Re: [PATCH] btrfs: fix 64bit mod compile error

[Geert Uytterhoeven <geert@linux-m68k.org>]

 	Hi Josef,

On Wed, 26 Jan 2022, Josef Bacik wrote:
> kernelbuild test bot complained about a 64bit % operation in the patch
>
> btrfs: add support for multiple global roots
>
> Fix this using div64_u64_rem.  This can be folded in to the original
> patch.
>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Thanks, this fixes the build error for me.

Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>

> --- a/fs/btrfs/block-group.c
> +++ b/fs/btrfs/block-group.c
> @@ -2452,6 +2452,7 @@ void btrfs_create_pending_block_groups(struct btrfs_trans_handle *trans)
> static u64 calculate_global_root_id(struct btrfs_fs_info *fs_info, u64 offset)
> {
> 	u64 div = SZ_1G;
> +	u64 index;
>
> 	if (!btrfs_fs_incompat(fs_info, EXTENT_TREE_V2))
> 		return BTRFS_FIRST_CHUNK_TREE_OBJECTID;
> @@ -2460,7 +2461,9 @@ static u64 calculate_global_root_id(struct btrfs_fs_info *fs_info, u64 offset)
> 	if (btrfs_super_total_bytes(fs_info->super_copy) <= (SZ_1G * 10ULL))
> 		div = SZ_128M;
>
> -	return (div_u64(offset, div) % fs_info->nr_global_roots);
> +	offset = div64_u64(offset, div);

On 32-bit, this is not implemented as a plain division, hence gcc is not
smart enough to notice that div is a power-of-two, and this can be
optimized to a shift.

Hence please make this explicit:

     if (btrfs_super_total_bytes(fs_info->super_copy) <= (SZ_1G * 10ULL))
             offset >>= ilog2(SZ_128M);
     else
 	    offset >>= ilog2(SZ_1G);

> +	div64_u64_rem(offset, fs_info->nr_global_roots, &index);

Does the number fs_info->nr_global_roots have special properties,
i.e. can this expensive modulo operation be replaced by a shift, too?

> +	return index;
> }
>
> struct btrfs_block_group *btrfs_make_block_group(struct btrfs_trans_handle *trans,

Gr{oetje,eeting}s,

 						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
 							    -- Linus Torvalds