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

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Message-ID: <20251120154405.7bcf9a6e@pumpkin>
Date: Thu, 20 Nov 2025 15:44:05 +0000
From: David Laight <david.laight.linux@...il.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
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 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.

	David