Re: Questions about exportfs

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Nov 27, 2008 at 02:51:40PM +0000, David Howells wrote:
> 
> Hi Christoph,
> 
> Can you answer some questions about exportfs please?
> 
>  (1) In linux/exportfs.h, the description of the encode_fh() op says that
>      "encode_fh() should return the number of bytes stored or a negative error
>      code such as %-ENOSPC".
> 
>      This, however, contradicts export_encode_fh(), which returns a member of
>      enum fid_type instead.  I presume the docs are wrong, or at least,
>      unclear.

No negative error codes, just 255 (or anything that would give 255 when
converted to u8, actually).
 
>  (2) Why does exportfs_encode_fh() return the type of the FID?  Why not embed
>      it in the fid struct?  It seems that the value has to be handed back to
>      the decode routine.

Take a look at the struct nfs_fhbase_new in include/linux/nfsd/nfsfh.h;
we have type information for all components in the beginning, packed
together, instead of embedding each into its part of array.

>  (3) Is fat_encode_fh() allowed to return a value not in the fid_type enum?

Yes.  That enum is just a bunch of common encoding schemes.  The values you
are using are entirely[*] up to specific filesystem; these ones are just for
common helper functions.

[*] other than "255 is reserved for encoding error"