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b=YJCyzbg2VBKutZnbfY5RGMyskD50lSS2TpqXqwLIjEQMZfJ1jHKWwl3AaxAi20GLL VsGf/m0Hd/URQaPfrKzsixUikbp0H2YSTNHc21IbAiVac7NXyTxzw6EzMAIj4PRCC1 IndCBmMBh777Pe6GqDfPwzb/JEaGHX4ZnkELgAgNN8DyDWkaVFMsLJZWHO2m6tNS+2 kTKs4EMqZxe9hCk3CmYekOfAHv1ElOEFy6WWzpQOgbFY+fkp4al9lyra46gHKRJY6h up9qpJfI5r2ydgFJqHfE3VchHNqJ7eQFa7A7bcQQumBV7jsWqu38n/x4dSFyI+6Su0 Az2gcwSK0iYSw== Message-ID: <8fcfe565-4371-47ee-a0de-a9d3e1efa0a1@kernel.org> Date: Thu, 5 Feb 2026 17:46:18 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table To: Lance Yang , Dave Hansen , Peter Zijlstra 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@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: <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> <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: 8bit X-Rspam-User: X-Rspamd-Queue-Id: 6996240012 X-Rspamd-Server: rspam07 X-Stat-Signature: 91raic3ikzs8rq6zgzc4mjaue6ppbjfb X-HE-Tag: 1770309990-530814 X-HE-Meta: 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 CWZr0wPe 1ae2C6Em8p1m9fxEMMCgzYekY4xJw//PQ3cSK819DGNrE1lDbN4hHtisgbfCLYATfz3y4BYDIAeKRBDtJRABs+w+FnCdZmboN2q1uNvY3GYFf0URqLL33Wm0q/LZWLOqXXCn6dLBvQrkA32CkRjK9/IY1xm2hAmspA39NjIBMzdSezBju2vJzDkMOla1F5gmJuoXayqlJMK76GPmNsmVzlpqOZOR7uLoRzxsGfpihvgcPYDutLaNLL+31YQ== 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 17:30, Lance Yang wrote: > > > On 2026/2/5 23:41, Dave Hansen wrote: >> On 2/5/26 07:31, Lance Yang wrote: >>> >>> Without IPIs or synchronize_rcu(), IIUC, we have no way to know if there >>> are ongoing concurrent lockless page-table walks — the walkers just >>> disable >>> IRQs and walk. >> >> 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. Right, and walking a page table that is suddenly no longer a page table is the real fun :) ... or trying to lookup the page of something that is not even a page. > >> Don't forget that we also have pesky concurrent lockless page-table >> walkers called CPUs. They're extra pesky in that they don't even stop >> for IPIs. ;) > > I assume those walkers that don't disable IRQs only read the PMD and > don't walk into the table; otherwise the current sync wouldn't work > for them. CPU page table walkers are much easier to control in that regard :) -- Cheers, David