From: Jeff Layton Subject: Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization Date: Fri, 12 May 2017 07:01:25 -0400 Message-ID: <1494586885.2787.3.camel@redhat.com> References: <20170329111507.GA18467@quack2.suse.cz> <1490810071.2678.6.camel@redhat.com> <20170330064724.GA21542@quack2.suse.cz> <1490872308.2694.1.camel@redhat.com> <20170330161231.GA9824@fieldses.org> <1490898932.2667.1.camel@redhat.com> <20170404183138.GC14303@fieldses.org> <878tnfiq7v.fsf@notabene.neil.brown.name> <20170405080551.GC8899@quack2.suse.cz> <20170405181409.GC28681@fieldses.org> <20170511185942.GD25434@fieldses.org> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: NeilBrown , Christoph Hellwig , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-xfs@vger.kernel.org To: "J. Bruce Fields" , Jan Kara Return-path: In-Reply-To: <20170511185942.GD25434@fieldses.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Thu, 2017-05-11 at 14:59 -0400, J. Bruce Fields wrote: > On Wed, Apr 05, 2017 at 02:14:09PM -0400, J. Bruce Fields wrote: > > On Wed, Apr 05, 2017 at 10:05:51AM +0200, Jan Kara wrote: > > > 1) Keep i_version as is, make clients also check for i_ctime. > > > > That would be a protocol revision, which we'd definitely rather avoid. > > > > But can't we accomplish the same by using something like > > > > ctime * (some constant) + i_version > > > > ? > > > > > Pro: No on-disk format changes. > > > Cons: After a crash, i_version can go backwards (but when file changes > > > i_version, i_ctime pair should be still different) or not, data can be > > > old or not. > > > > This is probably good enough for NFS purposes: typically on an NFS > > filesystem, results of a read in the face of a concurrent write open are > > undefined. And writers sync before close. > > > > So after a crash with a dirty inode, we're in a situation where an NFS > > client still needs to resend some writes, sync, and close. I'm OK with > > things being inconsistent during this window. > > > > I do expect things to return to normal once that client's has resent its > > writes--hence the worry about actually resuing old values after boot > > (such as if i_version regresses on boot and then increments back to the > > same value after further writes). Factoring in ctime fixes that. > > So for now I'm thinking of just doing something like the following. > > Only nfsd needs it for now, but it could be moved to a vfs helper for > statx, or for individual filesystems that want to do something > different. (The NFSv4 client will want to use the server's change > attribute instead, I think. And other filesystems might want to try > something more ambitious like Neil's proposal.) > > --b. > > diff --git a/fs/nfsd/nfs3xdr.c b/fs/nfsd/nfs3xdr.c > index 12feac6ee2fd..9636c9a60aba 100644 > diff --git a/fs/nfsd/nfsfh.h b/fs/nfsd/nfsfh.h > index f84fe6bf9aee..14f09f1ef605 100644 > --- a/fs/nfsd/nfsfh.h > +++ b/fs/nfsd/nfsfh.h > @@ -240,6 +240,16 @@ fh_clear_wcc(struct svc_fh *fhp) > fhp->fh_pre_saved = false; > } > > +static inline u64 nfsd4_change_attribute(struct inode *inode) > +{ > + u64 chattr; > + > + chattr = inode->i_ctime.tv_sec << 30; > + chattr += inode->i_ctime.tv_nsec; > + chattr += inode->i_version; > + return chattr; > +} > + > /* > * Fill in the pre_op attr for the wcc data > */ > @@ -253,7 +263,7 @@ fill_pre_wcc(struct svc_fh *fhp) > fhp->fh_pre_mtime = inode->i_mtime; > fhp->fh_pre_ctime = inode->i_ctime; > fhp->fh_pre_size = inode->i_size; > - fhp->fh_pre_change = inode->i_version; > + fhp->fh_pre_change = nfsd4_change_attribute(inode); > fhp->fh_pre_saved = true; > } > } > --- a/fs/nfsd/nfs3xdr.c > +++ b/fs/nfsd/nfs3xdr.c > @@ -260,7 +260,7 @@ void fill_post_wcc(struct svc_fh *fhp) > printk("nfsd: inode locked twice during operation.\n"); > > err = fh_getattr(fhp, &fhp->fh_post_attr); > - fhp->fh_post_change = d_inode(fhp->fh_dentry)->i_version; > + fhp->fh_post_change = nfsd4_change_attribute(d_inode(fhp->fh_dentry)); > if (err) { > fhp->fh_post_saved = false; > /* Grab the ctime anyway - set_change_info might use it */ > diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c > index 26780d53a6f9..a09532d4a383 100644 > --- a/fs/nfsd/nfs4xdr.c > +++ b/fs/nfsd/nfs4xdr.c > @@ -1973,7 +1973,7 @@ static __be32 *encode_change(__be32 *p, struct kstat *stat, struct inode *inode, > *p++ = cpu_to_be32(convert_to_wallclock(exp->cd->flush_time)); > *p++ = 0; > } else if (IS_I_VERSION(inode)) { > - p = xdr_encode_hyper(p, inode->i_version); > + p = xdr_encode_hyper(p, nfsd4_change_attribute(inode)); > } else { > *p++ = cpu_to_be32(stat->ctime.tv_sec); > *p++ = cpu_to_be32(stat->ctime.tv_nsec); Sorry I've been MIA on this discussion. I've had a very busy spring... This looks reasonable to me (modulo Jan's comment about casting tv_sec to u64). To be clear, I think this is mostly orthogonal to the changes that I was originally proposing, right? I think we can still benefit from only bumping and storing i_version values after they've been queried. -- Jeff Layton