On Thu, Sep 25, 2014 at 09:41:37AM -0400, Theodore Ts'o wrote: > On Thu, Sep 25, 2014 at 05:59:12PM +1000, Dave Chinner wrote: > > > Also I'm afraid we may quickly run out of > > > 32 available flags in xflags so we'd need to extend that. But all this > > > seems to be doable. > > > > The struct fsxattr was designed to be extensible - it has unused > > padding and enough space in the flags field to allow us to > > conditionally use that padding.... > > I agree that it would be useful for ext4 to support as much of the > XFS_IOC_GETXATTR/XFS_IOC_SETATTR as would make sense for ext4, and to > use that to set/get the project ID. (And that we should probably do > that as a separate set of patches that we could potentially go into > ext4 ahead of the project quota while it is undergoing testing and > review.) > > A few questions of Dave and other XFS folks: > > 1) If we only implement a partial set of the flags or other > functionality, are there going to be tools that get confused? i.e., > are there any userspace programs that will test for whether the ioctl > is supported, and then assume that some minimal set of functionality > must be implemented? No, I don't think they will get confused. The use of the flags is get/modify/set just like other flag setting functions. The extsize and projid fields are condition on the relevant flag being set on return from a get (i.e. projid is only valid if XFS_XFLAG_PROJID[_INHERIT] is set), and those fields are only considered valid on set if those flags are set by the application (or remain set as a result of the getxattr). Hence the applications that use the getxattr/setxattr interface correctly shouldn't care what set of flags and values the filesystem supports other than the specific flags the application needs the filesystem to understand. > 2) Unless I'm missing something, there is nothing that enforces that > fsx_pad must be zero. I assume that means that the only way you can > expand use of fields into that space is via a flag bit being consumed? Yup, that's exactly what I meant by "conditionally use the padding". Even if the padding was guaranteed to be zero, I'd strongly recommend a flag bit to indicate the application understands that the padding region has actual meaning to guard against buggy applications. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
