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

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Message-ID: <2cb4503d-3a3f-4f6c-8038-7b3d1c74b3c2@kernel.org>
Date: Thu, 5 Feb 2026 22:30:14 +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,
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 baolin.wang@...ux.alibaba.com, boris.ostrovsky@...cle.com, bp@...en8.de,
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Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table

On 2/5/26 18:06, David Hildenbrand (Arm) wrote:
> On 2/5/26 17:48, Matthew Wilcox wrote:
>> On Fri, Feb 06, 2026 at 12:30:56AM +0800, Lance Yang wrote:
>>>
>>> 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.

Now that I had dinner my memory comes back: for khugepaged, we have to 
make sure there is no concurrent GUP-fast before collapsing and 
(possibly) freeing the page table / re-depositing it.

-- 
Cheers,

David