Improve scalability of mntget/mntput by using per-cpu counters protected by the reader side of the brlock vfsmount_lock. If the mnt_hash field of the vfsmount structure is attached to a list, then it is mounted which contributes to its refcount, so the per-cpu counters need not be summed. MNT_PSEUDO keeps track of whether the vfsmount is actually a pseudo filesystem that will never be attached (such as sockfs). No extra atomics in the common case because atomic mnt refcount is now replaced with per-CPU spinlock. Code will be bigger and more complex however. With the previous per-cpu locking patch, mount lookups and common case refcounting are now per-cpu and should be ideally scalable. path lookups (and hence path_get/path_put) within the same vfsmount should now be more scalable, however this will often be hidden by dcache_lock on final dput, and d_lock on common path elements (eg. cwd or root dentry). Signed-off-by: Nick Piggin [Note: this is not for merging. Un-attached operation (lazy umount) may not be uncommon and will be slowed down and actually have worse scalablilty after this patch. I need to think about how to do fast refcounting with unattached mounts.] --- drivers/mtd/mtdchar.c | 1 fs/internal.h | 1 fs/libfs.c | 1 fs/namespace.c | 155 +++++++++++++++++++++++++++++++++++++++++++------- fs/pnode.c | 4 - include/linux/mount.h | 26 +------- 6 files changed, 144 insertions(+), 44 deletions(-) Index: linux-2.6/fs/namespace.c =================================================================== --- linux-2.6.orig/fs/namespace.c +++ linux-2.6/fs/namespace.c @@ -138,6 +138,64 @@ void mnt_release_group_id(struct vfsmoun mnt->mnt_group_id = 0; } +/* + * vfsmount lock must be held for read + */ +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 set_mnt_count(struct vfsmount *mnt, int n) +{ +#ifdef CONFIG_SMP + preempt_disable(); + (*per_cpu_ptr(mnt->mnt_count, smp_processor_id())) = n; + preempt_enable(); +#else + mnt->mnt_count = n; +#endif +} + +/* + * vfsmount lock must be held for read + */ +static inline void inc_mnt_count(struct vfsmount *mnt) +{ + add_mnt_count(mnt, 1); +} + +/* + * vfsmount lock must be held for read + */ +static inline void dec_mnt_count(struct vfsmount *mnt) +{ + add_mnt_count(mnt, -1); +} + +/* + * vfsmount lock must be held for write + */ +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); @@ -154,7 +212,15 @@ 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 + set_mnt_count(mnt, 1); + INIT_LIST_HEAD(&mnt->mnt_hash); INIT_LIST_HEAD(&mnt->mnt_child); INIT_LIST_HEAD(&mnt->mnt_mounts); @@ -166,7 +232,7 @@ 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 @@ -174,6 +240,8 @@ struct vfsmount *alloc_vfsmnt(const char return mnt; #ifdef CONFIG_SMP +out_free_mntcount: + free_percpu(mnt->mnt_count); out_free_devname: kfree(mnt->mnt_devname); #endif @@ -591,7 +659,7 @@ static struct vfsmount *clone_mnt(struct goto out_free; } - mnt->mnt_flags = old->mnt_flags; + WARN_ON(mnt->mnt_flags & MNT_WRITE_HOLD); atomic_inc(&sb->s_active); mnt->mnt_sb = sb; mnt->mnt_root = dget(root); @@ -638,6 +706,11 @@ static inline void __mntput(struct vfsmo /* * atomic_dec_and_lock() used to deal with ->mnt_count decrements * provides barriers, so count_mnt_writers() below is safe. AV + * XXX: We no longer have an atomic_dec_and_lock, so load of + * mnt_writers may be moved up into the vfsmount lock critical section? + * Do we need an smp_mb()? I don't see how it is possible because an + * elevated write count should also have elevated ref count so we'd + * never get here. */ WARN_ON(count_mnt_writers(mnt)); dput(mnt->mnt_root); @@ -648,45 +721,76 @@ static inline void __mntput(struct vfsmo void mntput_no_expire(struct vfsmount *mnt) { repeat: - if (atomic_add_unless(&mnt->mnt_count, -1, 1)) + if (likely(!list_empty(&mnt->mnt_hash) || + mnt->mnt_flags & MNT_PSEUDO)) { + br_read_lock(vfsmount_lock); + if (unlikely(list_empty(&mnt->mnt_hash) && + (!(mnt->mnt_flags & MNT_PSEUDO)))) { + br_read_unlock(vfsmount_lock); + goto repeat; + } + dec_mnt_count(mnt); + br_read_unlock(vfsmount_lock); return; + } + br_write_lock(vfsmount_lock); - if (!atomic_dec_and_test(&mnt->mnt_count)) { + dec_mnt_count(mnt); + if (count_mnt_count(mnt)) { br_write_unlock(vfsmount_lock); return; } - if (likely(!mnt->mnt_pinned)) { + if (unlikely(mnt->mnt_pinned)) { + add_mnt_count(mnt, mnt->mnt_pinned + 1); + mnt->mnt_pinned = 0; br_write_unlock(vfsmount_lock); - __mntput(mnt); - return; + acct_auto_close_mnt(mnt); + goto repeat; } - atomic_add(mnt->mnt_pinned + 1, &mnt->mnt_count); - mnt->mnt_pinned = 0; br_write_unlock(vfsmount_lock); - acct_auto_close_mnt(mnt); - goto