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

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Message-Id: <20250801141101.9f3555a172609cb64fde7f71@linux-foundation.org>
Date: Fri, 1 Aug 2025 14:11:01 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Sasha Levin <sashal@...nel.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 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:?)

I'd support creating a new changelog tag for this case.

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.