Re: [PATCH 39/44] mm: use min() instead of min_t()

[David Hildenbrand (Red Hat)]

On 11/20/25 16:44, David Laight wrote:
> On Thu, 20 Nov 2025 14:42:24 +0100
> "David Hildenbrand (Red Hat)" <david@xxxxxxxxxx> wrote:
>
> > > >   
> > > > > Signed-off-by: David Laight <david.laight.linux@xxxxxxxxx>
> > > > > ---
> > > > >    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