Subject: Re: [PATCH] jbd2: check superblock mapped prior to committing
To:     Jan Kara <jack@suse.cz>
Cc:     linux-ext4@vger.kernel.org, tytso@mit.edu,
        renzhen@linux.alibaba.com
References: <20190328091814.46168-1-jiufei.xue@linux.alibaba.com>
 <20190328095954.GE22915@quack2.suse.cz>
From:   Jiufei Xue <jiufei.xue@linux.alibaba.com>
Message-ID: <7bffa4dd-106a-7ab9-00a7-cab000304a39@linux.alibaba.com>
Date:   Thu, 28 Mar 2019 18:05:27 +0800
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Hi Jan,

On 2019/3/28 下午5:59, Jan Kara wrote:
> On Thu 28-03-19 17:18:14, Jiufei Xue wrote:
>> We hit a BUG at fs/buffer.c:3057 if we detached the nbd device
>> before unmounting ext4 filesystem.
>>
>> The typical chain of events leading to the BUG:
>> jbd2_write_superblock
>>   submit_bh
>>     submit_bh_wbc
>>       BUG_ON(!buffer_mapped(bh));
>>
>> The block device is removed and all the pages are invalidated. JBD2
>> was trying to write journal superblock to the block device which is
>> no longer present.
>>
>> Fix this by checking the journal superblock's buffer head prior to
>> submitting.
>>
>> Cc: stable@kernel.org
>> Reported-by: Eric Ren <renzhen@linux.alibaba.com>
>> Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
>> ---
>>  fs/jbd2/journal.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
>> index 382c030cc78b..90fd3ed0a232 100644
>> --- a/fs/jbd2/journal.c
>> +++ b/fs/jbd2/journal.c
>> @@ -1350,6 +1350,9 @@ static int jbd2_write_superblock(journal_t *journal, int write_flags)
>>  	journal_superblock_t *sb = journal->j_superblock;
>>  	int ret;
>>  
> Can you add a comment here like:
> 
> 	/* Buffer got discarded which means block device got invalidated */
> 
> to explain what's going on? Thanks! Otherwise the patch looks good to me so
> you can add:
> 
> Reviewed-by: Jan Kara <jack@suse.cz>
>

 
Thank you for your suggestion, I will add the comment and send version 2 soon.

Thanks,
Jiufei

> 								Honza
> 
>> +	if (!buffer_mapped(bh))
>> +		return -EIO;
>> +
> 
> 
>>  	trace_jbd2_write_superblock(journal, write_flags);
>>  	if (!(journal->j_flags & JBD2_BARRIER))
>>  		write_flags &= ~(REQ_FUA | REQ_PREFLUSH);
>> -- 
>> 2.19.1.856.g8858448bb
>>