Re: [PATCH v6 6/9] nfsd: use the getattr operation to fetch i_version

[NeilBrown]

On Wed, 05 Oct 2022, Jeff Layton wrote:
> On Tue, 2022-10-04 at 10:39 +1100, NeilBrown wrote:
> > On Fri, 30 Sep 2022, Jeff Layton wrote:
> > > Now that we can call into vfs_getattr to get the i_version field, use
> > > that facility to fetch it instead of doing it in nfsd4_change_attribute.
> > > 
> > > Neil also pointed out recently that IS_I_VERSION directory operations
> > > are always logged, and so we only need to mitigate the rollback problem
> > > on regular files. Also, we don't need to factor in the ctime when
> > > reexporting NFS or Ceph.
> > > 
> > > Set the STATX_VERSION (and BTIME) bits in the request when we're dealing
> > > with a v4 request. Then, instead of looking at IS_I_VERSION when
> > > generating the change attr, look at the result mask and only use it if
> > > STATX_VERSION is set. With this change, we can drop the fetch_iversion
> > > export operation as well.
> > > 
> > > Move nfsd4_change_attribute into nfsfh.c, and change it to only factor
> > > in the ctime if it's a regular file and the fs doesn't advertise
> > > STATX_ATTR_VERSION_MONOTONIC.
> > > 
> > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > ---
> > >  fs/nfs/export.c          |  7 -------
> > >  fs/nfsd/nfs4xdr.c        |  4 +++-
> > >  fs/nfsd/nfsfh.c          | 40 ++++++++++++++++++++++++++++++++++++++++
> > >  fs/nfsd/nfsfh.h          | 29 +----------------------------
> > >  fs/nfsd/vfs.h            |  7 ++++++-
> > >  include/linux/exportfs.h |  1 -
> > >  6 files changed, 50 insertions(+), 38 deletions(-)
> > > 
> > > diff --git a/fs/nfs/export.c b/fs/nfs/export.c
> > > index 01596f2d0a1e..1a9d5aa51dfb 100644
> > > --- a/fs/nfs/export.c
> > > +++ b/fs/nfs/export.c
> > > @@ -145,17 +145,10 @@ nfs_get_parent(struct dentry *dentry)
> > >  	return parent;
> > >  }
> > >  
> > > -static u64 nfs_fetch_iversion(struct inode *inode)
> > > -{
> > > -	nfs_revalidate_inode(inode, NFS_INO_INVALID_CHANGE);
> > > -	return inode_peek_iversion_raw(inode);
> > > -}
> > > -
> > >  const struct export_operations nfs_export_ops = {
> > >  	.encode_fh = nfs_encode_fh,
> > >  	.fh_to_dentry = nfs_fh_to_dentry,
> > >  	.get_parent = nfs_get_parent,
> > > -	.fetch_iversion = nfs_fetch_iversion,
> > >  	.flags = EXPORT_OP_NOWCC|EXPORT_OP_NOSUBTREECHK|
> > >  		EXPORT_OP_CLOSE_BEFORE_UNLINK|EXPORT_OP_REMOTE_FS|
> > >  		EXPORT_OP_NOATOMIC_ATTR,
> > > diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> > > index 1e9690a061ec..779c009314c6 100644
> > > --- a/fs/nfsd/nfs4xdr.c
> > > +++ b/fs/nfsd/nfs4xdr.c
> > > @@ -2869,7 +2869,9 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
> > >  			goto out;
> > >  	}
> > >  
> > > -	err = vfs_getattr(&path, &stat, STATX_BASIC_STATS, AT_STATX_SYNC_AS_STAT);
> > > +	err = vfs_getattr(&path, &stat,
> > > +			  STATX_BASIC_STATS | STATX_BTIME | STATX_VERSION,
> > > +			  AT_STATX_SYNC_AS_STAT);
> > >  	if (err)
> > >  		goto out_nfserr;
> > >  	if (!(stat.result_mask & STATX_BTIME))
> > > diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
> > > index a5b71526cee0..9168bc657378 100644
> > > --- a/fs/nfsd/nfsfh.c
> > > +++ b/fs/nfsd/nfsfh.c
> > > @@ -634,6 +634,10 @@ void fh_fill_pre_attrs(struct svc_fh *fhp)
> > >  		stat.mtime = inode->i_mtime;
> > >  		stat.ctime = inode->i_ctime;
> > >  		stat.size  = inode->i_size;
> > > +		if (v4 && IS_I_VERSION(inode)) {
> > > +			stat.version = inode_query_iversion(inode);
> > > +			stat.result_mask |= STATX_VERSION;
> > > +		}
> > 
> > This is increasingly ugly.  I wonder if it is justified at all...
> > 
> 
> I'm fine with dropping that. So if the getattrs fail, we should just not
> offer up pre/post attrs?
> 
> > >  	}
> > >  	if (v4)
> > >  		fhp->fh_pre_change = nfsd4_change_attribute(&stat, inode);
> > > @@ -665,6 +669,8 @@ void fh_fill_post_attrs(struct svc_fh *fhp)
> > >  	if (err) {
> > >  		fhp->fh_post_saved = false;
> > >  		fhp->fh_post_attr.ctime = inode->i_ctime;
> > > +		if (v4 && IS_I_VERSION(inode))
