exports: add allow_umount option for user un-mounting cached exports

After nfsd get exports cache, user cannot umounts the filesystem which base on for a reference of the vfsmnt.

This patch introduces a new option "allow_umount" for nfsd pining to the vfsmnt, not get a refernce. At all, user can umount now.

Signed-off-by Kinglong Mee <[email protected]>

---
 support/include/nfs/export.h | 3 ++-
 support/nfs/exports.c        | 6 ++++++
 utils/exportfs/exports.man   | 8 ++++++++
 3 files changed, 16 insertions(+), 1 deletion(-)

support/include/nfs/export.h +2 -1

index 1194255..ca4e0c1 100644
@@ -26,7 +26,8 @@
 #define	NFSEXP_CROSSMOUNT	0x4000
 #define NFSEXP_NOACL		0x8000 /* reserved for possible ACL related use */
 #define NFSEXP_V4ROOT		0x10000
-#define NFSEXP_PNFS            0x20000
+#define NFSEXP_PNFS		0x20000
+#define NFSEXP_ALLOW_UMOUNT	0x40000
 /*
  * All flags supported by the kernel before addition of the
  * export_features interface:

support/nfs/exports.c +6 -0

index 0aea6f1..ed5b984 100644
@@ -276,6 +276,8 @@ putexportent(struct exportent *ep)
 	if (ep->e_flags & NFSEXP_NOREADDIRPLUS)
 		fprintf(fp, "nordirplus,");
 	fprintf(fp, "%spnfs,", (ep->e_flags & NFSEXP_PNFS)? "" : "no_");
+	if (ep->e_flags & NFSEXP_ALLOW_UMOUNT)
+		fprintf(fp, "allow_umount,");
 	if (ep->e_flags & NFSEXP_FSID) {
 		fprintf(fp, "fsid=%d,", ep->e_fsid);
 	}

@@ -586,6 +588,10 @@ parseopts(char *cp, struct exportent *ep, int warn, int *had_subtree_opt_ptr)
 			setflags(NFSEXP_PNFS, active, ep);
 		else if (!strcmp(opt, "no_pnfs"))
 			clearflags(NFSEXP_PNFS, active, ep);
+		else if (!strcmp(opt, "allow_umount"))
+			setflags(NFSEXP_ALLOW_UMOUNT, active, ep);
+		else if (!strcmp(opt, "no_allow_umount"))
+			clearflags(NFSEXP_ALLOW_UMOUNT, active, ep);
 		else if (strncmp(opt, "anonuid=", 8) == 0) {
 			char *oe;
 			ep->e_anonuid = strtol(opt+8, &oe, 10);

utils/exportfs/exports.man +8 -0

index 9309246..d3947a4 100644
@@ -417,6 +417,14 @@ devices. The default can be explicitly requested with the
 .I no_pnfs
 option.
 
+.TP
+.IR allow_umount
+This option allows user un-mounting the filesystem where cached nfsd exports
+is based on. Otherwise, the kernel may returns -EBUSY and un-mounting fail.
+The default is without the
+.I allow_umount
+option.
+
 .SS User ID Mapping
 .PP
 .B nfsd