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

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Message-Id: <20251113153205.6507ecb308e7d09362905da7@linux-foundation.org>
Date: Thu, 13 Nov 2025 15:32:05 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Carlos Llamas <cmllamas@...gle.com>, "David Hildenbrand (Red Hat)"
 <david@...nel.org>, 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 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?