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

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Message-ID: <8fcfe565-4371-47ee-a0de-a9d3e1efa0a1@kernel.org>
Date: Thu, 5 Feb 2026 17:46:18 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Lance Yang <lance.yang@...ux.dev>, Dave Hansen <dave.hansen@...el.com>,
 Peter Zijlstra <peterz@...radead.org>
Cc: Liam.Howlett@...cle.com, akpm@...ux-foundation.org,
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 baolin.wang@...ux.alibaba.com, boris.ostrovsky@...cle.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, dev.jain@....com, hpa@...or.com,
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Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table

On 2/5/26 17:30, Lance Yang wrote:
> 
> 
> On 2026/2/5 23:41, Dave Hansen wrote:
>> On 2/5/26 07:31, Lance Yang wrote:
>>>
>>> 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?
> 
> 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.

Right, and walking a page table that is suddenly no longer a page table 
is the real fun :)

... or trying to lookup the page of something that is not even a page.

> 
>> 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. ;)
> 
> I assume those walkers that don't disable IRQs only read the PMD and
> don't walk into the table; otherwise the current sync wouldn't work
> for them.
CPU page table walkers are much easier to control in that regard :)

-- 
Cheers,

David