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![Nick Piggin](/assets/img/avatar_default.png)
The problem that this patch aims to fix is vfsmount refcounting scalability. We need to take a reference on the vfsmount for every successful path lookup, which often go to the same mount point. The fundamental difficulty is that a "simple" reference count can never be made scalable, because any time a reference is dropped, we must check whether that was the last reference. To do that requires communication with all other CPUs that may have taken a reference count. We can make refcounts more scalable in a couple of ways, involving keeping distributed counters, and checking for the global-zero condition less frequently. - check the global sum once every interval (this will delay zero detection for some interval, so it's probably a showstopper for vfsmounts). - keep a local count and only taking the global sum when local reaches 0 (this is difficult for vfsmounts, because we can't hold preempt off for the life of a reference, so a counter would need to be per-thread or tied strongly to a particular CPU which requires more locking). - keep a local difference of increments and decrements, which allows us to sum the total difference and hence find the refcount when summing all CPUs. Then, keep a single integer "long" refcount for slow and long lasting references, and only take the global sum of local counters when the long refcount is 0. This last scheme is what I implemented here. Attached mounts and process root and working directory references are "long" references, and everything else is a short reference. This allows scalable vfsmount references during path walking over mounted subtrees and unattached (lazy umounted) mounts with processes still running in them. This results in one fewer atomic op in the fastpath: mntget is now just a per-CPU inc, rather than an atomic inc; and mntput just requires a spinlock and non-atomic decrement in the common case. However code is otherwise bigger and heavier, so single threaded performance is basically a wash. Signed-off-by: Nick Piggin <npiggin@kernel.dk>
371 lines
9.3 KiB
C
371 lines
9.3 KiB
C
/*
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* linux/fs/pnode.c
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*
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* (C) Copyright IBM Corporation 2005.
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* Released under GPL v2.
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* Author : Ram Pai (linuxram@us.ibm.com)
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*
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*/
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#include <linux/mnt_namespace.h>
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#include <linux/mount.h>
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#include <linux/fs.h>
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#include "internal.h"
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#include "pnode.h"
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/* return the next shared peer mount of @p */
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static inline struct vfsmount *next_peer(struct vfsmount *p)
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{
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return list_entry(p->mnt_share.next, struct vfsmount, mnt_share);
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}
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static inline struct vfsmount *first_slave(struct vfsmount *p)
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{
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return list_entry(p->mnt_slave_list.next, struct vfsmount, mnt_slave);
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}
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static inline struct vfsmount *next_slave(struct vfsmount *p)
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{
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return list_entry(p->mnt_slave.next, struct vfsmount, mnt_slave);
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}
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/*
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* Return true if path is reachable from root
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*
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* namespace_sem is held, and mnt is attached
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*/
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static bool is_path_reachable(struct vfsmount *mnt, struct dentry *dentry,
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const struct path *root)
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{
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while (mnt != root->mnt && mnt->mnt_parent != mnt) {
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dentry = mnt->mnt_mountpoint;
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mnt = mnt->mnt_parent;
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}
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return mnt == root->mnt && is_subdir(dentry, root->dentry);
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}
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static struct vfsmount *get_peer_under_root(struct vfsmount *mnt,
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struct mnt_namespace *ns,
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const struct path *root)
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{
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struct vfsmount *m = mnt;
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do {
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/* Check the namespace first for optimization */
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if (m->mnt_ns == ns && is_path_reachable(m, m->mnt_root, root))
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return m;
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m = next_peer(m);
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} while (m != mnt);
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return NULL;
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}
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/*
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* Get ID of closest dominating peer group having a representative
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* under the given root.
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*
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* Caller must hold namespace_sem
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*/
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int get_dominating_id(struct vfsmount *mnt, const struct path *root)
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{
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struct vfsmount *m;
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for (m = mnt->mnt_master; m != NULL; m = m->mnt_master) {
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struct vfsmount *d = get_peer_under_root(m, mnt->mnt_ns, root);
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if (d)
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return d->mnt_group_id;
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}
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return 0;
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}
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static int do_make_slave(struct vfsmount *mnt)
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{
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struct vfsmount *peer_mnt = mnt, *master = mnt->mnt_master;
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struct vfsmount *slave_mnt;
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/*
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* slave 'mnt' to a peer mount that has the
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* same root dentry. If none is available then
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* slave it to anything that is available.
