Re: [PATCH] btrfs: fix signedness issue in min()

[David Sterba]

On Mon, Mar 17, 2025 at 02:16:37PM +0000, David Laight wrote:
> On Fri, 14 Mar 2025 16:54:41 +0100
> Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> 
> > From: Arnd Bergmann <arnd@xxxxxxxx>
> > 
> > Comparing a u64 to an loff_t causes a warning in min()
> > 
> > fs/btrfs/extent_io.c: In function 'extent_write_locked_range':
> > include/linux/compiler_types.h:557:45: error: call to '__compiletime_assert_588' declared with attribute error: min(folio_pos(folio) + folio_size(folio) - 1, end) signedness error
> > fs/btrfs/extent_io.c:2472:27: note: in expansion of macro 'min'
> >  2472 |                 cur_end = min(folio_pos(folio) + folio_size(folio) - 1, end);
> >       |                           ^~~
> > 
> > Use min_t() instead.
> 
> It would be slightly better to use min_unsigned() since, regardless of the types
> involved, it can't discard significant bits.
> 
> OTOH the real problem here is that both folio_pos() and folio_size() return signed types.

folio_size() returns size_t:

static inline size_t folio_size(const struct folio *folio)
{
	return PAGE_SIZE << folio_order(folio);
}

Otherwise the min_t with force u64 is ok and lots of min() use (in
btrfs) was converted to the typed variant in case the types don't match.