Re: [Ext2-devel] [PATCH 1/2] ext2/3: Support 2^32-1 blocks(Kernel)

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Sat, 2006-03-25 at 11:51 -0300, cascardo@minaslivre.org wrote:

> Regarding compatibility, there are plans to support xattr in Hurd and
> use them for these fields, translator and author. (I can't recall what
> i_mode_high is used for.) With respect to that, I'd appreciate if
> there is a recommendation to every ext2 implementation (not only
> Linux) that supports xattr, to support gnu.translator and gnu.author
> (I'll check about the i_mode_high and post about it asap.). 

What do you mean by "support", exactly?

There are 3 different bits of xattr design which matter here.  There's
the namespace exported to users via the *attr syscalls; there's the
encoding used on disk for those different namespaces; and there's the
exact semantics surrounding interpretation of the xattr contents.

Now, a non-Hurd system is not going to have any use for the gnu.* xattr
semantics, as translator is a Hurd-specific concept.  The user "gnu.*"
namespace is easy enough to teach to Linux: to simply reserve that
namespace, without actually implementing any part of it, I think it be
sufficient simply to claim the name in include/linux/xattr.h.

For ext2/3, though, the key is how to store gnu.* on disk.  Right now
the different namespaces that ext* stores on disk are enumerated in

	fs/ext[23]/xattr.h

which, for ext2, currently contains:

        /* Name indexes */
        /* Name indexes */
        #define EXT2_XATTR_INDEX_USER			1
        #define EXT2_XATTR_INDEX_POSIX_ACL_ACCESS	2
        #define EXT2_XATTR_INDEX_POSIX_ACL_DEFAULT	3
        #define EXT2_XATTR_INDEX_TRUSTED		4
        #define	EXT2_XATTR_INDEX_LUSTRE			5
        #define EXT2_XATTR_INDEX_SECURITY	        6

If you want to reserve a new semantically-significant portion of the
namespace for use in the Hurd by gnu.* xattrs, then you'd need to submit
an authoritative Linux patch to register a new name index on ext2;
reservation of such an xattr namespace index is in effect an on-disk
format decision so needs to be agreed between implementations.

> Regarding userland tools, it would be wise if they would still support
> old format filesystems, including those with fs creator set to
> Hurd. That would include supporting the oob block for translator when
> counting used/free blocks and other operations like copying a file
> using debugfs, for example.

Certainly; I don't think anybody is arguing against that, and I regard
such backwards compatibility as an absolute requirement.

--Stephen