Re: Clarification of statx->attributes_mask meaning?

[Eric Sandeen <sandeen@sandeen.net>]

On 11/30/20 9:20 PM, Theodore Y. Ts'o wrote:
> 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*.

Well, not trying to propose anything new, just trying to understand
the intent of the mask to get it set correctly for the dax attribute.

> 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.

I've been told that I'm over-complicating this, yes.

> 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.

Hah, indeed.

Sorry if I've over-complicated this, I'm honestly just confused now.

-Eric