Re: Clarification of statx->attributes_mask meaning?

["Theodore Y. Ts'o" <tytso@mit.edu>]

On Mon, Nov 30, 2020 at 05:29:47PM -0600, Eric Sandeen wrote:
> On 11/25/20 3:50 PM, David Howells wrote:
> > Darrick J. Wong <darrick.wong@oracle.com> wrote:
> > 
> >> mask=1 bit=0: "attribute not set on this file"
> >> mask=1 bit=1: "attribute is set on this file"
> >> mask=0 bit=0: "attribute doesn't fit into the design of this fs"
> > 
> > Or is "not supported by the filesystem driver in this kernel version".
> 
> For a concrete example, let's talk about the DAX statx attribute.
> 
> If the kernel is configured w/o DAX support, should the DAX attr be in the mask?
> If the block device has no DAX support, should the DAX attr be in the mask?
> If the filesystem is mounted with dax=never, should the DAX attr be in the mask?
> 
> About to send a patch for xfs which answers "no" to all of those, but I'm still
> not quite sure if that's what's expected.  I'll be sure to cc: dhowells, Ira, and
> others who may care...

So you're basically proposing that the mask is indicating whether or
not the attribute is supported by a particular on-disk file system
image and/or how it is currently configured/mounted --- and not
whether an attribute is supported by a particular file system
*implementation*.

For example, for ext4, if the extents feature is not enabled (for
example, when the ext4 file system code is used mount a file system
whose feature bitmask is consistent with a historic ext2 file system)
the extents flag should be cleared from the attribute mask?

This adds a fair amount of complexity to the file system since there
are a number of flags that might have similar issues --- for example,
FS_CASEFOLD_FL, and I could imagine for some file systems, where
different revisions might or might not support reflink FS_NOCOW_FL,
etc.

We should be really clear how applications are supposed to use the
attributes_mask.  Does it mean that they will always be able to set a
flag which is set in the attribute mask?  That can't be right, since
there will be a number of flags that may have some more complex checks
(you must be root, or the file must be zero length, etc.)  I'm a bit
unclear about what are the useful ways in which an attribute_mask can
be used by a userspace application --- and under what circumstances
might an application be depending on the semantics of attribute_mask,
so we don't accidentally give them an opportunity to complain and
whine, thus opening ourselves to another O_PONIES controversy.

> >> mask=0 bit=1: "filesystem is lying snake"
> > 
> > I like your phrasing :-)
> > 
> >> It's up to the fs driver and not the vfs to set attributes_mask, and
> >> therefore (as I keep pointing out to XiaoLi Feng) xfs_vn_getattr should
> >> be setting the mask.

... or maybe the on-disk file system is inconsistent....

       	     	 	      	     	- Ted