Re: [PATCH 10/15] block, dax: fix lifetime of in-kernel dax mappings

[Dan Williams]

On Wed, Oct 7, 2015 at 3:56 PM, Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote:
> Hi Dan,
>
> We've uncovered another issue during testing with these patches. We get a
> kernel panic sometimes just while using a DAX filesystem. I've traced the
> issue back to this patch. (There's a stack trace at the end of this email.)
>
> On 22/09/15 10:42 PM, Dan Williams wrote:
>>
>> +static void dax_unmap_bh(const struct buffer_head *bh, void __pmem *addr)
>> +{
>> +       struct block_device *bdev = bh->b_bdev;
>> +       struct request_queue *q = bdev->bd_queue;
>> +
>> +       if (IS_ERR(addr))
>> +               return;
>> +       blk_dax_put(q);
>>   }
>>
>> @@ -127,9 +159,8 @@ static ssize_t dax_io(struct inode *inode, struct
>> iov_iter *iter,
>>                         if (pos == bh_max) {
>>                                 bh->b_size = PAGE_ALIGN(end - pos);
>>                                 bh->b_state = 0;
>> -                               retval = get_block(inode, block, bh,
>> -                                                  iov_iter_rw(iter) ==
>> WRITE);
>> -                               if (retval)
>> +                               rc = get_block(inode, block, bh, rw ==
>> WRITE);
>> +                               if (rc)
>>                                         break;
>>                                 if (!buffer_size_valid(bh))
>>                                         bh->b_size = 1 << blkbits;
>> @@ -178,8 +213,9 @@ static ssize_t dax_io(struct inode *inode, struct
>> iov_iter *iter,
>>
>>         if (need_wmb)
>>                 wmb_pmem();
>> +       dax_unmap_bh(bh, kmap);
>>
>> -       return (pos == start) ? retval : pos - start;
>> +       return (pos == start) ? rc : pos - start;
>>   }
>
>
> The problem is if get_block fails and returns an error code, it will still
> call dax_unmap_bh which tries to dereference bh->b_bdev. However, seeing
> get_block failed, that pointer is NULL. Maybe a null check in dax_unmap_bh
> would be sufficient?

Thanks for the report, I have this fixed up in v2.  Will post shortly.
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