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b=UWJa58imC0523bdHUwOv4s43hDkqXGIQPMlrIIoBFoNK5Q4WIDQtxwtXe9LRgv7C3 1QOhGsuqa7ynp1ftSHwRBc4UYKNQPQHilUoIRYBu6uIlD7s6MVOdVRrnDVQMKnA0lt e6+/I/RmUZ3ndJGVodsAmXmCGSnYIDW3QpvPkGhYuqgaC5hXHneSYbLeskpQPGHxZ2 IwNCwiebzbU5H7TgTuurErSwRhUQVzMASWNT0ykO2LdV0Co4UK752mykmHWViMd30t Z7FcWNRAjEhgh2CYvI6dF9duOwmDfMiuXG3HQsi8YuTLX66jVqQuIVP3rADGoClRKg 7odoQAFZkaeLQ== Message-ID: <2cb4503d-3a3f-4f6c-8038-7b3d1c74b3c2@kernel.org> Date: Thu, 5 Feb 2026 22:30:14 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table From: "David Hildenbrand (Arm)" 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> <6a364356-5fea-4a6c-b959-ba3b22ce9c88@kernel.org> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 9255A140011 X-Stat-Signature: h38j57ptzkewwbi1c7q4hco1r1jzh5f3 X-Rspam-User: X-HE-Tag: 1770327027-590850 X-HE-Meta: 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 gAty6mQO ePDs34/8ON4Be7xImyLq8CMnUuZuvLHMEXSbg5Sz4vI0YyHKA88TTWAHwAh5KEiL3nWHNn6uFnm+4ImrAuAa2lTT7YHue7srdhcA/1t8U3JIuJ7qkfsli0ir7+51gvMIuoFGKhBVwuIlo0PJdRfKEZDfCur+yeP0MEqBCnZXhQaea73dloC+KmfxDK2fxTO0SES9Q61JYPUGViyNA+H8BnS8Um3OMjlCy65azrMYgc3ZztG4drbEWYGP1FQqZqZsqg+M3mdasM8Wcux2Vp/lDQJ1iFZHBtKHLezCX 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 18:06, David Hildenbrand (Arm) wrote: > On 2/5/26 17:48, Matthew Wilcox wrote: >> On Fri, Feb 06, 2026 at 12:30:56AM +0800, Lance Yang wrote: >>> >>> 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. Now that I had dinner my memory comes back: for khugepaged, we have to make sure there is no concurrent GUP-fast before collapsing and (possibly) freeing the page table / re-depositing it. -- Cheers, David