[PATCH] r/o bind mounts: track numbers of writers to mounts

This is the real meat of the entire series.  It actually
implements the tracking of the number of writers to a mount.
However, it causes scalability problems because there can be
hundreds of cpus doing open()/close() on files on the same mnt at
the same time.  Even an atomic_t in the mnt has massive scalaing
problems because the cacheline gets so terribly contended.

This uses a statically-allocated percpu variable.  All want/drop
operations are local to a cpu as long that cpu operates on the same
mount, and there are no writer count imbalances.  Writer count
imbalances happen when a write is taken on one cpu, and released
on another, like when an open/close pair is performed on two

Upon a remount,ro request, all of the data from the percpu
variables is collected (expensive, but very rare) and we determine
if there are any outstanding writers to the mount.

I've written a little benchmark to sit in a loop for a couple of
seconds in several cpus in parallel doing open/write/close loops.

http://sr71.net/~dave/linux/openbench.c

The code in here is a a worst-possible case for this patch.  It
does opens on a _pair_ of files in two different mounts in parallel.
This should cause my code to lose its "operate on the same mount"
optimization completely.  This worst-case scenario causes a 3%
degredation in the benchmark.

I could probably get rid of even this 3%, but it would be more
complex than what I have here, and I think this is getting into
acceptable territory.  In practice, I expect writing more than 3
bytes to a file, as well as disk I/O to mask any effects that this
has.

(To get rid of that 3%, we could have an #defined number of mounts
in the percpu variable.  So, instead of a CPU getting operate only
on percpu data when it accesses only one mount, it could stay on
percpu data when it only accesses N or fewer mounts.)

[AV] merged fix for __clear_mnt_mount() stepping on freed vfsmount

Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 237 insertions, 15 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 066b393578c1..e3ce18d91aad 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -17,6 +17,7 @@
 #include <linux/quotaops.h>
 #include <linux/acct.h>
 #include <linux/capability.h>
+#include <linux/cpumask.h>
 #include <linux/module.h>
 #include <linux/sysfs.h>
 #include <linux/seq_file.h>
@@ -55,6 +56,8 @@ static inline unsigned long hash(struct vfsmount *mnt, struct dentry *dentry)
         return tmp & (HASH_SIZE - 1);
 }
 
