linux-kernel - Re: [PATCH] mm/userfaultfd: fix kmap_local LIFO ordering for CONFIG

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Message-ID: <56331143-0532-4b6d-b23c-d15ca82f17c6@redhat.com>
Date: Mon, 4 Aug 2025 10:09:18 +0200
From: David Hildenbrand <david@...hat.com>
To: Sasha Levin <sashal@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>
Cc: surenb@...gle.com, aarcange@...hat.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/userfaultfd: fix kmap_local LIFO ordering for
 CONFIG_HIGHPTE

On 01.08.25 23:56, Sasha Levin wrote:
> On Fri, Aug 01, 2025 at 02:11:01PM -0700, Andrew Morton wrote:
>> On Thu, 31 Jul 2025 10:44:31 -0400 Sasha Levin <sashal@...nel.org> wrote:
>>
>>> With CONFIG_HIGHPTE on 32-bit ARM, move_pages_pte() maps PTE pages using
>>> kmap_local_page(), which requires unmapping in Last-In-First-Out order.
>>>
>>> The current code maps dst_pte first, then src_pte, but unmaps them in
>>> the same order (dst_pte, src_pte), violating the LIFO requirement.
>>> This causes the warning in kunmap_local_indexed():
>>>
>>>    WARNING: CPU: 0 PID: 604 at mm/highmem.c:622 kunmap_local_indexed+0x178/0x17c
>>>    addr \!= __fix_to_virt(FIX_KMAP_BEGIN + idx)
>>>
>>> Fix this by reversing the unmap order to respect LIFO ordering.
>>>
>>> This issue follows the same pattern as similar fixes:
>>> - commit eca6828403b8 ("crypto: skcipher - fix mismatch between mapping and unmapping order")
>>> - commit 8cf57c6df818 ("nilfs2: eliminate staggered calls to kunmap in nilfs_rename")
>>>
>>> Both of which addressed the same fundamental requirement that kmap_local
>>> operations must follow LIFO ordering.
>>>
>>> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
>>> Co-developed-by: Claude claude-opus-4-20250514
>>
>> Well this is innovative.  I doubt if Co-developed-by: is appropriate
>> for this (where's Claude's Signed-off-by:?)
> 
> Claude (or any other AI) can't legally sign off on code :)

I think we need a different tag than Co-developed-by. Avocado [1] seems 
to use

	Assisted-by:

Which I prefer over things like Generated-by o co-developed-by:

[1] 
https://avocado-framework.readthedocs.io/en/latest/guides/contributor/chapters/ai_policy.html

-- 
Cheers,

David / dhildenb