linux-kernel - Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table

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Message-ID: <c985a8ed-37ad-415e-b7b4-18a66b4da3fe@linux.dev>
Date: Thu, 5 Feb 2026 23:31:48 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Dave Hansen <dave.hansen@...el.com>,
 "David Hildenbrand (Arm)" <david@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>
Cc: Liam.Howlett@...cle.com, akpm@...ux-foundation.org,
 aneesh.kumar@...nel.org, arnd@...db.de, baohua@...nel.org,
 baolin.wang@...ux.alibaba.com, boris.ostrovsky@...cle.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, dev.jain@....com, hpa@...or.com,
 hughd@...gle.com, ioworker0@...il.com, jannh@...gle.com, jgross@...e.com,
 kvm@...r.kernel.org, linux-arch@...r.kernel.org,
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 lorenzo.stoakes@...cle.com, mingo@...hat.com, npache@...hat.com,
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Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table



On 2026/2/5 23:09, Dave Hansen wrote:
> On 2/5/26 07:01, Lance Yang wrote:
>> So for now, neither approach looks good: tracking on the read side adss
>> cost to GUP-fast, and syncing on the write side e.g. synchronize_rcu()
>> is too slow on large systems.
> 
> Which of the writers truly *need* synchronize_rcu()?
> 
> What are they doing with the memory that they can't move forward unless
> it's quiescent *now*?

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.