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

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Message-ID: <aI04CQZZzgCDO2A5@lappy>
Date: Fri, 1 Aug 2025 17:56:25 -0400
From: Sasha Levin <sashal@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: david@...hat.com, 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 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'd support creating a new changelog tag for this case.

This is in the context of a proposal on workflows@:
https://lore.kernel.org/workflows/20250728105634.GF787@pendragon.ideasonboard.com/T/#t

The Co-developed-by: usage wasn't my proposal, but it looked like the
majority of folks were okay with it.

Input is definitely welcome!

>And really, if AI was recruited in developing a kernel patch, it would
>be helpful if the changelog were to have a paragraph describing just
>how the AI assist was used.  At least, until everyone knows all about
>this?  You probably already have a presentation or a web page, so
>adding a link to that would suffice, thanks.

Kees actually has a good writeup about his experience with AI tooling
here: https://hachyderm.io/@kees/114907228284590439 , my experience is
fairly similar.

Kees logged his prompts as part of the patch he sent in
(https://lore.kernel.org/lkml/20250724080756.work.741-kees@kernel.org/)
which was interesting, but I didn't see much value in doing that beyond
the demo purposes as this is not really reproducible.

-- 
Thanks,
Sasha