VFS: Add shrink_submounts()

Allow a submount to be marked as being 'shrinkable' by means of the
vfsmount->mnt_flags, and then add a function 'shrink_submounts()' which
attempts to recursively unmount these submounts.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

2 files changed, 102 insertions, 25 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index bf478addb852..b22e469ab560 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1163,13 +1163,46 @@ static void expire_mount(struct vfsmount *mnt, struct list_head *mounts,
 }
 
 /*
+ * go through the vfsmounts we've just consigned to the graveyard to
+ * - check that they're still dead
+ * - delete the vfsmount from the appropriate namespace under lock
+ * - dispose of the corpse
+ */
+static void expire_mount_list(struct list_head *graveyard, struct list_head *mounts)
+{
+        struct namespace *namespace;
+        struct vfsmount *mnt;
+
+        while (!list_empty(graveyard)) {
+                LIST_HEAD(umounts);
+                mnt = list_entry(graveyard->next, struct vfsmount, mnt_expire);
+                list_del_init(&mnt->mnt_expire);
+
+                /* don't do anything if the namespace is dead - all the
+                 * vfsmounts from it are going away anyway */
+                namespace = mnt->mnt_namespace;
+                if (!namespace || !namespace->root)
+                        continue;
+                get_namespace(namespace);
+
+                spin_unlock(&vfsmount_lock);
+                down_write(&namespace_sem);
+                expire_mount(mnt, mounts, &umounts);
+                up_write(&namespace_sem);
+                release_mounts(&umounts);
+                mntput(mnt);
+                put_namespace(namespace);
+                spin_lock(&vfsmount_lock);
+        }
+}
+
+/*
  * process a list of expirable mountpoints with the intent of discarding any
  * mountpoints that aren't in use and haven't been touched since last we came
  * here
  */
 void mark_mounts_for_expiry(struct list_head *mounts)
 {
-        struct namespace *namespace;
         struct vfsmount *mnt, *next;
         LIST_HEAD(graveyard);
 
@@ -1193,38 +1226,79 @@ void mark_mounts_for_expiry(struct list_head *mounts)
                 list_move(&mnt->mnt_expire, &graveyard);
         }
 
-        /*
-         * go through the vfsmounts we've just consigned to the graveyard to
-         * - check that they're still dead
-         * - delete the vfsmount from the appropriate namespace under lock
-         * - dispose of the corpse
-         */
-        while (!list_empty(&graveyard)) {
-                LIST_HEAD(umounts);
-                mnt = list_entry(graveyard.next, struct vfsmount, mnt_expire);
-                list_del_init(&mnt->mnt_expire);
+        expire_mount_list(&graveyard, mounts);
 
-                /* don't do anything if the namespace is dead - all the
-                 * vfsmounts from it are going away anyway */
-                namespace = mnt->mnt_namespace;
-                if (!namespace || !namespace->root)
+        spin_unlock(&vfsmount_lock);
+}
+
+EXPORT_SYMBOL_GPL(mark_mounts_for_expiry);
+
+/*
+ * Ripoff of 'select_parent()'
+ *
+ * search the list of submounts for a given mountpoint, and move any
+ * shrinkable submounts to the 'graveyard' list.
+ */
+static int select_submounts(struct vfsmount *parent, struct list_head *graveyard)
+{
+        struct vfsmount *this_parent = parent;
+        struct list_head *next;
+        int found = 0;
+
+repeat:
+        next = this_parent->mnt_mounts.next;
+resume:
+        while (next != &this_parent->mnt_mounts) {
+                struct list_head *tmp = next;
+                struct vfsmount *mnt = list_entry(tmp, struct vfsmount, mnt_child);
+
+                next = tmp->next;
+                if (!(mnt->mnt_flags & MNT_SHRINKABLE))
                         continue;
-                get_namespace(namespace);
+                /*
+                 * Descend a level if the d_mounts list is non-empty.
+                 */
+                if (!list_empty(&mnt->mnt_mounts)) {
+                        this_parent = mnt;
+                        goto repeat;
+                }
 
-                spin_unlock(&vfsmount_lock);
-                down_write(&namespace_sem);
-                expire_mount(mnt, mounts, &umounts);
-                up_write(&namespace_sem);
-                release_mounts(&umounts);
-                mntput(mnt);
-                put_namespace(namespace);
-                spin_lock(&vfsmount_lock);
+                if (!propagate_mount_busy(mnt, 1)) {
+                        mntget(mnt);
+                        list_move_tail(&mnt->mnt_expire, graveyard);
+                        found++;
+                }
         }
+        /*
+         * All done at this level ... ascend and resume the search
+         */
+        if (this_parent != parent) {
+                next = this_parent->mnt_child.next;
+                this_parent = this_parent->mnt_parent;
+                goto resume;
+        }
+        return found;
+}
+
+/*
+ * process a list of expirable mountpoints with the intent of discarding any
+ * submounts of a specific parent mountpoint
+ */
+void shrink_submounts(struct vfsmount *mountpoint, struct list_head *mounts)
+{
+        LIST_HEAD(graveyard);
+        int found;
+
+        spin_lock(&vfsmount_lock);
+
+        /* extract submounts of 'mountpoint' from the expiration list */
+        while ((found = select_submounts(mountpoint, &graveyard)) != 0)
+                expire_mount_list(&graveyard, mounts);
 
         spin_unlock(&vfsmount_lock);
 }
 
-EXPORT_SYMBOL_GPL(mark_mounts_for_expiry);
+EXPORT_SYMBOL_GPL(shrink_submounts);
 
 /*
  * Some copy_from_user() implementations do not return the exact number of
diff --git a/include/linux/mount.h b/include/linux/mount.h
index aff68c3660f5..9b4e0071b92e 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -23,6 +23,8 @@
 #define MNT_NOATIME     0x08
 #define MNT_NODIRATIME  0x10
 
+#define MNT_SHRINKABLE  0x100
+
 #define MNT_SHARED      0x1000  /* if the vfsmount is a shared mount */
 #define MNT_UNBINDABLE  0x2000  /* if the vfsmount is a unbindable mount */
 #define MNT_PNODE_MASK  0x3000  /* propogation flag mask */
@@ -84,6 +86,7 @@ extern int do_add_mount(struct vfsmount *newmnt, struct nameidata *nd,
                         int mnt_flags, struct list_head *fslist);
 
 extern void mark_mounts_for_expiry(struct list_head *mounts);
+extern void shrink_submounts(struct vfsmount *mountpoint, struct list_head *mounts);
 
 extern spinlock_t vfsmount_lock;
 extern dev_t name_to_dev_t(char *name);