linux-kernel - Re: [PATCH] selftests/mm: fix division-by-zero in uffd-unit-tests

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Message-ID: <9d30836b-9ddb-4432-aa39-85e32c2ea645@kernel.org>
Date: Fri, 14 Nov 2025 09:11:40 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Carlos Llamas <cmllamas@...gle.com>, Peter Xu <peterx@...hat.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Shuah Khan <shuah@...nel.org>, Ujwal Kundur <ujwal.kundur@...il.com>,
 Brendan Jackman <jackmanb@...gle.com>, kernel-team@...roid.com,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org,
 "open list:MEMORY MANAGEMENT - USERFAULTFD" <linux-mm@...ck.org>,
 "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH] selftests/mm: fix division-by-zero in uffd-unit-tests

On 14.11.25 00:32, Andrew Morton wrote:
> On Thu, 13 Nov 2025 15:03:06 +0000 Lorenzo Stoakes <lorenzo.stoakes@...cle.com> wrote:
> 
>> On Thu, Nov 13, 2025 at 03:01:25PM +0000, Carlos Llamas wrote:
>>> On Thu, Nov 13, 2025 at 09:54:37AM +0000, Lorenzo Stoakes wrote:
>>>> On Thu, Nov 13, 2025 at 10:06:42AM +0100, David Hildenbrand (Red Hat) wrote:
>>>>> On 13.11.25 04:46, Carlos Llamas wrote:
>>>>>> Commit 4dfd4bba8578 ("selftests/mm/uffd: refactor non-composite global
>>>>>> vars into struct") moved some of the operations previously implemented
>>>>>> in uffd_setup_environment() earlier in the main test loop.
>>>>>>
>>>>>> The calculation of nr_pages, which involves a division by page_size, now
>>>>>> occurs before checking that default_huge_page_size() returns a non-zero
>>>>>> This leads to a division-by-zero error on systems with !CONFIG_HUGETLB.
>>>>>>
>>>>>> Fix this by relocating the non-zero page_size check before the nr_pages
>>>>>> calculation, as it was originally implemented.
>>>>>>
>>>>>> Cc: stable@...r.kernel.org
>>>>>
>>>>> Do we CC stable on unit tests? From my recollection, no.
>>>>
>>>> Yeah please let's not.
>>>
>>> Oops, I keep getting confused about this Cc stable thing. Please let me
>>> know if a v2 dropping the tag is needed.
>>
>> No need, Andrew should spot the objection and drop the tag :)
> 
> I actually add cc:stable tags to selftests fixes!
> 
> Because why not.  Why leave people running known-to-be-buggy code when
> our backporting processes are so well-functioning and lightweight?
> 
> I'm not getting the objection?

I thought we were so inconsistent on that that it ends up being a wasted 
effort to even try taking care about stability of selfests in stable trees.

-- 
Cheers

David