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b=fcYeMZIjFB4IOerLQoWZdRcGFgImN9mvMO7/6fSZZhrhmPrmVGuBWrwxPQlJ1yle7 HnL4PYMwl20DiMPZliZtSKsQUOgSKyGnZbXFBD/QfWINXD+7LbHne/nvN68R+6QDsf RDuKmDO1QhpWx4di20pWG3GGWxQYd8lO/42q84AinDfz/WMHVBxayoqhXYFFC/g9iQ fH2Jyh09BEtLvMc/k2mpvHRs7RDor4a1YCOkLr/xDboSxwps/whobr3hKOhMehm0Qd e0aYp8sIZhtkV0lAjd8WWnuApxIf94bDxhZlOIgnmMGGiBkcdFlj0bPn7J1JdQkeEu 1y9G/EYZ7Rsfw== Message-ID: <6a364356-5fea-4a6c-b959-ba3b22ce9c88@kernel.org> Date: Thu, 5 Feb 2026 18:06:59 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table To: Matthew Wilcox , Lance Yang Cc: Dave Hansen , Peter Zijlstra , Liam.Howlett@oracle.com, akpm@linux-foundation.org, aneesh.kumar@kernel.org, arnd@arndb.de, baohua@kernel.org, baolin.wang@linux.alibaba.com, boris.ostrovsky@oracle.com, bp@alien8.de, dave.hansen@linux.intel.com, dev.jain@arm.com, hpa@zytor.com, hughd@google.com, ioworker0@gmail.com, jannh@google.com, jgross@suse.com, kvm@vger.kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mingo@redhat.com, npache@redhat.com, npiggin@gmail.com, pbonzini@redhat.com, riel@surriel.com, ryan.roberts@arm.com, seanjc@google.com, shy828301@gmail.com, tglx@linutronix.de, virtualization@lists.linux.dev, will@kernel.org, x86@kernel.org, ypodemsk@redhat.com, ziy@nvidia.com References: <20260202133713.GF1395266@noisy.programming.kicks-ass.net> <540adec9-c483-460a-a682-f2076cf015c2@linux.dev> <20260202150957.GD1282955@noisy.programming.kicks-ass.net> <06d48a52-e4ec-47cd-b3fb-0fccd4dc49f4@kernel.org> <3026ad8d-92ad-4683-8c3e-733d4070d033@linux.dev> <64f3a75a-30ff-4bee-833c-be5dba05f72b@intel.com> <647cbe2e-a034-4a75-9492-21ea1708eccc@intel.com> <99237729-f2b0-4a7a-8213-65a2f1c57744@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 6BF02A001C X-Stat-Signature: 86yji1thxx9hr3eqmfhyuouyfccjk7bn X-HE-Tag: 1770311230-359623 X-HE-Meta: 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 yyxNE8bb 58udp2lb+/jWnkgtXaQExDTQ9c1bK5nNdDOd03CRZBGp4X12ia8jj7KQEsIYprMVFqAJfALeKDybsYsNnJRz7NFd3kTmIAjpU6GaWPbKGd0FwFpII7VS2G9EiWdfGDxTT10xZcuxrpEBqnM7CQNTRMYTYAO4uG+SD2VtzGSz2MyB8SWbavRgmD+t5Iw1CoNKlifWrkIbGOGQjlCDoKwGu4QlatQgZCd9eeD9ylhG3vRiZGjUASWw6GjAQD58Pl7UUyMmo 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 2/5/26 17:48, Matthew Wilcox wrote: > On Fri, Feb 06, 2026 at 12:30:56AM +0800, Lance Yang wrote: >> On 2026/2/5 23:41, Dave Hansen wrote: >>> Yeah, but one aim of RCU is ensuring that readers see valid data but not >>> necessarily the most up to date data. >>> >>> Are there cases where ongoing concurrent lockless page-table walks need >>> to see the writes and they can't tolerate seeing valid but slightly >>> stale data? >> >> The issue is we're about to free the page table (e.g. >> pmdp_collapse_flush()). >> >> We have to ensure no walker is still doing a lockless page-table walk >> when the page directories are freed, otherwise we get use-after-free. > > But can't we RCU-free the page table? Why do we need to wait for the > RCU readers to finish? For unsharing hugetlb PMD tables the problem is not the freeing but the reuse of the PMD table for other purposes in the last remaining user. It's complicated. For page table freeing, we only do it if we fail to allocate memory -- if we cannot use RCU IIRC. khugepaged, no idea. -- Cheers, David