Re: [PATCH 1/2] btrfs: use min_t() for mismatched type comparison

[David Laight <david.laight.linux@gmail.com>]

On Tue, 25 Feb 2025 20:44:10 +0100
Arnd Bergmann <arnd@kernel.org> wrote:

> From: Arnd Bergmann <arnd@arndb.de>
> 
> loff_t is a signed type, so using min() to compare it against a u64
> causes a compiler warning:
> 
> fs/btrfs/extent_io.c:2497:13: error: call to '__compiletime_assert_728' declared with 'error' attribute: min(folio_pos(folio) + folio_size(folio) - 1, end) signedness error
>  2497 |                 cur_end = min(folio_pos(folio) + folio_size(folio) - 1, end);

Isn't the actual problem that folio_pos() has the wrong return type.
I can't remember what loff_t is supposed to be for, but here you want
something that reduces to 'unsigned long'.

> Use min_t() instead.

If a signed variable is known to contain a non-negative value then
min_unsigned() is better.
In particular it will never discard upper bits.

Enough min_t() cause bugs (usually due to high bits being discarded when the
type of the destination (eg u8) is used) that is is tempting to start a 'duck shoot'
season against them.

> 
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202502211908.aCcQQyEY-lkp@intel.com/
> Fixes: aba063bf9336 ("btrfs: prepare extent_io.c for future larger folio support")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  fs/btrfs/extent_io.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index f0a1da40d641..7dc996e7e249 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -2485,7 +2485,7 @@ void extent_write_locked_range(struct inode *inode, const struct folio *locked_f
>  		 * code is just in case, but shouldn't actually be run.
>  		 */
>  		if (IS_ERR(folio)) {
> -			cur_end = min(round_down(cur, PAGE_SIZE) + PAGE_SIZE - 1, end);
> +			cur_end = min_t(u64, round_down(cur, PAGE_SIZE) + PAGE_SIZE - 1, end);

That one is fine and doesn't need changing.

>  			cur_len = cur_end + 1 - cur;
>  			btrfs_mark_ordered_io_finished(BTRFS_I(inode), NULL,
>  						       cur, cur_len, false);
> @@ -2494,7 +2494,7 @@ 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);

A subtle alternative to min_unsigned() is to change the 1 to 1ull.

	David

>  		cur_len = cur_end + 1 - cur;
>  
>  		ASSERT(folio_test_locked(folio));