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Received: from willy by casper.infradead.org with local (Exim 4.98.2 #2 (Red Hat Linux)) id 1vo2Wb-00000003tzE-36Oa; Thu, 05 Feb 2026 16:48:13 +0000 Date: Thu, 5 Feb 2026 16:48:13 +0000 From: Matthew Wilcox To: Lance Yang Cc: Dave Hansen , "David Hildenbrand (Arm)" , 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 Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table Message-ID: 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <99237729-f2b0-4a7a-8213-65a2f1c57744@linux.dev> X-Stat-Signature: d5i1z88exifo3os4yrim74co8et8k9kb X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 87D771C000D X-HE-Tag: 1770310118-449788 X-HE-Meta: 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 fDZlaCtv JLux8wnvsQFyB6T4rpU9hZYxIT+tMexHzw8fG2WDTKl8Cc0q9JnayWcz25B4hnhrjVMctEvE8l8/+1EUG/1gtnKKcFHHlg13HjQRLER9Vyfb7jTi2g2eLMI8VVWZ/PkJ/oI5tE5qp2AlTb5Ft/aCT1x9E92ySRzNqeBX+IkcStD2PPNM3kW++WZJ8KevaA9dJDoi/2Aqs7Nwnnrzmw0xT4Yt3DhWhZiiBuR1bFbLEzIuAATOwpgpIqGDmrvIHKh+5mRxT 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 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?