On Tue, Jan 07, 2014 at 12:04:30PM -0500, Theodore Ts'o wrote: > On Tue, Jan 07, 2014 at 07:49:35AM -0800, Christoph Hellwig wrote: > > On Tue, Jan 07, 2014 at 01:48:31PM +0100, Jan Kara wrote: > > > I have to say I'm not thrilled by the idea of juggling strings in > > > userspace and in kernel to set a flag for an inode... > > > > Nevermind the massive amounts of code that sit in the filesystem. > > The reason for this patch was to address what Dave Chinner has called > "a shitty interface"[1]. Using bitfields that need to be coordinated > across file systems, when sometimes a bit assignment is validly a fs > specific thing, and then later becomes something that gets shared > across file systems. > > [1] http://thread.gmane.org/gmane.linux.file-systems/80164/focus=80396 > > If we don't go about it this way, there are alternatives: we could > create new ioctls (or a new syscall) as we start running out of bits > used by FS_IOC_[GS]ETFLAGS. We can create new ioctls for bits which > are intended for fs-specific flags, which then later get promoted to > the new syscall when some functionality starts to get shared accross > other file systems (probably with a different bit assignment). This > is certainly less code, but it does mean more complexity outside of > the code when we try to coordinate new functionality across file > systems. I had thought of indexed inode flags as an alternative to the xattr/string parsing thing. Feature flags make their first appearance as part of a per-FS flag-space and are migrated to the common flag-space when there is demand. It would also avoid the need for each fs to create its own flag ioctl. On the other hand, someone suggested I try remaking IOC_[GS]ETFLAG as an xattr, so off I went. :) #define FS_IOC_FLAGS_COMMON 0 #define FS_IOC_FLAGS_COMMON2 1 #define FS_IOC_FLAGS_EXT4 0xEF53 struct inode_flag_ioctl { u32 flag; u32 value; /* or u64? */ }; #define FS_IOC_GETFLAGS2 _IOR('f', 12, struct inode_flag_ioctl); #define FS_IOC_SETFLAGS2 _IOW('f', 13, struct inode_flag_ioctl); foo() { struct inode_flag_ioctl if; if.flag = FS_IOC_FLAGS_COMMON; ioctl(fd, FS_IOC_GETFLAGS2, &if); printf("%d\n", if.value); if.flag = FS_IOC_FLAGS_EXT4; if.value = EXT5_BONGHITS_FL | EXT4_EA_INODE_FL; ioctl(fd, FS_IOC_SETFLAGS2, &if); } > Personally, I don't mind dealing with codepoint assignments, but my > impression is that this is a minority viewpoint. Al and Linus have > historically hated bitfields, and Al in the past has spoken favorably > of Plan 9's approach of using strings for the system interface. I prefer strings too, but I suppose one pays for the complexity. Given that all the flags so far seem to have been booleans, this could be good enough. > So while I have a preference towards using bitfields, as opposed to > using the xattr approach, what I'd really like is that we make a > decision, one way or another, about what's the best way to move > forward. Agreed. --D > > - Ted > -- > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
