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b=I/SR602yFdV2tzlaJOUFUht6cqmXFbDlNvkdpb8TiBZLqYQtp9nr9sR9trhMk9dFs uiy5i4ApupvlkMfhZIp6wryR73H+/vV/1JvDQBlQn2TruL+9EZ9lj9dPVV9WmdqyW9 qUabqRNLGg6XhymBVYvETuIGbydf3vqDvcGre4MuuTn46kmJRFLPcvCiixzq6V/nI0 PZ7L+roqY1Z7YgXnngGxQbQYFDDcTIFPsW3CW2yska+ACb5kqVviK+RJQBQhth5uVk YicdSNkZhtDzkupG4V7xrRO+KKNmWZGjxnaHzyMCjN5rJT+fsfHc7/WiL4GEkZCoc9 xsTai+DaQFGXg== Message-ID: Date: Thu, 5 Feb 2026 23:49:48 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table To: Dave Hansen , Matthew Wilcox , Lance Yang Cc: 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> 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: E03F8160004 X-Rspamd-Server: rspam07 X-Stat-Signature: ak41mf79t5qitco5pd8dcf6nzxppu6f7 X-HE-Tag: 1770331802-388257 X-HE-Meta: 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 9RSHvKuF HrEJK2oz9ImutydFN5o9ne23/5Ya5yjPBpu9TlpbwQR2lrRRwndK502Oym4ZElgwoCV+GPI2/8QHj11j17uxeRsm78WPdD3jV68ZxDto9BT0j1yG2rQzw36C3ZKt/rnLTpMdLEe7DWo7eyLEMB/K1zUx4i4BHlEdH9qlnjAwW2i9hJuzeEiVzmqMwY2vhiibiN/apxKvGNdFEv2Rfqlnp3ycuwtjLsn+A0Rfsa4CI22XYoPMjCc3j/ShETewbBPz/Nqe3KzdA/0KY0AsTef1pgrl/3Ss24Wvbaxmr 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 19:36, Dave Hansen wrote: > On 2/5/26 09:06, David Hildenbrand (Arm) wrote: >>> 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. > > Letting the previously-shared table get released to everything else in > the system sounds like a fixable problem. tlb_flush_unshared_tables() > talks about this, and it makes sense that once locks get dropped that > something else could get mapped in and start using the PMD. Yeah, I tried to document that carefully. > > The RCU way of fixing that would be to allocate new page table, replace > the old one, and RCU-free the old one. Read, Copy, Update. :) > > It does temporarily eat up an extra page, and cost an extra copy. But > neither of those seems expensive compared to IPI'ing the world. I played with many such ideas, including never reusing a page table again once it was once shared. All turned out rather horrible. RCU-way: replacing a shared page table involves updating all processes that share the page table :/ . I think another issue I stumbled into while trying to implement was around failing to allocate memory (but being required to make progress). It all turned to quite some complexity and inefficiency, so I had to give up on that. :) > >> For page table freeing, we only do it if we fail to allocate memory -- >> if we cannot use RCU IIRC. > > But that case is fine to be slow and use synchronize_rcu(). If you're > failing to allocate a single page, you're in a way slow path anyway. That's true. We could likely do that already and avoid the IPI broadcast there that was once reported to be a problem for RT applications. -- Cheers, David