Date:	Fri, 11 Sep 2015 17:29:57 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Jeff Layton <jlayton@...chiereds.net>
Cc:	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, HPDD-discuss@...ts.01.org,
	ceph-devel@...r.kernel.org, cluster-devel@...hat.com,
	fuse-devel@...ts.sourceforge.net, ocfs2-devel@....oracle.com
Subject: Re: [PATCH] nfsd: add a new EXPORT_OP_NOWCC flag to struct
 export_operations

On Fri, Sep 11, 2015 at 06:20:30AM -0400, Jeff Layton wrote:
> With NFSv3 nfsd will always attempt to send along WCC data to the
> client. This generally involves saving off the in-core inode information
> prior to doing the operation on the given filehandle, and then issuing a
> vfs_getattr to it after the op.
> 
> Some filesystems (particularly clustered or networked ones) have an
> expensive ->getattr inode operation. Atomicitiy is also often difficult
> or impossible to guarantee on such filesystems. For those, we're best
> off not trying to provide WCC information to the client at all, and to
> simply allow it to poll for that information as needed with a GETATTR
> RPC.
> 
> This patch adds a new flags field to struct export_operations, and
> defines a new EXPORT_OP_NOWCC flag that filesystems can use to indicate
> that nfsd should not attempt to provide WCC info in NFSv3 replies. It
> also adds a blurb about the new flags field and flag to the exporting
> documentation.
> 
> The server will also now skip collecting this information for NFSv2 as
> well, since that info is never used there anyway.
> 
> Note that this patch does not add this flag to any filesystem
> export_operations structures. This was originally developed to allow
> reexporting nfs via nfsd. That code is not (and may never be) suitable
> for merging into mainline.
> 
> Other filesystems may want to consider enabling this flag too. It's hard
> to tell however which ones have export operations to enable export via
> knfsd and which ones mostly rely on them for open-by-filehandle support,

Are there any in the latter class?  I'm not sure how or why you'd
support open-by-filehandle without supporting nfs exports.

> so I'm leaving that up to the individual maintainers to decide. I am
> cc'ing the relevant lists for those filesystems that I think may want to
> consider adding this though.

I'd definitely like to see evidence from maintainers of those
filesystems that this would be useful to them.

--b.

