Re: fsid change of ZFS?

On Wed, 24 Aug 2011, Rick Macklem wrote:

> "afs"

The current OpenAFS codebase uses the all-caps "AFS".  Judging by the 
omitted text, perhaps this should change.  (We also don't use VFS_SET to 
set it, which I filed a bug about.)

>
> and here is my current rendition of the patch. (I took Gleb's suggestion
> and switched to fnv_32_str(). I'll leave it that way unless there is a
> collision after adding any names people post to the above list.)
>
> It sounds like people have agreed that this is a reasonable solution.
> If hrs_at_ can confirm that testing shows it fixes the original problem
> (the ZFS file handles don't change when it's loaded at different times),
> I'll pass it along to re_at_.
>
> Thanks for the helpful suggestions, rick
>
> --- kern/vfs_init.c.sav	2011-06-11 18:58:33.000000000 -0400
> +++ kern/vfs_init.c	2011-08-24 16:15:24.000000000 -0400
> _at__at_ -39,6 +39,7 _at__at_ __FBSDID("$FreeBSD: head/sys/kern/vfs_in
>
> #include <sys/param.h>
> #include <sys/systm.h>
> +#include <sys/fnv_hash.h>
> #include <sys/kernel.h>
> #include <sys/linker.h>
> #include <sys/mount.h>
> _at__at_ -138,6 +139,8 _at__at_ vfs_register(struct vfsconf *vfc)
> 	struct sysctl_oid *oidp;
> 	struct vfsops *vfsops;
> 	static int once;
> +	struct vfsconf *tvfc;
> +	uint32_t hashval;
>
> 	if (!once) {
> 		vattr_null(&va_null);
> _at__at_ -152,7 +155,26 _at__at_ vfs_register(struct vfsconf *vfc)
> 	if (vfs_byname(vfc->vfc_name) != NULL)
> 		return EEXIST;
>
> -	vfc->vfc_typenum = maxvfsconf++;
> +	/*
> +	 * Calculate a hash on vfc_name to use for vfc_typenum. Unless
> +	 * a collision occurs, it is limited to 8bits since that is
> +	 * what ZFS uses from vfc_typenum and that also limits how sparsely
> +	 * distributed vfc_typenum becomes.
> +	 */
> +	hashval = fnv_32_str(vfc->vfc_name, FNV1_32_INIT);
> +	hashval &= 0xff;
> +	do {
> +		/* Look for and fix any collision. */
> +		TAILQ_FOREACH(tvfc, &vfsconf, vfc_list) {
> +			if (hashval == tvfc->vfc_typenum) {
> +				hashval++; /* Can exceed 8bits, if needed. */

If we're confident that we won't ever fully fill the hash table, I would 
think that this should wrap around back to zero (or one?) instead of 
overflowing.

Do we need to care about something attempting to add the same vfc_name 
twice?  This code will happily add a second entry at the next available 
index.

> +				break;
> +			}
> +		}
> +	} while (tvfc != NULL);
> +	vfc->vfc_typenum = hashval;
> +	if (vfc->vfc_typenum >= maxvfsconf)
> +		maxvfsconf = vfc->vfc_typenum + 1;

I guess we're holding off on killing maxvfsconf until after 9.0 is out?

-Ben

> 	TAILQ_INSERT_TAIL(&vfsconf, vfc, vfc_list);
>
> 	/*
>
>
>