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Subject[patch 05/33] fs: scale mntget/mntput
Improve scalability of mntget/mntput by using per-cpu counters protected
by the reader side of the brlock vfsmount_lock. mnt_mounted keeps track of
whether the vfsmount is actually attached to the tree so we can shortcut
expensive checks in mntput.
---
fs/libfs.c | 1
fs/namespace.c | 122 +++++++++++++++++++++++++++++++++++++++++++-------
fs/pnode.c | 2
include/linux/mount.h | 33 ++++---------
4 files changed, 121 insertions(+), 37 deletions(-)

Index: linux-2.6/fs/namespace.c
===================================================================
--- linux-2.6.orig/fs/namespace.c
+++ linux-2.6/fs/namespace.c
@@ -177,6 +177,49 @@ void mnt_release_group_id(struct vfsmoun
mnt->mnt_group_id = 0;
}

+static inline void add_mnt_count(struct vfsmount *mnt, int n)
+{
+#ifdef CONFIG_SMP
+ (*per_cpu_ptr(mnt->mnt_count, smp_processor_id())) += n;
+#else
+ mnt->mnt_count += n;
+#endif
+}
+
+static inline void inc_mnt_count(struct vfsmount *mnt)
+{
+#ifdef CONFIG_SMP
+ (*per_cpu_ptr(mnt->mnt_count, smp_processor_id()))++;
+#else
+ mnt->mnt_count++;
+#endif
+}
+
+static inline void dec_mnt_count(struct vfsmount *mnt)
+{
+#ifdef CONFIG_SMP
+ (*per_cpu_ptr(mnt->mnt_count, smp_processor_id()))--;
+#else
+ mnt->mnt_count--;
+#endif
+}
+
+unsigned int count_mnt_count(struct vfsmount *mnt)
+{
+#ifdef CONFIG_SMP
+ unsigned int count = 0;
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ count += *per_cpu_ptr(mnt->mnt_count, cpu);
+ }
+
+ return count;
+#else
+ return mnt->mnt_count;
+#endif
+}
+
struct vfsmount *alloc_vfsmnt(const char *name)
{
struct vfsmount *mnt = kmem_cache_zalloc(mnt_cache, GFP_KERNEL);
@@ -193,7 +236,13 @@ struct vfsmount *alloc_vfsmnt(const char
goto out_free_id;
}

- atomic_set(&mnt->mnt_count, 1);
+#ifdef CONFIG_SMP
+ mnt->mnt_count = alloc_percpu(int);
+ if (!mnt->mnt_count)
+ goto out_free_devname;
+#else
+ mnt->mnt_count = 0;
+#endif
INIT_LIST_HEAD(&mnt->mnt_hash);
INIT_LIST_HEAD(&mnt->mnt_child);
INIT_LIST_HEAD(&mnt->mnt_mounts);
@@ -205,14 +254,19 @@ struct vfsmount *alloc_vfsmnt(const char
#ifdef CONFIG_SMP
mnt->mnt_writers = alloc_percpu(int);
if (!mnt->mnt_writers)
- goto out_free_devname;
+ goto out_free_mntcount;
#else
mnt->mnt_writers = 0;
#endif
+ preempt_disable();
+ inc_mnt_count(mnt);
+ preempt_enable();
}
return mnt;

#ifdef CONFIG_SMP
+out_free_mntcount:
+ free_percpu(mnt->mnt_count);
out_free_devname:
kfree(mnt->mnt_devname);
#endif
@@ -526,9 +580,11 @@ static void detach_mnt(struct vfsmount *
old_path->mnt = mnt->mnt_parent;
mnt->mnt_parent = mnt;
mnt->mnt_mountpoint = mnt->mnt_root;
- list_del_init(&mnt->mnt_child);
list_del_init(&mnt->mnt_hash);
+ list_del_init(&mnt->mnt_child);
old_path->dentry->d_mounted--;
+ WARN_ON(mnt->mnt_mounted != 1);
+ mnt->mnt_mounted--;
}

void mnt_set_mountpoint(struct vfsmount *mnt, struct dentry *dentry,
@@ -545,6 +601,8 @@ static void attach_mnt(struct vfsmount *
list_add_tail(&mnt->mnt_hash, mount_hashtable +
hash(path->mnt, path->dentry));
list_add_tail(&mnt->mnt_child, &path->mnt->mnt_mounts);
+ WARN_ON(mnt->mnt_mounted != 0);
+ mnt->mnt_mounted++;
}

