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Fri, 12 Sep 2025 08:19:08 -0400 (EDT) Date: Fri, 12 Sep 2025 13:19:05 +0100 From: Kiryl Shutsemau To: Nico Pache Cc: linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, david@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, dev.jain@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, 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, richard.weiyang@gmail.com, lance.yang@linux.dev, vbabka@suse.cz, rppt@kernel.org, jannh@google.com, pfalcato@suse.de Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support Message-ID: References: <20250912032810.197475-1-npache@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250912032810.197475-1-npache@redhat.com> X-Rspamd-Queue-Id: C18CFA000F X-Rspamd-Server: rspam05 X-Stat-Signature: 76f113qjszh7bb6qr8zcfra4bhaqmizt X-Rspam-User: X-HE-Tag: 1757679552-346060 X-HE-Meta: U2FsdGVkX1/g6SXrvjSG4ptV5UsPJAD7SiqJg8isv5rXlh0fClJDDYnBjIJjrvM0pB67g6G2VNO7uxboT+u2upNaIPghP1I2sJ5bF2tjbgT1L/RQAetPNQ3nkDlfUvtgjIE1jiAJbxAjQNeQkym5iJkLdUeltYSvDuCrwMJ1DJNHdCI0kg9g/JDSBDOET7dpGhbx+LniBjXmxBtDDxpSE/nookwveCnod8LcK7lou+9hbIy1w2rK0nv3ajvs8NKKHhbevK+nyuki0duUTIhjw2o7hOK8yJ8buBdkoKQlo2NSMKMRDW3DGbRt8SU/I6iiWZt3kXQCpZcQgoKW2z8Pg4r5ij5BasXmx20op3koHvMu7f/s7n2MaOoBiyqf0KxvMvOh8Ztq2+VLJNbc/f1w3VVDe/Mh8szj3e7VUg6LmDvtccxOM5C6iMhWqscLlwz7PjMqpq07YfUdkxUbb1T9xd0jEqFsr4GwhNeJ2ftSNB9HAoXMBY/TNkMMn7PEXgTwYVFnugp7W3sGBxbBxDtKhyboT1jBXIC+T5InrqG7Ipzwl/UM6wJ3w3mc6utQdbp0QBZrvemnsFD0Jju7USTWJqNkiQvQPwRjK9YAmyjnmqoTIBFNqyxrB0XGoGAOhsR3S0hShFOzRCDeMogiSsPw+bdVCitFmRpbQQd15VrJ8jFAuzQOZBoKVOM+fKyDAuMYg+MN+i8oW0n2Wd6MkKiV8yADniN/1pxLKjHTVNQFpNS5u5QWw++FZSbIhKaYRYsupKszTZMwW1UrZ15hTgHHYUOvUtpeUIO53bCD4XGyLo7PdGBFHQTpjGMIc/sdmmLDw2iXVoJbSQ4YQgmusFaDN7FLdl+kEbRa9mlHIQlTZrpI5m/t6ZgkcgbxUQ9H4cnV03BN4y1o1igAOtJmsDAODbPmN5HTG+KMeGv8Fw0ThqGf+JrnTYh4y2e9UYXYMhj0G/A9elfkXCi+aysVpuC AaXNmF5+ 3wAeFFE71ahOoivK1T8kI1VMmv0n+fX1eX973mCkP7uTnX+/RKpKXqgtNHupjwd6Rf5HkPQoeHNP7npF+QtOh2OQX6P94ARxnWjYnU4k8h+9b9EVvZDzZFj4rtH8sGnV4bWPjTNARu6PAqQ8c5hmNCGjfhU9wth/MVYPa5BQPpCC2RN/Mdo5W9sRpECiajlmoaL5SzRlQ2Jfst0AxYpLSdlzmR6CLQndKAqqOmMazEcu6ViIL5F5bW2XQJmCMKFHl7WVCYmMVd0E0znARwkHl/O6zDN8385VR9qsrb0L+XpPvil/LFpQSF03b2dByrAGUrgadvcmalzkIH1mXgmnEvCmr/ApfP8Kxkir6 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 Thu, Sep 11, 2025 at 09:27:55PM -0600, Nico Pache wrote: > The following series provides khugepaged with the capability to collapse > anonymous memory regions to mTHPs. > > To achieve this we generalize the khugepaged functions to no longer depend > on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual > pages that are occupied (!none/zero). After the PMD scan is done, we do > binary recursion on the bitmap to find the optimal mTHP sizes for the PMD > range. The restriction on max_ptes_none is removed during the scan, to make > sure we account for the whole PMD range. When no mTHP size is enabled, the > legacy behavior of khugepaged is maintained. max_ptes_none will be scaled > by the attempted collapse order to determine how full a mTHP must be to be > eligible for the collapse to occur. If a mTHP collapse is attempted, but > contains swapped out, or shared pages, we don't perform the collapse. It is > now also possible to collapse to mTHPs without requiring the PMD THP size > to be enabled. > > When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on > 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for > mTHP collapses to prevent collapse "creep" behavior. This prevents > constantly promoting mTHPs to the next available size, which would occur > because a collapse introduces more non-zero pages that would satisfy the > promotion condition on subsequent scans. Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count all-zeros 4k as none_or_zero? It mirrors the logic of shrinker. -- Kiryl Shutsemau / Kirill A. Shutemov