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Subject: Re: [PATCH] fat: Avoid oops when bdi->io_pages==0
To:     OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        fsdevel <linux-fsdevel@vger.kernel.org>
References: <87ft85osn6.fsf@mail.parknet.co.jp>
 <b4e1f741-989c-6c9d-b559-4c1ada88c499@kernel.dk>
 <87o8mq6aao.fsf@mail.parknet.co.jp>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <4010690f-20ad-f7ba-b595-2e07b0fa2d94@kernel.dk>
Date:   Mon, 31 Aug 2020 10:39:26 -0600
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On 8/31/20 10:37 AM, OGAWA Hirofumi wrote:
> Jens Axboe <axboe@kernel.dk> writes:
> 
>> On Sat, Aug 29, 2020 at 7:08 PM OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> wrote:
>>>
>>> On one system, there was bdi->io_pages==0. This seems to be the bug of
>>> a driver somewhere, and should fix it though. Anyway, it is better to
>>> avoid the divide-by-zero Oops.
>>>
>>> So this check it.
>>>
>>> Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
>>> Cc: <stable@vger.kernel.org>
>>> ---
>>>  fs/fat/fatent.c |    2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
>>> index f7e3304..98a1c4f 100644
>>> --- a/fs/fat/fatent.c   2020-08-30 06:52:47.251564566 +0900
>>> +++ b/fs/fat/fatent.c   2020-08-30 06:54:05.838319213 +0900
>>> @@ -660,7 +660,7 @@ static void fat_ra_init(struct super_blo
>>>         if (fatent->entry >= ent_limit)
>>>                 return;
>>>
>>> -       if (ra_pages > sb->s_bdi->io_pages)
>>> +       if (sb->s_bdi->io_pages && ra_pages > sb->s_bdi->io_pages)
>>>                 ra_pages = rounddown(ra_pages, sb->s_bdi->io_pages);
>>>         reada_blocks = ra_pages << (PAGE_SHIFT - sb->s_blocksize_bits + 1);
>>
>> I don't think we should work-around this here. What device is this on?
>> Something like the below may help.
> 
> The reported bug is from nvme stack, and the below patch (I submitted
> same patch to you) fixed the reported case though. But I didn't verify
> all possible path, so I'd liked to use safer side.
> 
> If block layer can guarantee io_pages!=0 instead, and can apply to
> stable branch (5.8+). It would work too.

We really should ensure that ->io_pages is always set, imho, instead of
having to work-around it in other spots.

-- 
Jens Axboe