/*
@@ -567,6 +625,8 @@ static void commit_tree(struct vfsmount
list_add_tail(&mnt->mnt_hash, mount_hashtable +
hash(parent, mnt->mnt_mountpoint));
list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
+ WARN_ON(mnt->mnt_mounted != 0);
+ mnt->mnt_mounted++;
touch_mnt_namespace(n);
}

@@ -670,50 +730,80 @@ static inline void __mntput(struct vfsmo

void mntput_no_expire(struct vfsmount *mnt)
{
-repeat:
- /* open-code atomic_dec_and_lock for the vfsmount lock */
- if (atomic_add_unless(&mnt->mnt_count, -1, 1))
+ if (likely(mnt->mnt_mounted)) {
+ vfsmount_read_lock();
+ if (unlikely(!mnt->mnt_mounted)) {
+ vfsmount_read_unlock();
+ goto repeat;
+ }
+ dec_mnt_count(mnt);
+ BUG_ON(count_mnt_count(mnt) == 0);
+ vfsmount_read_unlock();
+
return;
+ }
+
+repeat:
vfsmount_write_lock();
- if (!atomic_dec_and_test(&mnt->mnt_count)) {
+ BUG_ON(mnt->mnt_mounted);
+ dec_mnt_count(mnt);
+ if (count_mnt_count(mnt)) {
vfsmount_write_unlock();
return;
}
-
if (likely(!mnt->mnt_pinned)) {
vfsmount_write_unlock();
__mntput(mnt);
return;
}
- atomic_add(mnt->mnt_pinned + 1, &mnt->mnt_count);
+ add_mnt_count(mnt, mnt->mnt_pinned + 1);
mnt->mnt_pinned = 0;
vfsmount_write_unlock();
acct_auto_close_mnt(mnt);
security_sb_umount_close(mnt);
goto repeat;
}
-
EXPORT_SYMBOL(mntput_no_expire);

+void mntput(struct vfsmount *mnt)
+{
+ if (mnt) {
+ /* avoid cacheline pingpong */
+ if (unlikely(mnt->mnt_expiry_mark))
+ mnt->mnt_expiry_mark = 0;
+ mntput_no_expire(mnt);
+ }
+}
+EXPORT_SYMBOL(mntput);
+
+struct vfsmount *mntget(struct vfsmount *mnt)
+{
+ if (mnt) {
+ preempt_disable();
+ inc_mnt_count(mnt);
+ preempt_enable();
+ }
+ return mnt;
+}
+EXPORT_SYMBOL(mntget);
+
void mnt_pin(struct vfsmount *mnt)
{
vfsmount_write_lock();
mnt->mnt_pinned++;
vfsmount_write_unlock();
}
-
EXPORT_SYMBOL(mnt_pin);

void mnt_unpin(struct vfsmount *mnt)
{
vfsmount_write_lock();
if (mnt->mnt_pinned) {
- atomic_inc(&mnt->mnt_count);
+ inc_mnt_count(mnt);
mnt->mnt_pinned--;
}
vfsmount_write_unlock();
}
-
EXPORT_SYMBOL(mnt_unpin);

static inline void mangle(struct seq_file *m, const char *s)
@@ -996,7 +1086,7 @@ int may_umount_tree(struct vfsmount *mnt

vfsmount_read_lock();
for (p = mnt; p; p = next_mnt(p, mnt)) {
- actual_refs += atomic_read(&p->mnt_count);
+ actual_refs += count_mnt_count(p);
minimum_refs += 2;
}
vfsmount_read_unlock();
@@ -1076,6 +1166,8 @@ void umount_tree(struct vfsmount *mnt, i
__touch_mnt_namespace(p->mnt_ns);
p->mnt_ns = NULL;
list_del_init(&p->mnt_child);
+ WARN_ON(p->mnt_mounted != 1);
+ p->mnt_mounted--;
if (p->mnt_parent != p) {
p->mnt_parent->mnt_ghosts++;
p->mnt_mountpoint->d_mounted--;
@@ -1107,7 +1199,7 @@ static int do_umount(struct vfsmount *mn
flags & (MNT_FORCE | MNT_DETACH))
return -EINVAL;

