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

Qu Wenruo
2023-09-19T00:37:22Z

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@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.

In fact, we already have an check in btrfs_validate_super(), if the
num_devices is over 1<<31, we would reject the fs.

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.

Thanks,
Qu