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@xxxxxxxxxx> 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
