Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization

["J. Bruce Fields" <bfields@fieldses.org>]

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);