Add a facility for vgone() to inform the set of subscribed mounts

about vnode reclamation. Typical use is for the bypass mounts like
nullfs to get a notification about lower vnode going away.

Now, vgone() calls new VFS op vfs_reclaim_lowervp() with an argument
lowervp which is reclaimed. It is possible to register several
reclamation event listeners, to correctly handle the case of several
nullfs mounts over the same directory.

For the filesystem not having nullfs mounts over it, the overhead
added is a single mount interlock lock/unlock in the vnode reclamation
path.

In collaboration with:	pho
MFC after:	3 weeks

1 files changed, 55 insertions, 0 deletions

diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index 631d3f2..06d16c0 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -2688,6 +2688,58 @@ vgone(struct vnode *vp)
 	VI_UNLOCK(vp);
 }
 
+static void
+vgonel_reclaim_lowervp_vfs(struct mount *mp __unused,
+    struct vnode *lowervp __unused)
+{
+}
+
+/*
+ * Notify upper mounts about reclaimed vnode.
+ */
+static void
+vgonel_reclaim_lowervp(struct vnode *vp)
+{
+	static struct vfsops vgonel_vfsops = {
+		.vfs_reclaim_lowervp = vgonel_reclaim_lowervp_vfs
+	};
+	struct mount *mp, *ump, *mmp;
+
+	mp = vp->v_mount;
+	if (mp == NULL)
+		return;
+
+	MNT_ILOCK(mp);
+	if (TAILQ_EMPTY(&mp->mnt_uppers))
+		goto unlock;
+	MNT_IUNLOCK(mp);
+	mmp = malloc(sizeof(struct mount), M_TEMP, M_WAITOK | M_ZERO);
+	mmp->mnt_op = &vgonel_vfsops;
+	mmp->mnt_kern_flag |= MNTK_MARKER;
+	MNT_ILOCK(mp);
+	mp->mnt_kern_flag |= MNTK_VGONE_UPPER;
+	for (ump = TAILQ_FIRST(&mp->mnt_uppers); ump != NULL;) {
+		if ((ump->mnt_kern_flag & MNTK_MARKER) != 0) {
+			ump = TAILQ_NEXT(ump, mnt_upper_link);
+			continue;
+		}
+		TAILQ_INSERT_AFTER(&mp->mnt_uppers, ump, mmp, mnt_upper_link);
+		MNT_IUNLOCK(mp);
+		VFS_RECLAIM_LOWERVP(ump, vp);
+		MNT_ILOCK(mp);
+		ump = TAILQ_NEXT(mmp, mnt_upper_link);
+		TAILQ_REMOVE(&mp->mnt_uppers, mmp, mnt_upper_link);
+	}
+	free(mmp, M_TEMP);
+	mp->mnt_kern_flag &= ~MNTK_VGONE_UPPER;
+	if ((mp->mnt_kern_flag & MNTK_VGONE_WAITER) != 0) {
+		mp->mnt_kern_flag &= ~MNTK_VGONE_WAITER;
+		wakeup(&mp->mnt_uppers);
+	}
+unlock:
+	MNT_IUNLOCK(mp);
+}
+
 /*
  * vgone, with the vp interlock held.
  */
@@ -2712,6 +2764,7 @@ vgonel(struct vnode *vp)
 	if (vp->v_iflag & VI_DOOMED)
 		return;
 	vp->v_iflag |= VI_DOOMED;
+
 	/*
 	 * Check to see if the vnode is in use.  If so, we have to call
 	 * VOP_CLOSE() and VOP_INACTIVE().
@@ -2719,6 +2772,8 @@ vgonel(struct vnode *vp)
 	active = vp->v_usecount;
 	oweinact = (vp->v_iflag & VI_OWEINACT);
 	VI_UNLOCK(vp);
+	vgonel_reclaim_lowervp(vp);
+
 	/*
 	 * Clean out any buffers associated with the vnode.
 	 * If the flush fails, just toss the buffers.