On Mon, Apr 30, 2007 at 09:39:06PM -0700, Nicholas Miell wrote: > On Tue, 2007-05-01 at 14:22 +1000, David Chinner wrote: > > On Mon, Apr 30, 2007 at 04:44:01PM -0600, Andreas Dilger wrote: > > > This is actually for future use. Any flags that are added into this > > > range must be understood by both sides or it should be considered an > > > error. Flags outside the FIEMAP_FLAG_INCOMPAT do not necessarily need > > > to be supported. If it turns out that 8 bits is too small a range for > > > INCOMPAT flags, then we can make 0x01000000 an incompat flag that means > > > e.g. 0x00ff0000 are also incompat flags also. > > > > Ah, ok. So it's not really a set of "compatibility" flags, it's more a > > "compulsory" set. Under those terms, i don't really see why this is > > necessary - either the filesystem will understand the flags or it will > > return EINVAL or ignore them... > > > > > I'm assuming that all flags that will be in the original FIEMAP proposal > > > will be understood by the implementations. Most filesystems can safely > > > ignore FLAG_HSM_READ, for example, since they don't support HSM, and for > > > that matter FLAG_SYNC is probably moot for most filesystems also because > > > they do block allocation at preprw time. > > > > Exactly my point - so why do we really need to encode a compulsory set of > > > > Because flags have meaning, independent of whether or not the filesystem > understands them. And if the filesystem chooses to ignore critically > important flags (instead of returning EINVAL), bad things may happen. > > So, either the filesystem will understand the flag or iff the unknown flag > is in the incompat set, it will return EINVAL or else the unknown flag will > be safely ignored.
My point was that there is a difference between specification and implementation - if the specification says something is compulsory, then they must be implemented in the filesystem. This is easy enough to ensure by code review - we don't need additional interface complexity for this.... Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group - To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