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*/
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while ((peer_mnt = next_peer(peer_mnt)) != mnt &&
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peer_mnt->mnt_root != mnt->mnt_root) ;
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if (peer_mnt == mnt) {
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peer_mnt = next_peer(mnt);
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if (peer_mnt == mnt)
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peer_mnt = NULL;
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}
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if (IS_MNT_SHARED(mnt) && list_empty(&mnt->mnt_share))
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mnt_release_group_id(mnt);
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list_del_init(&mnt->mnt_share);
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mnt->mnt_group_id = 0;
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if (peer_mnt)
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master = peer_mnt;
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if (master) {
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list_for_each_entry(slave_mnt, &mnt->mnt_slave_list, mnt_slave)
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slave_mnt->mnt_master = master;
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list_move(&mnt->mnt_slave, &master->mnt_slave_list);
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list_splice(&mnt->mnt_slave_list, master->mnt_slave_list.prev);
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INIT_LIST_HEAD(&mnt->mnt_slave_list);
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} else {
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struct list_head *p = &mnt->mnt_slave_list;
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while (!list_empty(p)) {
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slave_mnt = list_first_entry(p,
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struct vfsmount, mnt_slave);
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list_del_init(&slave_mnt->mnt_slave);
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slave_mnt->mnt_master = NULL;
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}
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}
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mnt->mnt_master = master;
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CLEAR_MNT_SHARED(mnt);
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return 0;
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}
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/*
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* vfsmount lock must be held for write
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*/
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void change_mnt_propagation(struct vfsmount *mnt, int type)
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{
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if (type == MS_SHARED) {
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set_mnt_shared(mnt);
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return;
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}
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do_make_slave(mnt);
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if (type != MS_SLAVE) {
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list_del_init(&mnt->mnt_slave);
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mnt->mnt_master = NULL;
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if (type == MS_UNBINDABLE)
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mnt->mnt_flags |= MNT_UNBINDABLE;
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else
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mnt->mnt_flags &= ~MNT_UNBINDABLE;
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}
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}
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/*
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* get the next mount in the propagation tree.
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* @m: the mount seen last
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* @origin: the original mount from where the tree walk initiated
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*
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* Note that peer groups form contiguous segments of slave lists.
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* We rely on that in get_source() to be able to find out if
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* vfsmount found while iterating with propagation_next() is
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* a peer of one we'd found earlier.
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*/
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static struct vfsmount *propagation_next(struct vfsmount *m,
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struct vfsmount *origin)
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{
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/* are there any slaves of this mount? */
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if (!IS_MNT_NEW(m) && !list_empty(&m->mnt_slave_list))
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return first_slave(m);
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while (1) {
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struct vfsmount *next;
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struct vfsmount *master = m->mnt_master;
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if (master == origin->mnt_master) {
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next = next_peer(m);
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return ((next == origin) ? NULL : next);
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} else if (m->mnt_slave.next != &master->mnt_slave_list)
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return next_slave(m);
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/* back at master */
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m = master;
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}
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}
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/*
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* return the source mount to be used for cloning
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*
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* @dest the current destination mount
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* @last_dest the last seen destination mount
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* @last_src the last seen source mount
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* @type return CL_SLAVE if the new mount has to be
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* cloned as a slave.
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*/
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static struct vfsmount *get_source(struct vfsmount *dest,
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struct vfsmount *last_dest,
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struct vfsmount *last_src,
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int *type)
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{
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struct vfsmount *p_last_src = NULL;
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struct vfsmount *p_last_dest = NULL;
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while (last_dest != dest->mnt_master) {
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p_last_dest = last_dest;
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p_last_src = last_src;
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last_dest = last_dest->mnt_master;
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last_src = last_src->mnt_master;
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}
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if (p_last_dest) {
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do {
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p_last_dest = next_peer(p_last_dest);
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} while (IS_MNT_NEW(p_last_dest));
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/* is that a peer of the earlier? */
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if (dest == p_last_dest) {
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*type = CL_MAKE_SHARED;
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return p_last_src;
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}
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}
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/* slave of the earlier, then */
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*type = CL_SLAVE;
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/* beginning of peer group among the slaves? */
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if (IS_MNT_SHARED(dest))
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*type |= CL_MAKE_SHARED;
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return last_src;
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}
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/*
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* mount 'source_mnt' under the destination 'dest_mnt' at
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* dentry 'dest_dentry'. And propagate that mount to
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* all the peer and slave mounts of 'dest_mnt'.
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* Link all the new mounts into a propagation tree headed at
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* source_mnt. Also link all the new mounts using ->mnt_list
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* headed at source_mnt's ->mnt_list
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*
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* @dest_mnt: destination mount.
