linux-kernel - Re: [PATCH 39/44] mm: use min() instead of min

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <9910c06a-f613-4c1a-b30b-ad8ad2d75c08@kernel.org>
Date: Fri, 21 Nov 2025 09:24:15 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: David Laight <david.laight.linux@...il.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Axel Rasmussen <axelrasmussen@...gle.com>, Christoph Lameter
 <cl@...two.org>, Dennis Zhou <dennis@...nel.org>,
 Johannes Weiner <hannes@...xchg.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Mike Rapoport <rppt@...nel.org>, Tejun Heo <tj@...nel.org>,
 Yuanchu Xie <yuanchu@...gle.com>
Subject: Re: [PATCH 39/44] mm: use min() instead of min_t()

On 11/20/25 16:44, David Laight wrote:
> On Thu, 20 Nov 2025 14:42:24 +0100
> "David Hildenbrand (Red Hat)" <david@...nel.org> wrote:
> 
>>>>   
>>>>>
>>>>> Signed-off-by: David Laight <david.laight.linux@...il.com>
>>>>> ---
>>>>>    mm/gup.c      | 4 ++--
>>>>>    mm/memblock.c | 2 +-
>>>>>    mm/memory.c   | 2 +-
>>>>>    mm/percpu.c   | 2 +-
>>>>>    mm/truncate.c | 3 +--
>>>>>    mm/vmscan.c   | 2 +-
>>>>>    6 files changed, 7 insertions(+), 8 deletions(-)
>>>>>
>>>>> diff --git a/mm/gup.c b/mm/gup.c
>>>>> index a8ba5112e4d0..55435b90dcc3 100644
>>>>> --- a/mm/gup.c
>>>>> +++ b/mm/gup.c
>>>>> @@ -237,8 +237,8 @@ static inline struct folio *gup_folio_range_next(struct page *start,
>>>>>    	unsigned int nr = 1;
>>>>>
>>>>>    	if (folio_test_large(folio))
>>>>> -		nr = min_t(unsigned int, npages - i,
>>>>> -			   folio_nr_pages(folio) - folio_page_idx(folio, next));
>>>>> +		nr = min(npages - i,
>>>>> +			 folio_nr_pages(folio) - folio_page_idx(folio, next));
>>>>
>>>> There's no cases where any of these would discard significant bits. But we
>>>> ultimately cast to unisnged int anyway (nr) so not sure this achieves anything.
>>>
>>> The (implicit) cast to unsigned int is irrelevant - that happens after the min().
>>> The issue is that 'npages' is 'unsigned long' so can (in theory) be larger than 4G.
>>> Ok that would be a 16TB buffer, but someone must have decided that npages might
>>> not fit in 32 bits otherwise they wouldn't have used 'unsigned long'.
>>
>> See commit fa17bcd5f65e ("mm: make folio page count functions return
>> unsigned") why that function used to return "long" instead of "unsigned
>> int" and how we changed it to "unsigned long".
>>
>> Until that function actually returns something that large might take a
>> while, so no need to worry about that right now.
> 
> Except that it gives a false positive on a compile-time test that finds a
> few real bugs.
> 
> I've been (slowly) fixing 'allmodconfig' and found 'goodies' like:
> 	min_t(u32, MAX_UINT, expr)
> and
> 	min_t(u8, expr, 255)
> 

:)

> Pretty much all the min_t(unsigned xxx) that compile when changed to min()
> are safe changes and might fix an obscure bug.
> Probably 99% make no difference.
> 
> So I'd like to get rid of the ones that make no difference.

No objection from my side if using min() is the preferred way now and 
introduces no observable changes.

-- 
Cheers

David