Re: [PATCH] mm: Fix a NULL dereference crash while accessing bdev->bd_disk

[Wei Fang <fangwei1@huawei.com>]

Hi, Jan,

On 2016/11/28 18:07, Jan Kara wrote:
> Good catch but I don't like sprinkling checks like this into the writeback
> code and furthermore we don't want to call into writeback code when block
> device is in the process of being destroyed which is what would happen with
> your patch. That is a bug waiting to happen...

Agreed. Need another way to fix this problem. I looked through the
writeback cgroup code in __filemap_fdatawrite_range(), found if we
turn on CONFIG_CGROUP_WRITEBACK, a new crash will happen.

Thanks,
Wei

> As I'm looking into the code, we need a serialization between bdev writeback
> and blkdev_put(). That should be doable if we use writeback_single_inode()
> for writing bdev inode instead of simple filemap_fdatawrite() and then use
> inode_wait_for_writeback() in blkdev_put() but it needs some careful
> thought.
> 
> Frankly that whole idea of tearing block devices down on last close is a
> major headache and keeps biting us. I'm wondering whether it is still worth
> it these days...
> 
> 								Honza
> 
>> ---
>>  mm/filemap.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index 235021e..d607677 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -334,8 +334,9 @@ int __filemap_fdatawrite_range(struct address_space *mapping, loff_t start,
>>  		.range_end = end,
>>  	};
>>  
>> -	if (!mapping_cap_writeback_dirty(mapping))
>> -		return 0;
>> +	if (!sb_is_blkdev_sb(mapping->host->i_sb))
>> +		if (!mapping_cap_writeback_dirty(mapping))
>> +			return 0;
>>  
>>  	wbc_attach_fdatawrite_inode(&wbc, mapping->host);
>>  	ret = do_writepages(mapping, &wbc);
>> -- 
>> 2.4.11
>>

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