Re: [PATCH] xfs: return correct XFS_IOC_DIOINFO for DAX inode

[Eric Sandeen <sandeen@redhat.com>]

On 4/2/19 12:56 PM, Darrick J. Wong wrote:
> On Tue, Apr 02, 2019 at 10:44:38AM -0500, Eric Sandeen wrote:
>> pmem is byte addressable, and indeed byte-aligned DIO works on
>> a DAX file.  So, teach XFS_IOC_DIOINFO to return the correct
>> alignment information if IS_DAX(inode).
> 
> If it's a DAX filesystem, do we want to try to steer people towards
> things like 2MB pages since (in theory) we can get away with fewer page
> table mappings?  And (seeing as that's mmap that cares, not directio)
> would advertising preferential page mapping sizes be more appropriate
> advertised in a different ioctl?

The xfsctl(3) manpage documents XFS_IOC_DIOINFO as providing the
minimum/required alignments to avoid DIO failure.  Says nothing about
optimal.  So, if you'd like to advertise preferences, it seems like
this is not the interface to use...

-Eric

> --D
> 
>> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
>> ---
>>
>> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
>> index 6ecdbb3..35eae7d 100644
>> --- a/fs/xfs/xfs_ioctl.c
>> +++ b/fs/xfs/xfs_ioctl.c
>> @@ -1919,12 +1919,21 @@ xfs_file_ioctl(
>>  	}
>>  	case XFS_IOC_DIOINFO: {
>>  		struct dioattr	da;
>> -		xfs_buftarg_t	*target =
>> -			XFS_IS_REALTIME_INODE(ip) ?
>> -			mp->m_rtdev_targp : mp->m_ddev_targp;
>>  
>> -		da.d_mem =  da.d_miniosz = target->bt_logical_sectorsize;
>> -		da.d_maxiosz = INT_MAX & ~(da.d_miniosz - 1);
>> +		if (IS_DAX(inode)) {
>> +			/* pmem is byte addressable */
>> +			da.d_mem = 1;
>> +			da.d_miniosz = 1;
>> +			da.d_maxiosz = INT_MAX;
>> +		} else {
>> +			xfs_buftarg_t	*target =
>> +				XFS_IS_REALTIME_INODE(ip) ?
>> +				mp->m_rtdev_targp : mp->m_ddev_targp;
>> +
>> +			da.d_mem = target->bt_logical_sectorsize;
>> +			da.d_miniosz = target->bt_logical_sectorsize;
>> +			da.d_maxiosz = INT_MAX & ~(da.d_miniosz - 1);
>> +		}
>>  
>>  		if (copy_to_user(arg, &da, sizeof(da)))
>>  			return -EFAULT;
>>