- if (atomic_read(&mnt->mnt_count) != 2)
+ if (count_mnt_count(mnt) != 2)
return -EBUSY;

if (!xchg(&mnt->mnt_expiry_mark, 1))
Index: linux-2.6/include/linux/mount.h
===================================================================
--- linux-2.6.orig/include/linux/mount.h
+++ linux-2.6/include/linux/mount.h
@@ -56,20 +56,20 @@ struct vfsmount {
struct mnt_namespace *mnt_ns; /* containing namespace */
int mnt_id; /* mount identifier */
int mnt_group_id; /* peer group identifier */
- /*
- * We put mnt_count & mnt_expiry_mark at the end of struct vfsmount
- * to let these frequently modified fields in a separate cache line
- * (so that reads of mnt_flags wont ping-pong on SMP machines)
- */
- atomic_t mnt_count;
int mnt_expiry_mark; /* true if marked for expiry */
int mnt_pinned;
int mnt_ghosts;
+ int mnt_mounted;
#ifdef CONFIG_SMP
int *mnt_writers;
#else
int mnt_writers;
#endif
+#ifdef CONFIG_SMP
+ int *mnt_count;
+#else
+ int mnt_count;
+#endif
};

static inline int *get_mnt_writers_ptr(struct vfsmount *mnt)
@@ -81,13 +81,6 @@ static inline int *get_mnt_writers_ptr(s
#endif
}

-static inline struct vfsmount *mntget(struct vfsmount *mnt)
-{
- if (mnt)
- atomic_inc(&mnt->mnt_count);
- return mnt;
-}
-
struct file; /* forward dec */

extern void vfsmount_read_lock(void);
@@ -95,23 +88,21 @@ extern void vfsmount_read_unlock(void);
extern void vfsmount_write_lock(void);
extern void vfsmount_write_unlock(void);

+extern unsigned int count_mnt_count(struct vfsmount *mnt);
+
extern int mnt_want_write(struct vfsmount *mnt);
extern int mnt_want_write_file(struct file *file);
extern int mnt_clone_write(struct vfsmount *mnt);
extern void mnt_drop_write(struct vfsmount *mnt);
+
extern void mntput_no_expire(struct vfsmount *mnt);
+extern struct vfsmount *mntget(struct vfsmount *mnt);
+extern void mntput(struct vfsmount *mnt);
+
extern void mnt_pin(struct vfsmount *mnt);
extern void mnt_unpin(struct vfsmount *mnt);
extern int __mnt_is_readonly(struct vfsmount *mnt);

-static inline void mntput(struct vfsmount *mnt)
-{
- if (mnt) {
- mnt->mnt_expiry_mark = 0;
- mntput_no_expire(mnt);
- }
-}
-
extern struct vfsmount *do_kern_mount(const char *fstype, int flags,
const char *name, void *data);

Index: linux-2.6/fs/pnode.c
===================================================================
--- linux-2.6.orig/fs/pnode.c
+++ linux-2.6/fs/pnode.c
@@ -279,7 +279,7 @@ out:
*/
static inline int do_refcount_check(struct vfsmount *mnt, int count)
{
- int mycount = atomic_read(&mnt->mnt_count) - mnt->mnt_ghosts;
+ int mycount = count_mnt_count(mnt) - mnt->mnt_ghosts;
return (mycount > count);
}

Index: linux-2.6/fs/libfs.c
===================================================================
--- linux-2.6.orig/fs/libfs.c
+++ linux-2.6/fs/libfs.c
@@ -244,6 +244,7 @@ int get_sb_pseudo(struct file_system_typ
d_instantiate(dentry, root);
s->s_root = dentry;
s->s_flags |= MS_ACTIVE;
+ mnt->mnt_mounted++; /* never unmounted, shortcut mntget (XXX: OK?) */
simple_set_mnt(mnt, s);
return 0;




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