+#define MNT_WRITER_UNDERFLOW_LIMIT -(1<<16)
+
 struct vfsmount *alloc_vfsmnt(const char *name)
 {
         struct vfsmount *mnt = kmem_cache_zalloc(mnt_cache, GFP_KERNEL);
@@ -68,6 +71,7 @@ struct vfsmount *alloc_vfsmnt(const char *name)
                 INIT_LIST_HEAD(&mnt->mnt_share);
                 INIT_LIST_HEAD(&mnt->mnt_slave_list);
                 INIT_LIST_HEAD(&mnt->mnt_slave);
+                atomic_set(&mnt->__mnt_writers, 0);
                 if (name) {
                         int size = strlen(name) + 1;
                         char *newname = kmalloc(size, GFP_KERNEL);
@@ -88,6 +92,92 @@ struct vfsmount *alloc_vfsmnt(const char *name)
  * we can determine when writes are able to occur to
  * a filesystem.
  */
+/*
+ * __mnt_is_readonly: check whether a mount is read-only
+ * @mnt: the mount to check for its write status
+ *
+ * This shouldn't be used directly ouside of the VFS.
+ * It does not guarantee that the filesystem will stay
+ * r/w, just that it is right *now*.  This can not and
+ * should not be used in place of IS_RDONLY(inode).
+ * mnt_want/drop_write() will _keep_ the filesystem
+ * r/w.
+ */
+int __mnt_is_readonly(struct vfsmount *mnt)
+{
+        return (mnt->mnt_sb->s_flags & MS_RDONLY);
+}
+EXPORT_SYMBOL_GPL(__mnt_is_readonly);
+
+struct mnt_writer {
+        /*
+         * If holding multiple instances of this lock, they
+         * must be ordered by cpu number.
+         */
+        spinlock_t lock;
+        struct lock_class_key lock_class; /* compiles out with !lockdep */
+        unsigned long count;
+        struct vfsmount *mnt;
+} ____cacheline_aligned_in_smp;
+static DEFINE_PER_CPU(struct mnt_writer, mnt_writers);
+
+static int __init init_mnt_writers(void)
+{
+        int cpu;
+        for_each_possible_cpu(cpu) {
+                struct mnt_writer *writer = &per_cpu(mnt_writers, cpu);
+                spin_lock_init(&writer->lock);
+                lockdep_set_class(&writer->lock, &writer->lock_class);
+                writer->count = 0;
+        }
+        return 0;
+}
+fs_initcall(init_mnt_writers);
+
+static void unlock_mnt_writers(void)
+{
+        int cpu;
+        struct mnt_writer *cpu_writer;
+
+        for_each_possible_cpu(cpu) {
+                cpu_writer = &per_cpu(mnt_writers, cpu);
+                spin_unlock(&cpu_writer->lock);
+        }
+}
+
+static inline void __clear_mnt_count(struct mnt_writer *cpu_writer)
+{
+        if (!cpu_writer->mnt)
+                return;
+        /*
+         * This is in case anyone ever leaves an invalid,
+         * old ->mnt and a count of 0.
+         */
+        if (!cpu_writer->count)
+                return;
+        atomic_add(cpu_writer->count, &cpu_writer->mnt->__mnt_writers);
+        cpu_writer->count = 0;
+}
+ /*
+ * must hold cpu_writer->lock
+ */
+static inline void use_cpu_writer_for_mount(struct mnt_writer *cpu_writer,
+                                          struct vfsmount *mnt)
+{
+        if (cpu_writer->mnt == mnt)
+                return;
+        __clear_mnt_count(cpu_writer);
+        cpu_writer->mnt = mnt;
+}
+
+/*
+ * Most r/o checks on a fs are for operations that take
+ * discrete amounts of time, like a write() or unlink().
+ * We must keep track of when those operations start
+ * (for permission checks) and when they end, so that
+ * we can determine when writes are able to occur to
+ * a filesystem.
+ */
 /**
  * mnt_want_write - get write access to a mount
  * @mnt: the mount on which to take a write
@@ -100,12 +190,77 @@ struct vfsmount *alloc_vfsmnt(const char *name)
  */
 int mnt_want_write(struct vfsmount *mnt)
 {
-        if (__mnt_is_readonly(mnt))
-                return -EROFS;
-        return 0;
+        int ret = 0;
+        struct mnt_writer *cpu_writer;
+
+        cpu_writer = &get_cpu_var(mnt_writers);
+        spin_lock(&cpu_writer->lock);
+        if (__mnt_is_readonly(mnt)) {
+                ret = -EROFS;
+                goto out;
+        }
+        use_cpu_writer_for_mount(cpu_writer, mnt);
+        cpu_writer->count++;
+out:
+        spin_unlock(&cpu_writer->lock);
+        put_cpu_var(mnt_writers);
+        return ret;
 }
 EXPORT_SYMBOL_GPL(mnt_want_write);
 