repeat; + __mntput(mnt); } EXPORT_SYMBOL(mntput_no_expire); +void mntput(struct vfsmount *mnt) +{ + if (mnt) { + /* avoid cacheline pingpong, hope gcc doesn't get "smart" */ + 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) { br_write_lock(vfsmount_lock); mnt->mnt_pinned++; br_write_unlock(vfsmount_lock); } - EXPORT_SYMBOL(mnt_pin); void mnt_unpin(struct vfsmount *mnt) { br_write_lock(vfsmount_lock); if (mnt->mnt_pinned) { - atomic_inc(&mnt->mnt_count); + inc_mnt_count(mnt); mnt->mnt_pinned--; } br_write_unlock(vfsmount_lock); } - EXPORT_SYMBOL(mnt_unpin); static inline void mangle(struct seq_file *m, const char *s) @@ -982,12 +1086,13 @@ int may_umount_tree(struct vfsmount *mnt int minimum_refs = 0; struct vfsmount *p; - br_read_lock(vfsmount_lock); + /* write lock needed for count_mnt_count */ + br_write_lock(vfsmount_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; } - br_read_unlock(vfsmount_lock); + br_write_unlock(vfsmount_lock); if (actual_refs > minimum_refs) return 0; @@ -1014,10 +1119,10 @@ int may_umount(struct vfsmount *mnt) { int ret = 1; down_read(&namespace_sem); - br_read_lock(vfsmount_lock); + br_write_lock(vfsmount_lock); if (propagate_mount_busy(mnt, 2)) ret = 0; - br_read_unlock(vfsmount_lock); + br_write_unlock(vfsmount_lock); up_read(&namespace_sem); return ret; } @@ -1099,8 +1204,16 @@ static int do_umount(struct vfsmount *mn flags & (MNT_FORCE | MNT_DETACH)) return -EINVAL; - if (atomic_read(&mnt->mnt_count) != 2) + /* + * probably don't strictly need the lock here if we examined + * all race cases, but it's a slowpath. + */ + br_write_lock(vfsmount_lock); + if (count_mnt_count(mnt) != 2) { + br_write_lock(vfsmount_lock); return -EBUSY; + } + br_write_unlock(vfsmount_lock); if (!xchg(&mnt->mnt_expiry_mark, 1)) return -EAGAIN; Index: linux-2.6/include/linux/mount.h =================================================================== --- linux-2.6.orig/include/linux/mount.h +++ linux-2.6/include/linux/mount.h @@ -31,6 +31,7 @@ struct mnt_namespace; #define MNT_SHRINKABLE 0x100 #define MNT_WRITE_HOLD 0x200 +#define MNT_PSEUDO 0x400 #define MNT_SHARED 0x1000 /* if the vfsmount is a shared mount */ #define MNT_UNBINDABLE 0x2000 /* if the vfsmount is a unbindable mount */ @@ -67,19 +68,15 @@ 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; #ifdef CONFIG_SMP int __percpu *mnt_writers; + int __percpu *mnt_count; #else int mnt_writers; + int mnt_count; #endif }; @@ -92,13 +89,6 @@ 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 int mnt_want_write(struct vfsmount *mnt); @@ -106,18 +96,12 @@ extern int mnt_want_write_file(struct fi 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 void mntput(struct vfsmount *mnt); +extern struct vfsmount *mntget(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 @@ -288,7 +288,7 @@ 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); } @@ -300,7 +300,7 @@ static inline int do_refcount_check(stru * Check if any of these mounts that **do not have submounts** * have more references than 'refcnt'. If so return busy. * - * vfsmount lock must be held for read or write + * vfsmount lock must be held for write */ int propagate_mount_busy(struct vfsmount *mnt, int refcnt) { Index: linux-2.6/fs/internal.h =================================================================== --- linux-2.6.orig/fs/internal.h +++ linux-2.6/fs/internal.h @@ -63,6 +63,7 @@ extern int copy_mount_string(const void extern void free_vfsmnt(struct vfsmount *); extern struct vfsmount *alloc_vfsmnt(const char *); +extern unsigned int count_mnt_count(struct vfsmount *mnt); extern struct vfsmount *__lookup_mnt(struct vfsmount *, struct dentry *, int); extern void mnt_set_mountpoint(struct vfsmount *, struct dentry *, struct vfsmount *); Index: linux-2.6/fs/libfs.c =================================================================== --- linux-2.6.orig/fs/libfs.c +++ linux-2.6/fs/libfs.c @@ -241,6 +241,7 @@ int get_sb_pseudo(struct file_system_typ d_instantiate(dentry, root); s->s_root = dentry; s->s_flags |= MS_ACTIVE; + mnt->mnt_flags |= MNT_PSEUDO; simple_set_mnt(mnt, s); return 0; Index: linux-2.6/drivers/mtd/mtdchar.c =================================================================== --- linux-2.6.orig/drivers/mtd/mtdchar.c +++ linux-2.6/drivers/mtd/mtdchar.c @@ -1048,6 +1048,7 @@ err_unregister_chdev: static void __exit cleanup_mtdchar(void) { unregister_mtd_user(&mtdchar_notifier); + mtd_inode_mnt &= ~MNT_PSEUDO; mntput(mtd_inode_mnt); unregister_filesystem(&mtd_inodefs_type); __unregister_chrdev(MTD_CHAR_MAJOR, 0, 1 << MINORBITS, "mtd"); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/