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

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Message-ID: <aYTJzft5cuqD9akC@casper.infradead.org>
Date: Thu, 5 Feb 2026 16:48:13 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Lance Yang <lance.yang@...ux.dev>
Cc: Dave Hansen <dave.hansen@...el.com>,
	"David Hildenbrand (Arm)" <david@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, Liam.Howlett@...cle.com,
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	baohua@...nel.org, baolin.wang@...ux.alibaba.com,
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Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table

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?