Re: [HPDD-discuss] [PATCH] nfsd: add a new EXPORT_OP_NOWCC flag to struct export_operations

[Jeff Layton]

On Sat, 12 Sep 2015 04:41:33 +0000
"Dilger, Andreas" <andreas.dilger@xxxxxxxxx> wrote:

> On 2015/09/11, 4:20 AM, "HPDD-discuss on behalf of Jeff Layton"
> <hpdd-discuss-bounces@xxxxxxxxxxxx on behalf of jlayton@xxxxxxxxxxxxxxx>
> 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,
> >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.
> >
> >Cc: HPDD-discuss@xxxxxxxxxxxx
> >Cc: ceph-devel@xxxxxxxxxxxxxxx
> >Cc: cluster-devel@xxxxxxxxxx
> >Cc: fuse-devel@xxxxxxxxxxxxxxxxxxxxx
> >Cc: ocfs2-devel@xxxxxxxxxxxxxx
> >Signed-off-by: Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx>
> >---
> > 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 */
> 
> This increases the size of svc_fh because it splits the four unsigned
> chars.
> You could change all of these (fh_locked, fh_want_write,
> fh_{pre,post}saved)
> to be bools to avoid that and make it more clear they are only used as
> booleans (I verified that they all are only assigned 0 or 1).
> 

I don't think it matters, at least not on x86_64. bools and chars both
require a byte. pahole does show this adding a new hole, but that's
just because this brings the code up to 5 flags and the next field
(fh_pre_size) needs to be aligned.

I do agree that replacing those other unsigned chars with bools is more
clear however. Maybe we should even replace them all with a single
unsigned int and use bitops to set flags in there. That would be more
space efficient now that we're at 5 flags.

> Cheers, Andreas
> 
> > 
> > #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
> >
> >_______________________________________________
> >HPDD-discuss mailing list
> >HPDD-discuss@xxxxxxxxxxxx
> >https://lists.01.org/mailman/listinfo/hpdd-discuss
> >
> 
> 
> Cheers, Andreas


-- 
Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
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