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

[Qu Wenruo]

On 2023/9/19 23:28, David Sterba wrote:
> On Tue, Sep 19, 2023 at 10:07:00AM +0930, Qu Wenruo wrote:
> > On 2023/9/19 01:54, David Sterba wrote:
> > > 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@xxxxxxx> wrote:
> > > > > > Fix modpost error due to 64bit division on 32bit systems in
> > > > > > btrfs_insert_striped_mirrored_raid_extents.
> > > > > >
> > > > > > Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> > > > > > Signed-off-by: Johannes Thumshirn <johannes.thumshirn@xxxxxxx>
> > > > >
> > > > > 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.
> >
> > In fact, we already have an check in btrfs_validate_super(), if the
> > num_devices is over 1<<31, we would reject the fs.
>
> No, it's just a warning in that case.

We can make it a proper reject.

> > I think we should be safe to further reduce the threshold.
> >
> > U16_MAX sounds a valid and sane value to me.
> > If no rejection I can send out a patch for this.
> >
> > And later change internal rw_devices/num_devices to u16.
>
> U16 does not make sense here, it's not a native int type on many
> architectures and generates awkward assembly code. We use it in
> justified cases where it's saving space in structures that are allocated
> thousand times. The arbitrary limit 65536 is probably sane but not
> much different than 1<<31, practically not hit and was useful to
> note fuzzed superblocks.

OK, we can make it unsigned int (mostly u32) for fs_info::*_devices, but
still do extra limits on things like device add to limit it to U16_MAX.

Would this be a better solution?
At least it would still half the width while keep it native to most (if
not all) archs.

Thanks,
Qu