Re: [PATCH 1/4] btrfs: fix 64bit division in btrfs_insert_striped_mirrored_raid_extents

[David Sterba <dsterba@suse.cz>]

On Mon, Sep 18, 2023 at 03:03:10PM +0000, Johannes Thumshirn wrote:
> On 18.09.23 16:19, Geert Uytterhoeven wrote:
> > Hi Johannes,
> > 
> > On Mon, Sep 18, 2023 at 4:14 PM Johannes Thumshirn
> > <johannes.thumshirn@wdc.com> wrote:
> >> Fix modpost error due to 64bit division on 32bit systems in
> >> btrfs_insert_striped_mirrored_raid_extents.
> >>
> >> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> >> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> > 
> > Thanks for your patch!
> > 
> >> --- a/fs/btrfs/raid-stripe-tree.c
> >> +++ b/fs/btrfs/raid-stripe-tree.c
> >> @@ -148,10 +148,10 @@ static int btrfs_insert_striped_mirrored_raid_extents(
> >>   {
> >>          struct btrfs_io_context *bioc;
> >>          struct btrfs_io_context *rbioc;
> >> -       const int nstripes = list_count_nodes(&ordered->bioc_list);
> >> -       const int index = btrfs_bg_flags_to_raid_index(map_type);
> >> -       const int substripes = btrfs_raid_array[index].sub_stripes;
> >> -       const int max_stripes = trans->fs_info->fs_devices->rw_devices / substripes;
> >> +       const size_t nstripes = list_count_nodes(&ordered->bioc_list);
> >> +       const enum btrfs_raid_types index = btrfs_bg_flags_to_raid_index(map_type);
> >> +       const u8 substripes = btrfs_raid_array[index].sub_stripes;
> >> +       const int max_stripes = div_u64(trans->fs_info->fs_devices->rw_devices, substripes);
> > 
> > What if the quotient does not fit in a signed 32-bit value?
> 
> Then you've bought a lot of HDDs ;-)
> 
> Jokes aside, yes this is theoretically correct. Dave can you fix 
> max_stripes up to be u64 when applying?

I think we can keep it int, or unsigned int if needed, we can't hit such
huge values for rw_devices. The 'theoretically' would fit for a machine
with infinite resources, otherwise the maximum number of devices I'd
expect is a few thousand.