Date: Tue, 23 Aug 2011 16:11:20 -0400 (EDT) From: Rick Macklem <rmacklem@uoguelph.ca> To: Pawel Jakub Dawidek <pjd@FreeBSD.org> Cc: Hiroki Sato <hrs@FreeBSD.org>, current@FreeBSD.org, Benjamin Kaduk <kaduk@MIT.EDU> Subject: Re: fsid change of ZFS? Message-ID: <1614657395.247867.1314130280524.JavaMail.root@erie.cs.uoguelph.ca> In-Reply-To: <20110823151041.GA1697@garage.freebsd.pl>
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Pawel Jakub Dawidek wrote: > On Tue, Aug 23, 2011 at 10:09:41AM -0400, Rick Macklem wrote: > > Ok, I'll admit I wasn't very fond of a fixed table that would > > inevitably > > get out of date someday, either. > > > > I didn't think hashing for the cases not in the table was worth the > > effort, > > but doing a hash instead of a table seems reasonable. > > > > I see that ZFS only uses the low order 8 bits, so I'll try and come > > up > > with an 8bit hash solution and will post a patch for testing/review > > soon. > > > > I don't think the vfs_sysctl() is that great a concern, given that > > it > > appears to be deprecated already anyhow. (With an 8bit hash, > > vfs_typenum > > won't be that sparse.) I'll also make sure that whatever hash I use > > doesn't collide for the current list of file names (although I will > > include > > code that handles a collision in the patch). > > Sounds great. Thanks! > Here's the patch. (Hiroki could you please test this, thanks, rick.) ps: If the white space gets trashed, the same patch is at: http://people.freebsd.org/~rmacklem/fsid.patch --- kern/vfs_init.c.sav 2011-06-11 18:58:33.000000000 -0400 +++ kern/vfs_init.c 2011-08-23 15:55:30.000000000 -0400 @@ -39,6 +39,7 @@ __FBSDID("$FreeBSD: head/sys/kern/vfs_in #include <sys/param.h> #include <sys/systm.h> +#include <sys/hash.h> #include <sys/kernel.h> #include <sys/linker.h> #include <sys/mount.h> @@ -138,6 +139,8 @@ 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); @@ -152,7 +155,27 @@ 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 = hash32_str(vfc->vfc_name, 0); + hashval = ((hashval & 0xff) + ((hashval >> 8) & 0xff) + + ((hashval >> 16) & 0xff) + (hashval >> 24)) & 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. */ + break; + } + } + } while (tvfc != NULL); + vfc->vfc_typenum = hashval; + if (vfc->vfc_typenum >= maxvfsconf) + maxvfsconf = vfc->vfc_typenum + 1; TAILQ_INSERT_TAIL(&vfsconf, vfc, vfc_list); /*
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