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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <647cbe2e-a034-4a75-9492-21ea1708eccc@intel.com>
Date: Thu, 5 Feb 2026 07:41:16 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Lance Yang <lance.yang@...ux.dev>,
 "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,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 lorenzo.stoakes@...cle.com, mingo@...hat.com, npache@...hat.com,
 npiggin@...il.com, pbonzini@...hat.com, riel@...riel.com,
 ryan.roberts@....com, seanjc@...gle.com, shy828301@...il.com,
 tglx@...utronix.de, virtualization@...ts.linux.dev, will@...nel.org,
 x86@...nel.org, ypodemsk@...hat.com, ziy@...dia.com
Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table

On 2/5/26 07:31, Lance Yang wrote:
> 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.

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?

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. ;)