From: "NeilBrown" Subject: Re: [opensuse] nfs_update_inode: inode X mode changed, Y to Z Date: Wed, 26 Mar 2008 10:09:00 +1100 (EST) Message-ID: <45922.192.168.1.70.1206486540.squirrel@neil.brown.name> References: <9a8748490803121513w285cd45rb6b26a3d842cac1b@mail.gmail.com> <20080312221511.GC31632@fieldses.org> <9a8748490803121516u36395872i70cc88b0439adc74@mail.gmail.com> <18394.1501.991087.80264@notabene.brown> <47DAEFD0.9020407@m2000.com> <47E92F8E.7030504@m2000.com> <20080325190943.GF2237@fieldses.org> <32953.192.168.1.70.1206477121.squirrel@neil.brown.name> <20080325212425.GA20257@josefsipek.net> <34178.192.168.1.70.1206481102.squirrel@neil.brown.name> <20080325221321.GC20257@josefsipek.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Cc: "J. Bruce Fields" , xfs@oss.sgi.com, "Adam Schrotenboer" , "Jesper Juhl" , "Trond Myklebust" , linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, "Thomas Daniel" , "Frederic Revenu" , "Jeff Doan" To: "Josef 'Jeff' Sipek" Return-path: Received: from ns2.suse.de ([195.135.220.15]:60724 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760731AbYCYXJL (ORCPT ); Tue, 25 Mar 2008 19:09:11 -0400 In-Reply-To: <20080325221321.GC20257-PM1Ls4bqFqUFEYicpp4bmg@public.gmane.org> Sender: linux-nfs-owner@vger.kernel.org List-ID: On Wed, March 26, 2008 9:13 am, Josef 'Jeff' Sipek wrote: > On Wed, Mar 26, 2008 at 08:38:22AM +1100, NeilBrown wrote: > ... >> However you still need to do something about the generation number. It >> must be set to something. > > Right. > >> When you allocate an inode that doesn't currently exist on the device, >> you obviously cannot increment the old value and use that. > > Makes sense. > >> However you can do a lot better than always using 0. > > I looked at the code (xfs_ialloc.c:xfs_ialloc_ag_alloc) > > 290 /* > 291 * Set initial values for the inodes in this buffer. > 292 */ > 293 xfs_biozero(fbuf, 0, ninodes << > args.mp->m_sb.sb_inodelog); > 294 for (i = 0; i < ninodes; i++) { > 295 free = XFS_MAKE_IPTR(args.mp, fbuf, i); > 296 free->di_core.di_magic = > cpu_to_be16(XFS_DINODE_MAGIC); > 297 free->di_core.di_version = version; > 298 free->di_next_unlinked = > cpu_to_be32(NULLAGINO); > 299 xfs_ialloc_log_di(tp, fbuf, i, > 300 XFS_DI_CORE_BITS | > XFS_DI_NEXT_UNLINKED); > 301 } > > xfs_biozero(...) turns into a memset(buf, 0, len), and since the loop that > follows doesn't change the generation number, it'll stay 0. > >> The simplest would be to generate a 'random' number (get_random_bytes). >> Slightly better would be to generate a random number at boot time >> and use that, incrementing it each time it is used to set the >> generation number for an inode. > > I'm not familiar enough with NFS, do you want something that's > monotonically > increasing or do you just test for inequality? If it is inequality, why > not > just use something like the jiffies - that should be unique enough. > What we need is for the "filehandle" to be stable and unique. By 'stable' I mean that every time I get the filehandle for a particular file, I get the same string of bytes. By 'uniqie' I mean that if I get two filehandles for two different files, they must differ in at least one bit. If a file is deleted and the inode is re-used for a new file, then the old and new files are different and must have different file handles. The filehandle is traditionally generated from the inode number and a generation number, but the filesystem can actually do whatever it likes. xfs does it with xfs_fs_encode_fh(). Certainly you could initialise the i_generation to jiffies in xfs_ialloc_ag_alloc. That would be a suitable fix. get_random_bytes might be better, but the difference probably wouldn't be noticeable. NeilBrown