> > > +			fhp->fh_post_attr.version = inode_query_iversion(inode);
> > 
> > ... ditto ...
> > 
> > >  	} else
> > >  		fhp->fh_post_saved = true;
> > >  	if (v4)
> > > @@ -754,3 +760,37 @@ enum fsid_source fsid_source(const struct svc_fh *fhp)
> > >  		return FSIDSOURCE_UUID;
> > >  	return FSIDSOURCE_DEV;
> > >  }
> > > +
> > > +/*
> > > + * We could use i_version alone as the change attribute.  However, i_version
> > > + * can go backwards on a regular file after an unclean shutdown.  On its own
> > > + * that doesn't necessarily cause a problem, but if i_version goes backwards
> > > + * and then is incremented again it could reuse a value that was previously
> > > + * used before boot, and a client who queried the two values might incorrectly
> > > + * assume nothing changed.
> > > + *
> > > + * By using both ctime and the i_version counter we guarantee that as long as
> > > + * time doesn't go backwards we never reuse an old value. If the filesystem
> > > + * advertises STATX_ATTR_VERSION_MONOTONIC, then this mitigation is not needed.
> > > + *
> > > + * We only need to do this for regular files as well. For directories, we
> > > + * assume that the new change attr is always logged to stable storage in some
> > > + * fashion before the results can be seen.
> > > + */
> > > +u64 nfsd4_change_attribute(struct kstat *stat, struct inode *inode)
> > > +{
> > > +	u64 chattr;
> > > +
> > > +	if (stat->result_mask & STATX_VERSION) {
> > > +		chattr = stat->version;
> > > +
> > > +		if (S_ISREG(inode->i_mode) &&
> > > +		    !(stat->attributes & STATX_ATTR_VERSION_MONOTONIC)) {
> > 
> > I would really rather that the fs got to make this decision.
> > If it can guarantee that the i_version is monotonic even over a crash
> > (which is probably can for directory, and might need changes to do for
> > files) then it sets STATX_ATTR_VERSION_MONOTONIC and nfsd trusts it
> > completely.
> > If it cannot, then it doesn't set the flag.
> > i.e. the S_ISREG() test should be in the filesystem, not in nfsd.
> > 
> 
> This sounds reasonable, but for one thing.
> 
> From RFC 7862:
> 
>    While Section 5.4 of [RFC5661] discusses
>    per-file system attributes, it is expected that the value of
>    change_attr_type will not depend on the value of "homogeneous" and
>    will only change in the event of a migration.
> 
> The change_attr_type4 must be the same for all filehandles under a
> particular filesystem.
> 
> If we do what you suggest though, then it's easily possible for the fs
> to set STATX_ATTR_VERSION_MONOTONIC on directories but not files. If we
> later want to allow nfsd to advertise a change_attr_type4, we won't be
> able to rely on the STATX_ATTR_VERSION_MONOTONIC to tell us how to fill
> that out.
> 
> Maybe that's ok. I suppose we could add a new field to the export
> options that filesystems can set to advertise what sort of change attr
> they offer?
> 

There are 3 cases:
1/ a file/dir which advertises MONOTONIC is easy to handle.
2/ an IS_I_VERSION file/dir that does not advertise MONOTONIC will only fail
   to be MONOTONIC across unclean restart (correct?).  nfsd can
   compensate using an xattr on the root to count crashes, or just adding ctime.
3/ a non-IS_I_VERSION fs that does not advertise MONOTONIC cannot
   be compensated for by nfsd.

If we ever want nfsd to advertise MONOTONIC, then we must be able to
reject non-IS_I_VERSION filesystems that don't advertise MONOTONIC on
all files.

Maybe we need a global nfsd option which defaults to "monotoric" and
causes those files to be rejected, but can be set to "non-monotonic" and
then allows all files to be exported.

It would be nice to make it easy to run multiple nfsd instances each on a
different IP address.  Each can then have different options.  This could
also be used to reexport an NFS mount using unmodified filehandles.

Currently you need a network namespace to create a new nfsd.  I wonder
if that is a little too much of a barrier.  But maybe we could automate
the creation of working network namespaces for nfsd....

NeilBrown