+static void lock_mnt_writers(void)
+{
+        int cpu;
+        struct mnt_writer *cpu_writer;
+
+        for_each_possible_cpu(cpu) {
+                cpu_writer = &per_cpu(mnt_writers, cpu);
+                spin_lock(&cpu_writer->lock);
+                __clear_mnt_count(cpu_writer);
+                cpu_writer->mnt = NULL;
+        }
+}
+
+/*
+ * These per-cpu write counts are not guaranteed to have
+ * matched increments and decrements on any given cpu.
+ * A file open()ed for write on one cpu and close()d on
+ * another cpu will imbalance this count.  Make sure it
+ * does not get too far out of whack.
+ */
+static void handle_write_count_underflow(struct vfsmount *mnt)
+{
+        if (atomic_read(&mnt->__mnt_writers) >=
+            MNT_WRITER_UNDERFLOW_LIMIT)
+                return;
+        /*
+         * It isn't necessary to hold all of the locks
+         * at the same time, but doing it this way makes
+         * us share a lot more code.
+         */
+        lock_mnt_writers();
+        /*
+         * vfsmount_lock is for mnt_flags.
+         */
+        spin_lock(&vfsmount_lock);
+        /*
+         * If coalescing the per-cpu writer counts did not
+         * get us back to a positive writer count, we have
+         * a bug.
+         */
+        if ((atomic_read(&mnt->__mnt_writers) < 0) &&
+            !(mnt->mnt_flags & MNT_IMBALANCED_WRITE_COUNT)) {
+                printk(KERN_DEBUG "leak detected on mount(%p) writers "
+                                "count: %d\n",
+                        mnt, atomic_read(&mnt->__mnt_writers));
+                WARN_ON(1);
+                /* use the flag to keep the dmesg spam down */
+                mnt->mnt_flags |= MNT_IMBALANCED_WRITE_COUNT;
+        }
+        spin_unlock(&vfsmount_lock);
+        unlock_mnt_writers();
+}
+
 /**
  * mnt_drop_write - give up write access to a mount
  * @mnt: the mount on which to give up write access
@@ -116,23 +271,61 @@ EXPORT_SYMBOL_GPL(mnt_want_write);
  */
 void mnt_drop_write(struct vfsmount *mnt)
 {
+        int must_check_underflow = 0;
+        struct mnt_writer *cpu_writer;
+
+        cpu_writer = &get_cpu_var(mnt_writers);
+        spin_lock(&cpu_writer->lock);
+
+        use_cpu_writer_for_mount(cpu_writer, mnt);
+        if (cpu_writer->count > 0) {
+                cpu_writer->count--;
+        } else {
+                must_check_underflow = 1;
+                atomic_dec(&mnt->__mnt_writers);
+        }
+
+        spin_unlock(&cpu_writer->lock);
+        /*
+         * Logically, we could call this each time,
+         * but the __mnt_writers cacheline tends to
+         * be cold, and makes this expensive.
+         */
+        if (must_check_underflow)
+                handle_write_count_underflow(mnt);
+        /*
+         * This could be done right after the spinlock
+         * is taken because the spinlock keeps us on
+         * the cpu, and disables preemption.  However,
+         * putting it here bounds the amount that
+         * __mnt_writers can underflow.  Without it,
+         * we could theoretically wrap __mnt_writers.
+         */
+        put_cpu_var(mnt_writers);
 }
 EXPORT_SYMBOL_GPL(mnt_drop_write);
 
-/*
- * __mnt_is_readonly: check whether a mount is read-only
- * @mnt: the mount to check for its write status
- *
- * This shouldn't be used directly ouside of the VFS.
- * It does not guarantee that the filesystem will stay
- * r/w, just that it is right *now*.  This can not and
- * should not be used in place of IS_RDONLY(inode).
- */
-int __mnt_is_readonly(struct vfsmount *mnt)
+int mnt_make_readonly(struct vfsmount *mnt)
 {
-        return (mnt->mnt_sb->s_flags & MS_RDONLY);
+        int ret = 0;
+
+        lock_mnt_writers();
+        /*
+         * With all the locks held, this value is stable
+         */
+        if (atomic_read(&mnt->__mnt_writers) > 0) {
+                ret = -EBUSY;
+                goto out;
+        }
+        /*
+         * actually set mount's r/o flag here to make
+         * __mnt_is_readonly() true, which keeps anyone
+         * from doing a successful mnt_want_write().
+         */
+out:
+        unlock_mnt_writers();
+        return ret;
 }
-EXPORT_SYMBOL_GPL(__mnt_is_readonly);
 
 int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)
 {
@@ -325,7 +518,36 @@ static struct vfsmount *clone_mnt(struct vfsmount *old, struct dentry *root,
 
 static inline void __mntput(struct vfsmount *mnt)
 {
+        int cpu;
         struct super_block *sb = mnt->mnt_sb;
+        /*
+         * We don't have to hold all of the locks at the
+         * same time here because we know that we're the
+         * last reference to mnt and that no new writers
+         * can come in.
+         */
+        for_each_possible_cpu(cpu) {
+                struct mnt_writer *cpu_writer = &per_cpu(mnt_writers, cpu);
+                if (cpu_writer->mnt != mnt)
+                        continue;
+                spin_lock(&cpu_writer->lock);
+                atomic_add(cpu_writer->count, &mnt->__mnt_writers);
+                cpu_writer->count = 0;
+                /*
+                 * Might as well do this so that no one
+                 * ever sees the pointer and expects
+                 * it to be valid.
+                 */
+                cpu_writer->mnt = NULL;
+                spin_unlock(&cpu_writer->lock);
+        }
+        /*
+         * This probably indicates that somebody messed
+         * up a mnt_want/drop_write() pair.  If this
+         * happens, the filesystem was probably unable
+         * to make r/w->r/o transitions.
+         */
+        WARN_ON(atomic_read(&mnt->__mnt_writers));
         dput(mnt->mnt_root);
         free_vfsmnt(mnt);
         deactivate_super(sb);