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

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Date:   Mon, 18 Sep 2023 15:03:10 +0000
From:   Johannes Thumshirn <Johannes.Thumshirn@....com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
CC:     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 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?