nfscl: Add a "has acquired a delegation" flag for delegations

A problem was reported via email, where a large (130000+) accumulation
of NFSv4 opens on an NFSv4 mount caused significant lock contention
on the mutex used to protect the client mount's open/lock state.
Although the root cause for the accumulation of opens was not
resolved, it is obvious that the NFSv4 client is not designed to
handle 100000+ opens efficiently.

For a common case where delegations are not being issued by the
NFSv4 server, the code acquires the mutex lock for open/lock state,
finds the delegation list empty and just unlocks the mutex and returns.
This patch adds an NFS mount point flag that is set when a delegation
is issued for the mount.  Then the patched code checks for this flag
before acquiring the open/lock mutex, avoiding the need to acquire
the lock for the case where delegations are not being issued by the
NFSv4 server.
This change appears to be performance neutral for a small number
of opens, but should reduce lock contention for a large number of opens
for the common case where server is not issuing delegations.

This commit should not affect the high level semantics of delegation
handling.

MFC after:      2 weeks

2 files changed, 42 insertions, 3 deletions

diff --git a/sys/fs/nfsclient/nfs_clstate.c b/sys/fs/nfsclient/nfs_clstate.c
index 5e4ac2ae9d4d..0659e77289e9 100644
--- a/sys/fs/nfsclient/nfs_clstate.c
+++ b/sys/fs/nfsclient/nfs_clstate.c
@@ -438,20 +438,32 @@ nfscl_deleg(mount_t mp, struct nfsclclient *clp, u_int8_t *nfhp,
     int fhlen, struct ucred *cred, NFSPROC_T *p, struct nfscldeleg **dpp)
 {
 	struct nfscldeleg *dp = *dpp, *tdp;
+	struct nfsmount *nmp;
 
+	KASSERT(mp != NULL, ("nfscl_deleg: mp NULL"));
+	nmp = VFSTONFS(mp);
 	/*
 	 * First, if we have received a Read delegation for a file on a
 	 * read/write file system, just return it, because they aren't
 	 * useful, imho.
 	 */
-	if (mp != NULL && dp != NULL && !NFSMNT_RDONLY(mp) &&
+	if (dp != NULL && !NFSMNT_RDONLY(mp) &&
 	    (dp->nfsdl_flags & NFSCLDL_READ)) {
-		(void) nfscl_trydelegreturn(dp, cred, VFSTONFS(mp), p);
+		nfscl_trydelegreturn(dp, cred, nmp, p);
 		free(dp, M_NFSCLDELEG);
 		*dpp = NULL;
 		return (0);
 	}
 
+	/*
+	 * Since a delegation might be added to the mount,
+	 * set NFSMNTP_DELEGISSUED now.  If a delegation already
+	 * exagain ists, setting this flag is harmless.
+	 */
+	NFSLOCKMNT(nmp);
+	nmp->nm_privflag |= NFSMNTP_DELEGISSUED;
+	NFSUNLOCKMNT(nmp);
+
 	/* Look for the correct deleg, based upon FH */
 	NFSLOCKCLSTATE();
 	tdp = nfscl_finddeleg(clp, nfhp, fhlen);
@@ -3356,12 +3368,20 @@ nfscl_delegreturnvp(vnode_t vp, NFSPROC_T *p)
 	struct nfscldeleg *dp;
 	struct ucred *cred;
 	struct nfsnode *np;
+	struct nfsmount *nmp;
 
+	nmp = VFSTONFS(vp->v_mount);
+	NFSLOCKMNT(nmp);
+	if ((nmp->nm_privflag & NFSMNTP_DELEGISSUED) == 0) {
+		NFSUNLOCKMNT(nmp);
+		return;
+	}
+	NFSUNLOCKMNT(nmp);
 	np = VTONFS(vp);
 	cred = newnfs_getcred();
 	dp = NULL;
 	NFSLOCKCLSTATE();
-	clp = VFSTONFS(vp->v_mount)->nm_clp;
+	clp = nmp->nm_clp;
 	if (clp != NULL)
 		dp = nfscl_finddeleg(clp, np->n_fhp->nfh_fh,
 		    np->n_fhp->nfh_len);
@@ -4500,6 +4520,12 @@ nfscl_nodeleg(vnode_t vp, int writedeleg)
 	nmp = VFSTONFS(vp->v_mount);
 	if (!NFSHASNFSV4(nmp))
 		return (1);
+	NFSLOCKMNT(nmp);
+	if ((nmp->nm_privflag & NFSMNTP_DELEGISSUED) == 0) {
+		NFSUNLOCKMNT(nmp);
+		return (1);
+	}
+	NFSUNLOCKMNT(nmp);
 	NFSLOCKCLSTATE();
 	clp = nfscl_findcl(nmp);
 	if (clp == NULL) {
@@ -4856,6 +4882,12 @@ nfscl_delegmodtime(vnode_t vp)
 	nmp = VFSTONFS(vp->v_mount);
 	if (!NFSHASNFSV4(nmp))
 		return;
+	NFSLOCKMNT(nmp);
+	if ((nmp->nm_privflag & NFSMNTP_DELEGISSUED) == 0) {
+		NFSUNLOCKMNT(nmp);
+		return;
+	}
+	NFSUNLOCKMNT(nmp);
 	NFSLOCKCLSTATE();
 	clp = nfscl_findcl(nmp);
 	if (clp == NULL) {
@@ -4885,6 +4917,12 @@ nfscl_deleggetmodtime(vnode_t vp, struct timespec *mtime)
 	nmp = VFSTONFS(vp->v_mount);
 	if (!NFSHASNFSV4(nmp))
 		return;
+	NFSLOCKMNT(nmp);
+	if ((nmp->nm_privflag & NFSMNTP_DELEGISSUED) == 0) {
+		NFSUNLOCKMNT(nmp);
+		return;
+	}
+	NFSUNLOCKMNT(nmp);
 	NFSLOCKCLSTATE();
 	clp = nfscl_findcl(nmp);
 	if (clp == NULL) {
diff --git a/sys/fs/nfsclient/nfsmount.h b/sys/fs/nfsclient/nfsmount.h
index 57adcd8f2fca..f8ea8c9a1418 100644
--- a/sys/fs/nfsclient/nfsmount.h
+++ b/sys/fs/nfsclient/nfsmount.h
@@ -115,6 +115,7 @@ struct	nfsmount {
 #define	NFSMNTP_NOXATTR		0x00000080
 #define	NFSMNTP_NOADVISE	0x00000100
 #define	NFSMNTP_NOALLOCATE	0x00000200
+#define	NFSMNTP_DELEGISSUED	0x00000400
 
 /* New mount flags only used by the kernel via nmount(2). */
 #define	NFSMNT_TLS		0x00000001