Re: [PATCH 4/6] Ext4: fail if we try to use hole punch

[Ted Ts'o <tytso@mit.edu>]

> >There is no simple way to test if a filesystem supports hole punching or not so
> >the check has to be done per fs.  Thanks,
> 
> Could put a flag word in superblock_operations.  Filesystems which
> support punching (or other features) can enable it there.

No, it couldn't be in super_operations.  It may vary on a per-inode
basis for some file systems, such as ext4 (depending on whether the
inode is extent-mapped or indirect-block mapped).

So at least for ext4 we'd need to call into fallocate() function
anyway, once we add support.  I suppose if other file systems really
want it, we could add a flag to the super block ops structure, so they
don't have do the "do we support the punch" operation.  I can go
either way on that; although if we think the majority of file systems
are going support punch in the long-term, then it might not be worth
it to add such a flag.

						- Ted