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* @dest_dentry: destination dentry.
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* @source_mnt: source mount.
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* @tree_list : list of heads of trees to be attached.
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*/
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int propagate_mnt(struct vfsmount *dest_mnt, struct dentry *dest_dentry,
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struct vfsmount *source_mnt, struct list_head *tree_list)
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{
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struct vfsmount *m, *child;
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int ret = 0;
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struct vfsmount *prev_dest_mnt = dest_mnt;
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struct vfsmount *prev_src_mnt = source_mnt;
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LIST_HEAD(tmp_list);
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LIST_HEAD(umount_list);
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for (m = propagation_next(dest_mnt, dest_mnt); m;
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m = propagation_next(m, dest_mnt)) {
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int type;
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struct vfsmount *source;
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if (IS_MNT_NEW(m))
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continue;
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source = get_source(m, prev_dest_mnt, prev_src_mnt, &type);
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if (!(child = copy_tree(source, source->mnt_root, type))) {
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ret = -ENOMEM;
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list_splice(tree_list, tmp_list.prev);
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goto out;
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}
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if (is_subdir(dest_dentry, m->mnt_root)) {
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mnt_set_mountpoint(m, dest_dentry, child);
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list_add_tail(&child->mnt_hash, tree_list);
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} else {
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/*
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* This can happen if the parent mount was bind mounted
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* on some subdirectory of a shared/slave mount.
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*/
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list_add_tail(&child->mnt_hash, &tmp_list);
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}
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prev_dest_mnt = m;
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prev_src_mnt = child;
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}
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out:
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br_write_lock(vfsmount_lock);
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while (!list_empty(&tmp_list)) {
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child = list_first_entry(&tmp_list, struct vfsmount, mnt_hash);
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umount_tree(child, 0, &umount_list);
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}
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br_write_unlock(vfsmount_lock);
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release_mounts(&umount_list);
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return ret;
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}
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/*
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* return true if the refcount is greater than count
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*/
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static inline int do_refcount_check(struct vfsmount *mnt, int count)
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{
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int mycount = mnt_get_count(mnt) - mnt->mnt_ghosts;
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return (mycount > count);
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}
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/*
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* check if the mount 'mnt' can be unmounted successfully.
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* @mnt: the mount to be checked for unmount
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* NOTE: unmounting 'mnt' would naturally propagate to all
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* other mounts its parent propagates to.
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* Check if any of these mounts that **do not have submounts**
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* have more references than 'refcnt'. If so return busy.
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*
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* vfsmount lock must be held for write
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*/
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int propagate_mount_busy(struct vfsmount *mnt, int refcnt)
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{
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struct vfsmount *m, *child;
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struct vfsmount *parent = mnt->mnt_parent;
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int ret = 0;
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if (mnt == parent)
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return do_refcount_check(mnt, refcnt);
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/*
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* quickly check if the current mount can be unmounted.
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* If not, we don't have to go checking for all other
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* mounts
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*/
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if (!list_empty(&mnt->mnt_mounts) || do_refcount_check(mnt, refcnt))
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return 1;
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for (m = propagation_next(parent, parent); m;
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m = propagation_next(m, parent)) {
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child = __lookup_mnt(m, mnt->mnt_mountpoint, 0);
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if (child && list_empty(&child->mnt_mounts) &&
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(ret = do_refcount_check(child, 1)))
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break;
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}
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return ret;
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}
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/*
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* NOTE: unmounting 'mnt' naturally propagates to all other mounts its
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* parent propagates to.
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*/
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static void __propagate_umount(struct vfsmount *mnt)
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{
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struct vfsmount *parent = mnt->mnt_parent;
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struct vfsmount *m;
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BUG_ON(parent == mnt);
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for (m = propagation_next(parent, parent); m;
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m = propagation_next(m, parent)) {
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struct vfsmount *child = __lookup_mnt(m,
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mnt->mnt_mountpoint, 0);
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/*
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* umount the child only if the child has no
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* other children
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*/
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if (child && list_empty(&child->mnt_mounts))
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list_move_tail(&child->mnt_hash, &mnt->mnt_hash);
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}
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}
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/*
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* collect all mounts that receive propagation from the mount in @list,
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* and return these additional mounts in the same list.
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* @list: the list of mounts to be unmounted.
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*
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* vfsmount lock must be held for write
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*/
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int propagate_umount(struct list_head *list)
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{
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struct vfsmount *mnt;
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list_for_each_entry(mnt, list, mnt_hash)
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__propagate_umount(mnt);
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return 0;
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}
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