622sm10534150f8f.69.2025.01.20.04.54.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Jan 2025 04:54:15 -0800 (PST) Message-ID: <37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com> Date: Mon, 20 Jan 2025 13:54:13 +0100 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: [RFC 00/11] khugepaged: mTHP support To: Nico Pache , Ryan Roberts Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, anshuman.khandual@arm.com, catalin.marinas@arm.com, cl@gentwo.org, vbabka@suse.cz, mhocko@suse.com, apopple@nvidia.com, dave.hansen@linux.intel.com, will@kernel.org, baohua@kernel.org, jack@suse.cz, srivatsa@csail.mit.edu, haowenchao22@gmail.com, hughd@google.com, aneesh.kumar@kernel.org, yang@os.amperecomputing.com, peterx@redhat.com, ioworker0@gmail.com, wangkefeng.wang@huawei.com, ziy@nvidia.com, jglisse@google.com, surenb@google.com, vishal.moola@gmail.com, zokeefe@google.com, zhengqi.arch@bytedance.com, jhubbard@nvidia.com, 21cnbao@gmail.com, willy@infradead.org, kirill.shutemov@linux.intel.com, aarcange@redhat.com, raquini@redhat.com, dev.jain@arm.com, sunnanyong@huawei.com, usamaarif642@gmail.com, audra@redhat.com, akpm@linux-foundation.org References: <20250108233128.14484-1-npache@redhat.com> <40a65c5e-af98-45f9-a254-7e054b44dc95@arm.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 >> I think the 1 problem that emerged during review of Dev's series, which we don't >> have a proper solution to yet, is the issue of "creep", where regions can be >> collapsed to progressively higher orders through iterative scans. At each >> collapse, the required thresholds (e.g. max_ptes_none) are met, and the collapse >> effectively adds more non-none ptes so the next scan will then collapse to even >> higher order. Does your solution suffer from this (theoretical/edge case) issue? >> If not, how did you solve? > > Yes sadly it suffers from the same issue. bringing max_ptes_none much > lower as a default would "help". Can we just keep it simple and only support max_ptes_none = 511 ("pagefault behavior" -- PMD_NR_PAGES - 1) or max_ptes_none = 0 ("deferred behavior") and document that the other weird configurations will make mTHP skip, because "weird and unexpetced" ? :) -- Cheers, David / dhildenb