Re: INFO: task hung in generic_file_write_iter

[Tetsuo Handa]

On 2019/01/03 2:26, Jan Kara wrote:
> On Thu 03-01-19 01:07:25, Tetsuo Handa wrote:
>> On 2019/01/02 23:40, Jan Kara wrote:
>>> I had a look into this and the only good explanation for this I have is
>>> that sb->s_blocksize is different from (1 << sb->s_bdev->bd_inode->i_blkbits).
>>> If that would happen, we'd get exactly the behavior syzkaller observes
>>> because grow_buffers() would populate different page than
>>> __find_get_block() then looks up.
>>>
>>> However I don't see how that's possible since the filesystem has the block
>>> device open exclusively and blkdev_bszset() makes sure we also have
>>> exclusive access to the block device before changing the block device size.
>>> So changing block device block size after filesystem gets access to the
>>> device should be impossible. 
>>>
>>> Anyway, could you perhaps add to your debug patch a dump of 'size' passed
>>> to __getblk_slow() and bdev->bd_inode->i_blkbits? That should tell us
>>> whether my theory is right or not. Thanks!
>>>
>>
>> OK. Andrew, will you add (or fold into) this change?
>>
>> From e6f334380ad2c87457bfc2a4058316c47f75824a Mon Sep 17 00:00:00 2001
>> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
>> Date: Thu, 3 Jan 2019 01:03:35 +0900
>> Subject: [PATCH] fs/buffer.c: dump more info for __getblk_gfp() stall problem
>>
>> We need to dump more variables on top of
>> "fs/buffer.c: add debug print for __getblk_gfp() stall problem".
>>
>> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
>> Cc: Jan Kara <jack@xxxxxxx>
>> ---
>>  fs/buffer.c | 9 +++++++--
>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/buffer.c b/fs/buffer.c
>> index 580fda0..a50acac 100644
>> --- a/fs/buffer.c
>> +++ b/fs/buffer.c
>> @@ -1066,9 +1066,14 @@ static sector_t blkdev_max_block(struct block_device *bdev, unsigned int size)
>>  #ifdef CONFIG_DEBUG_AID_FOR_SYZBOT
>>  		if (!time_after(jiffies, current->getblk_stamp + 3 * HZ))
>>  			continue;
>> -		printk(KERN_ERR "%s(%u): getblk(): executed=%x bh_count=%d bh_state=%lx\n",
>> +		printk(KERN_ERR "%s(%u): getblk(): executed=%x bh_count=%d bh_state=%lx "
>> +		       "bdev_super_blocksize=%lu size=%u "
>> +		       "bdev_super_blocksize_bits=%u bdev_inode_blkbits=%u\n",
>>  		       current->comm, current->pid, current->getblk_executed,
>> -		       current->getblk_bh_count, current->getblk_bh_state);
>> +		       current->getblk_bh_count, current->getblk_bh_state,
>> +		       bdev->bd_super->s_blocksize, size,
>> +		       bdev->bd_super->s_blocksize_bits,
>> +		       bdev->bd_inode->i_blkbits);
> 
> Well, bd_super may be NULL if there's no filesystem mounted so it would be
> safer to check for this rather than blindly dereferencing it... Otherwise
> the change looks good to me.

I see. Let's be cautious here.