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From:   OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH] fat: Avoid oops when bdi->io_pages==0
References: <87ft85osn6.fsf@mail.parknet.co.jp>
        <b4e1f741-989c-6c9d-b559-4c1ada88c499@kernel.dk>
Date:   Tue, 01 Sep 2020 01:37:51 +0900
In-Reply-To: <b4e1f741-989c-6c9d-b559-4c1ada88c499@kernel.dk> (Jens Axboe's
        message of "Mon, 31 Aug 2020 09:22:15 -0600")
Message-ID: <87o8mq6aao.fsf@mail.parknet.co.jp>
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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.

Thanks.

> diff --git a/block/blk-core.c b/block/blk-core.c
> index d9d632639bd1..10c08ac50697 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -539,6 +539,7 @@ struct request_queue *blk_alloc_queue(int node_id)
>  		goto fail_stats;
>  
>  	q->backing_dev_info->ra_pages = VM_READAHEAD_PAGES;
> +	q->backing_dev_info->io_pages = VM_READAHEAD_PAGES;
>  	q->backing_dev_info->capabilities = BDI_CAP_CGROUP_WRITEBACK;
>  	q->node = node_id;


-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>