linux-kernel - Re: [PATCH 1/4] btrfs: fix 64bit division in btrfs_insert_striped_mirrored_raid

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Message-ID: <a364f344-b718-48ff-9e2a-484c5ded6e7f@gmx.com>
Date:   Wed, 20 Sep 2023 07:20:49 +0930
From:   Qu Wenruo <quwenruo.btrfs@....com>
To:     dsterba@...e.cz
Cc:     Johannes Thumshirn <Johannes.Thumshirn@....com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, Qu Wenru <wqu@...e.com>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] btrfs: fix 64bit division in
 btrfs_insert_striped_mirrored_raid_extents



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@....com> wrote:
>>>>>> Fix modpost error due to 64bit division on 32bit systems in
>>>>>> btrfs_insert_striped_mirrored_raid_extents.
>>>>>>
>>>>>> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
>>>>>> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@....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.
>
> 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