linux-kernel - Re: [PATCH v6 4/9] nfs: report the inode version in getattr if requested

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Date:   Tue, 04 Oct 2022 05:43:50 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     NeilBrown <neilb@...e.de>
Cc:     tytso@....edu, adilger.kernel@...ger.ca, djwong@...nel.org,
        david@...morbit.com, trondmy@...merspace.com,
        viro@...iv.linux.org.uk, zohar@...ux.ibm.com, xiubli@...hat.com,
        chuck.lever@...cle.com, lczerner@...hat.com, jack@...e.cz,
        bfields@...ldses.org, brauner@...nel.org, fweimer@...hat.com,
        linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, ceph-devel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH v6 4/9] nfs: report the inode version in getattr if
 requested

On Tue, 2022-10-04 at 10:29 +1100, NeilBrown wrote:
> On Fri, 30 Sep 2022, Jeff Layton wrote:
> > Allow NFS to report the i_version in getattr requests. Since the cost to
> > fetch it is relatively cheap, do it unconditionally and just set the
> > flag if it looks like it's valid. Also, conditionally enable the
> > MONOTONIC flag when the server reports its change attr type as such.
> > 
> > Signed-off-by: Jeff Layton <jlayton@...nel.org>
> > ---
> >  fs/nfs/inode.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
> > index bea7c005119c..5cb7017e5089 100644
> > --- a/fs/nfs/inode.c
> > +++ b/fs/nfs/inode.c
> > @@ -830,6 +830,8 @@ static u32 nfs_get_valid_attrmask(struct inode *inode)
> >  		reply_mask |= STATX_UID | STATX_GID;
> >  	if (!(cache_validity & NFS_INO_INVALID_BLOCKS))
> >  		reply_mask |= STATX_BLOCKS;
> > +	if (!(cache_validity & NFS_INO_INVALID_CHANGE))
> > +		reply_mask |= STATX_VERSION;
> >  	return reply_mask;
> >  }
> >  
> > @@ -848,7 +850,7 @@ int nfs_getattr(struct user_namespace *mnt_userns, const struct path *path,
> >  
> >  	request_mask &= STATX_TYPE | STATX_MODE | STATX_NLINK | STATX_UID |
> >  			STATX_GID | STATX_ATIME | STATX_MTIME | STATX_CTIME |
> > -			STATX_INO | STATX_SIZE | STATX_BLOCKS;
> > +			STATX_INO | STATX_SIZE | STATX_BLOCKS | STATX_VERSION;
> >  
> >  	if ((query_flags & AT_STATX_DONT_SYNC) && !force_sync) {
> >  		if (readdirplus_enabled)
> > @@ -877,7 +879,7 @@ int nfs_getattr(struct user_namespace *mnt_userns, const struct path *path,
> >  	/* Is the user requesting attributes that might need revalidation? */
> >  	if (!(request_mask & (STATX_MODE|STATX_NLINK|STATX_ATIME|STATX_CTIME|
> >  					STATX_MTIME|STATX_UID|STATX_GID|
> > -					STATX_SIZE|STATX_BLOCKS)))
> > +					STATX_SIZE|STATX_BLOCKS|STATX_VERSION)))
> >  		goto out_no_revalidate;
> >  
> >  	/* Check whether the cached attributes are stale */
> > @@ -915,6 +917,10 @@ int nfs_getattr(struct user_namespace *mnt_userns, const struct path *path,
> >  
> >  	generic_fillattr(&init_user_ns, inode, stat);
> >  	stat->ino = nfs_compat_user_ino64(NFS_FILEID(inode));
> > +	stat->version = inode_peek_iversion_raw(inode);
> 
> This looks wrong.
> 1/ it includes the I_VERSION_QUERIED bit, which should be hidden.
> 2/ it doesn't set that bit.
> 
> I understand that the bit was already set when the generic code called
> inode_query_iversion(), but it might have changed if we needed to
> refresh the attrs.
> 
> I'm beginning to think I shouldn't have approved the 3/9 patch.  The
> stat->version shouldn't be set in vfs_getattr_nosec() - maybe in
> generic_fillattr(), but not a lot of point.
> 

NFS (and Ceph), do not set the SB_I_VERSION flag and they don't use the
QUERIED bit. These are "server managed" implementations of i_version.
The server is responsible for incrementing the value, and we just store
the result in the i_version field and present it when needed. That's why
the patch for NFS is using the "raw" API.

> > +	stat->attributes_mask |= STATX_ATTR_VERSION_MONOTONIC;
> > +	if (server->change_attr_type != NFS4_CHANGE_TYPE_IS_UNDEFINED)
> > +		stat->attributes |= STATX_ATTR_VERSION_MONOTONIC;
> 
> So if the server tells us that the change attrs is based on time
> metadata, we accept that it will be monotonic (and RFC7862 encourages
> this), even though we seem to worry about timestamps going backwards
> (which we know that can)...  Interesting.
> 
> 

I followed suit from nfs_inode_attrs_cmp(). It seems to treat any value
that isn't UNDEFINED as MONOTONIC, though it does use a less strict
comparator for NFS4_CHANGE_TYPE_IS_TIME_METADATA. It may make sense to
carve that out as an exception.

This is probably an indicator that we need a more strict definition for
STATX_ATTR_VERSION_MONOTONIC.


> 
> >  	if (S_ISDIR(inode->i_mode))
> >  		stat->blksize = NFS_SERVER(inode)->dtsize;
> >  out:
> > -- 
> > 2.37.3
> > 
> > 

-- 
Jeff Layton <jlayton@...nel.org>