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

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Message-ID: <1dfde42a-838c-4dc5-8ea8-0b501df37af8@intel.com>
Date: Thu, 5 Feb 2026 09:00:34 -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>
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Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table

On 2/5/26 08:30, Lance Yang wrote:
...
>> 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 isn't this already solved by the existing RCU freeing approach
documented above tlb_remove_table_smp_sync()?

This seems like a rather classic way to use RCU: wait to free until RCU
says there can't be a reader any more. You don't have to sit there and
wait for it, you just use call_rcu() which will hold off the free until
it's safe.