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

[David Laight]

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.

	David

> 
> Fixes: f286b1c72175 ("btrfs: prepare extent_io.c for future larger folio support")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  fs/btrfs/extent_io.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index c2451194be66..88bced0bfa51 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -2468,7 +2468,7 @@ void extent_write_locked_range(struct inode *inode, const struct folio *locked_f
>  			continue;
>  		}
>  
> -		cur_end = min(folio_pos(folio) + folio_size(folio) - 1, end);
> +		cur_end = min_t(u64, folio_pos(folio) + folio_size(folio) - 1, end);
>  		cur_len = cur_end + 1 - cur;
>  
>  		ASSERT(folio_test_locked(folio));