Re: [PATCH 2/2] statx: move STATX_ATTR_DAX attribute handling to filesystems

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Dec 01, 2020 at 10:59:36AM -0600, Eric Sandeen wrote:
> It's a bit odd to set STATX_ATTR_DAX into the statx attributes in the VFS;
> while the VFS can detect the current DAX state, it is the filesystem which
> actually sets S_DAX on the inode, and the filesystem is the place that
> knows whether DAX is something that the "filesystem actually supports" [1]
> so that the statx attributes_mask can be properly set.
> 
> So, move STATX_ATTR_DAX attribute setting to the individual dax-capable
> filesystems, and update the attributes_mask there as well.
> 
> [1] 3209f68b3ca4 statx: Include a mask for stx_attributes in struct statx
> 
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> ---
>  fs/ext2/inode.c   | 6 +++++-
>  fs/ext4/inode.c   | 5 ++++-
>  fs/stat.c         | 3 ---
>  fs/xfs/xfs_iops.c | 5 ++++-
>  4 files changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> index 11c5c6fe75bb..3550783a6ea0 100644
> --- a/fs/ext2/inode.c
> +++ b/fs/ext2/inode.c
> @@ -1653,11 +1653,15 @@ int ext2_getattr(const struct path *path, struct kstat *stat,
>  		stat->attributes |= STATX_ATTR_IMMUTABLE;
>  	if (flags & EXT2_NODUMP_FL)
>  		stat->attributes |= STATX_ATTR_NODUMP;
> +	if (IS_DAX(inode))
> +		stat->attributes |= STATX_ATTR_DAX;
> +
>  	stat->attributes_mask |= (STATX_ATTR_APPEND |
>  			STATX_ATTR_COMPRESSED |
>  			STATX_ATTR_ENCRYPTED |
>  			STATX_ATTR_IMMUTABLE |
> -			STATX_ATTR_NODUMP);
> +			STATX_ATTR_NODUMP |
> +			STATX_ATTR_DAX);
>  
>  	generic_fillattr(inode, stat);
>  	return 0;
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 0d8385aea898..848a0f2b154e 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -5550,13 +5550,16 @@ int ext4_getattr(const struct path *path, struct kstat *stat,
>  		stat->attributes |= STATX_ATTR_NODUMP;
>  	if (flags & EXT4_VERITY_FL)
>  		stat->attributes |= STATX_ATTR_VERITY;
> +	if (IS_DAX(inode))
> +		stat->attributes |= STATX_ATTR_DAX;
>  
>  	stat->attributes_mask |= (STATX_ATTR_APPEND |
>  				  STATX_ATTR_COMPRESSED |
>  				  STATX_ATTR_ENCRYPTED |
>  				  STATX_ATTR_IMMUTABLE |
>  				  STATX_ATTR_NODUMP |
> -				  STATX_ATTR_VERITY);
> +				  STATX_ATTR_VERITY |
> +				  STATX_ATTR_DAX);
>  
>  	generic_fillattr(inode, stat);
>  	return 0;
> diff --git a/fs/stat.c b/fs/stat.c
> index dacecdda2e79..5bd90949c69b 100644
> --- a/fs/stat.c
> +++ b/fs/stat.c
> @@ -80,9 +80,6 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
>  	if (IS_AUTOMOUNT(inode))
>  		stat->attributes |= STATX_ATTR_AUTOMOUNT;
>  
> -	if (IS_DAX(inode))
> -		stat->attributes |= STATX_ATTR_DAX;
> -
>  	if (inode->i_op->getattr)
>  		return inode->i_op->getattr(path, stat, request_mask,
>  					    query_flags);
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index 1414ab79eacf..56deda7042fd 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -575,10 +575,13 @@ xfs_vn_getattr(
>  		stat->attributes |= STATX_ATTR_APPEND;
>  	if (ip->i_d.di_flags & XFS_DIFLAG_NODUMP)
>  		stat->attributes |= STATX_ATTR_NODUMP;
> +	if (IS_DAX(inode))
> +		stat->attributes |= STATX_ATTR_DAX;
>  
>  	stat->attributes_mask |= (STATX_ATTR_IMMUTABLE |
>  				  STATX_ATTR_APPEND |
> -				  STATX_ATTR_NODUMP);
> +				  STATX_ATTR_NODUMP |
> +				  STATX_ATTR_DAX);

TBH I preferred your previous iteration on this, which only set the DAX
bit in the attributes_mask if the underlying storage was pmem and the
blocksize was correct, etc. since it made it easier to distinguish
between a filesystem where you /could/ have DAX (but it wasn't currently
enabled) and a filesystem where it just isn't possible.

That might be enough to satisfy any critics who want to be able to
detect DAX support from an installer program.

--D

>  
>  	switch (inode->i_mode & S_IFMT) {
>  	case S_IFBLK:
> -- 
> 2.17.0
> 
>