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: <6a364356-5fea-4a6c-b959-ba3b22ce9c88@kernel.org>
Date: Thu, 5 Feb 2026 18:06:59 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Matthew Wilcox <willy@...radead.org>, Lance Yang <lance.yang@...ux.dev>
Cc: Dave Hansen <dave.hansen@...el.com>, Peter Zijlstra
 <peterz@...radead.org>, 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 17:48, Matthew Wilcox wrote:
> On Fri, Feb 06, 2026 at 12:30:56AM +0800, Lance Yang wrote:
>> On 2026/2/5 23:41, Dave Hansen wrote:
>>> 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.
> 
> 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.

-- 
Cheers,

David