Re: [PATCH 1/3] ext4: Add alignment check for DAX mount

[Boaz Harrosh]

On 05/01/2016 08:31 PM, Christoph Hellwig wrote:
> On Fri, Apr 29, 2016 at 02:39:33PM -0600, Toshi Kani wrote:
>> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
>> index 304c712..90a8670 100644
>> --- a/fs/ext4/super.c
>> +++ b/fs/ext4/super.c
>> @@ -3421,6 +3421,12 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>>  					"error: unsupported blocksize for dax");
>>  			goto failed_mount;
>>  		}
>> +		if (sb->s_bdev->bd_part->start_sect % (PAGE_SIZE / 512) ||
>> +		    sb->s_bdev->bd_part->nr_sects % (PAGE_SIZE / 512)) {
>> +			ext4_msg(sb, KERN_ERR,
>> +					"error: unaligned partition for dax");
>> +			goto failed_mount;
>> +		}
>>  		if (!sb->s_bdev->bd_disk->fops->direct_access) {
>>  			ext4_msg(sb, KERN_ERR,
>>  					"error: device does not support dax");
> 
> Factor your new checks and the ->direct_access into a new helper. It
> should take the block device as file systems might have multiple
> underlying devices.

This already exists as part of bdev_direct_access()

All the code needs to do is call bdev_direct_access() on sector ZERO and
see if it is successful. If the alignment is broken it will fail.

Infact the FS does not even need the second if (!...bd_disk->fops->direct_access)
check either because it is also done inside bdev_direct_access(). The only check
needed is calling bdev_direct_access() and the generic layer does all the
checks needed already.

Cheers
Boaz