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Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vmu77-0000000GYC9-2S0m; Mon, 02 Feb 2026 13:37:14 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 22BE63008E2; Mon, 02 Feb 2026 14:37:13 +0100 (CET) Date: Mon, 2 Feb 2026 14:37:13 +0100 From: Peter Zijlstra To: Lance Yang Cc: 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@intel.com, dave.hansen@linux.intel.com, david@kernel.org, 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: <20260202133713.GF1395266@noisy.programming.kicks-ass.net> References: <20260202095414.GE2995752@noisy.programming.kicks-ass.net> <20260202110329.74397-1-lance.yang@linux.dev> <20260202125030.GB1395266@noisy.programming.kicks-ass.net> <4700e7ba-8456-4a93-9e28-7e5a3ca2a1be@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4700e7ba-8456-4a93-9e28-7e5a3ca2a1be@linux.dev> X-Stat-Signature: pfq53xwq5rc3zqiy6kdx1tuy8acngexo X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: B46D720003 X-HE-Tag: 1770039443-732661 X-HE-Meta: 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 TvH6H9nh LjmStb4ZX8DyW+XLoR2+HZ5u8g4pQ2A9j29f5mP+cAqI491wsN7LVGg77HCvBbeT6ebICkwxWV/Ts2bJ5nAcH2SR8pYrrm/SmfMkPVaRK2JlLWqiiJnAqm3TQMjPQ4vnqik189RwF4/2MLxdbkIuRbvWB83909Yq7pplFAaltiqOsFFK25JOYZlITHJbeknfTmeBTLa0R9/xXktOGQpwsnUmhuA== 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 Mon, Feb 02, 2026 at 09:07:10PM +0800, Lance Yang wrote: > > > Right, but if we can use full RCU for PT_RECLAIM, why can't we do so > > > unconditionally and not add overhead? > > > > The sync (IPI) is mainly needed for unshare (e.g. hugetlb) and collapse > > (khugepaged) paths, regardless of whether table free uses RCU, IIUC. > > In addition: We need the sync when we modify page tables (e.g. unshare, > collapse), not only when we free them. RCU can defer freeing but does > not prevent lockless walkers from seeing concurrent in-place > modifications, so we need the IPI to synchronize with those walkers > first. Currently PT_RECLAIM=y has no IPI; are you saying that is broken? If not, then why do we need this at all?