> 
> Cc: HPDD-discuss@...ts.01.org
> Cc: ceph-devel@...r.kernel.org
> Cc: cluster-devel@...hat.com
> Cc: fuse-devel@...ts.sourceforge.net
> Cc: ocfs2-devel@....oracle.com
> Signed-off-by: Jeff Layton <jeff.layton@...marydata.com>
> ---
>  Documentation/filesystems/nfs/Exporting | 27 +++++++++++++++++++++++++++
>  fs/nfsd/nfs3xdr.c                       |  5 ++++-
>  fs/nfsd/nfsfh.c                         | 14 ++++++++++++++
>  fs/nfsd/nfsfh.h                         |  5 ++++-
>  include/linux/exportfs.h                |  2 ++
>  5 files changed, 51 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/filesystems/nfs/Exporting b/Documentation/filesystems/nfs/Exporting
> index 520a4becb75c..fa636cde3907 100644
> --- a/Documentation/filesystems/nfs/Exporting
> +++ b/Documentation/filesystems/nfs/Exporting
> @@ -138,6 +138,11 @@ struct which has the following members:
>      to find potential names, and matches inode numbers to find the correct
>      match.
>  
> +  flags
> +    Some filesystems may need to be handled differently than others. The
> +    export_operations struct also includes a flags field that allows the
> +    filesystem to communicate such information to nfsd. See the Export
> +    Operations Flags section below for more explanation.
>  
>  A filehandle fragment consists of an array of 1 or more 4byte words,
>  together with a one byte "type".
> @@ -147,3 +152,25 @@ generated by encode_fh, in which case it will have been padded with
>  nuls.  Rather, the encode_fh routine should choose a "type" which
>  indicates the decode_fh how much of the filehandle is valid, and how
>  it should be interpreted.
> +
> +Export Operations Flags
> +-----------------------
> +In addition to the operation vector pointers, struct export_operations also
> +contains a "flags" field that allows the filesystem to communicate to nfsd
> +that it may want to do things differently when dealing with it. The
> +following flags are defined:
> +
> +  EXPORT_OP_NOWCC
> +    RFC 1813 recommends that servers always send weak cache consistency
> +    (WCC) data to the client after each operation. The server should
> +    atomically collect attributes about the inode, do an operation on it,
> +    and then collect the attributes afterward. This allows the client to
> +    skip issuing GETATTRs in some situations but means that the server
> +    is calling vfs_getattr for almost all RPCs. On some filesystems
> +    (particularly those that are clustered or networked) this is expensive
> +    and atomicity is difficult to guarantee. This flag indicates to nfsd
> +    that it should skip providing WCC attributes to the client in NFSv3
> +    replies when doing operations on this filesystem. Consider enabling
> +    this on filesystems that have an expensive ->getattr inode operation,
> +    or when atomicity between pre and post operation attribute collection
> +    is impossible to guarantee.
> diff --git a/fs/nfsd/nfs3xdr.c b/fs/nfsd/nfs3xdr.c
> index 01dcd494f781..c30c8c604e2a 100644
> --- a/fs/nfsd/nfs3xdr.c
> +++ b/fs/nfsd/nfs3xdr.c
> @@ -203,7 +203,7 @@ static __be32 *
>  encode_post_op_attr(struct svc_rqst *rqstp, __be32 *p, struct svc_fh *fhp)
>  {
>  	struct dentry *dentry = fhp->fh_dentry;
> -	if (dentry && d_really_is_positive(dentry)) {
> +	if (!fhp->fh_no_wcc && dentry && d_really_is_positive(dentry)) {
>  	        __be32 err;
>  		struct kstat stat;
>  
> @@ -256,6 +256,9 @@ void fill_post_wcc(struct svc_fh *fhp)
>  {
>  	__be32 err;
>  
> +	if (fhp->fh_no_wcc)
> +		return;
> +
>  	if (fhp->fh_post_saved)
>  		printk("nfsd: inode locked twice during operation.\n");
>  
> diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
> index 350041a40fe5..29ae37f62b9b 100644
> --- a/fs/nfsd/nfsfh.c
> +++ b/fs/nfsd/nfsfh.c
> @@ -267,6 +267,16 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst *rqstp, struct svc_fh *fhp)
>  
>  	fhp->fh_dentry = dentry;
>  	fhp->fh_export = exp;
> +
> +	switch (rqstp->rq_vers) {
> +	case 3:
> +		if (!(dentry->d_sb->s_export_op->flags & EXPORT_OP_NOWCC))
> +			break;
> +		/* Fallthrough */
> +	case 2:
> +		fhp->fh_no_wcc = true;
> +	}
> +
>  	return 0;
>  out:
>  	exp_put(exp);
> @@ -535,6 +545,9 @@ fh_compose(struct svc_fh *fhp, struct svc_export *exp, struct dentry *dentry,
>  	 */
>  	 set_version_and_fsid_type(fhp, exp, ref_fh);
>  
> +	/* If we have a ref_fh, then copy the fh_no_wcc setting from it. */
> +	fhp->fh_no_wcc = ref_fh ? ref_fh->fh_no_wcc : false;
> +
>  	if (ref_fh == fhp)
>  		fh_put(ref_fh);
>  
> @@ -641,6 +654,7 @@ fh_put(struct svc_fh *fhp)
>  		exp_put(exp);
>  		fhp->fh_export = NULL;
>  	}
> +	fhp->fh_no_wcc = false;
>  	return;
>  }
>  
> diff --git a/fs/nfsd/nfsfh.h b/fs/nfsd/nfsfh.h
> index 1e90dad4926b..9ddead4d98f8 100644
> --- a/fs/nfsd/nfsfh.h
> +++ b/fs/nfsd/nfsfh.h
> @@ -32,6 +32,7 @@ typedef struct svc_fh {
>  
>  	unsigned char		fh_locked;	/* inode locked by us */
>  	unsigned char		fh_want_write;	/* remount protection taken */
> +	bool			fh_no_wcc;	/* no wcc data needed */
>  
>  #ifdef CONFIG_NFSD_V3
>  	unsigned char		fh_post_saved;	/* post-op attrs saved */
> @@ -51,7 +52,6 @@ typedef struct svc_fh {
>  	struct kstat		fh_post_attr;	/* full attrs after operation */
>  	u64			fh_post_change; /* nfsv4 change; see above */
>  #endif /* CONFIG_NFSD_V3 */
> -
>  } svc_fh;
>  
>  enum nfsd_fsid {
> @@ -225,6 +225,9 @@ fill_pre_wcc(struct svc_fh *fhp)
>  {
>  	struct inode    *inode;
>  
> +	if (fhp->fh_no_wcc)
> +		return;
> +
>  	inode = d_inode(fhp->fh_dentry);
>  	if (!fhp->fh_pre_saved) {
>  		fhp->fh_pre_mtime = inode->i_mtime;
> diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
> index fa05e04c5531..600c3fccc999 100644
> --- a/include/linux/exportfs.h
> +++ b/include/linux/exportfs.h
> @@ -214,6 +214,8 @@ struct export_operations {
>  			  bool write, u32 *device_generation);
>  	int (*commit_blocks)(struct inode *inode, struct iomap *iomaps,
>  			     int nr_iomaps, struct iattr *iattr);
> +#define	EXPORT_OP_NOWCC		(0x1)	/* Don't collect wcc data for NFSv3 replies */
> +	unsigned long	flags;
>  };
>  
>  extern int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
> -- 
> 2.4.3
> 
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