From: Dave Chinner Subject: Re: [PATCH 4/4] Adds ioctl interface support for ext4 project Date: Fri, 26 Sep 2014 08:22:21 +1000 Message-ID: <20140925222221.GI4945@dastard> References: <1411567470-31799-1-git-send-email-lixi@ddn.com> <1411567470-31799-5-git-send-email-lixi@ddn.com> <20140924162507.GC27000@quack.suse.cz> <20140924162634.GA16886@infradead.org> <20140924170105.GE27000@quack.suse.cz> <20140925075912.GG4758@dastard> <20140925134136.GE4592@thunk.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , Christoph Hellwig , adilger-m1MBpc4rdrD3fQ9qLvQP4Q@public.gmane.org, linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, xfs-VZNHf3L845pBDgjK7y7TUQ@public.gmane.org, dmonakhov-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org, viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org, Li Xi , linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Theodore Ts'o Return-path: Content-Disposition: inline In-Reply-To: <20140925134136.GE4592-AKGzg7BKzIDYtjvyW6yDsg@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-ext4.vger.kernel.org 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-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org