[PATCH 28/30] r/o bind mounts: track numbers of writers to mounts

Dave Hansen
2008-02-15T22:53:04Z

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.)

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>
---

linux-2.6.git-dave/fs/namespace.c | 240 +++++++++++++++++++++++++++++--
linux-2.6.git-dave/include/linux/mount.h | 7
2 files changed, 232 insertions(+), 15 deletions(-)

diff -puN fs/namespace.c~r-o-bind-mounts-track-number-of-mount-writers fs/namespace.c
--- linux-2.6.git/fs/namespace.c~r-o-bind-mounts-track-number-of-mount-writers 2008-02-15 13:25:59.000000000 -0800
+++ linux-2.6.git-dave/fs/namespace.c 2008-02-15 13:25:59.000000000 -0800
@@ -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
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
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 +92,86 @@ struct vfsmount *alloc_vfsmnt(const char
* 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;
+ 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 +184,77 @@ struct vfsmount *alloc_vfsmnt(const char
*/
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 +265,61 @@ 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)
{
@@ -327,7 +514,30 @@ static struct vfsmount *clone_mnt(struct

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;
+ 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);
diff -puN include/linux/mount.h~r-o-bind-mounts-track-number-of-mount-writers include/linux/mount.h
--- linux-2.6.git/include/linux/mount.h~r-o-bind-mounts-track-number-of-mount-writers 2008-02-15 13:25:59.000000000 -0800
+++ linux-2.6.git-dave/include/linux/mount.h 2008-02-15 13:25:59.000000000 -0800
@@ -14,6 +14,7 @@

#include <linux/types.h>
#include <linux/list.h>
+#include <linux/nodemask.h>
#include <linux/spinlock.h>
#include <asm/atomic.h>

@@ -30,6 +31,7 @@ struct mnt_namespace;
#define MNT_RELATIME 0x20

#define MNT_SHRINKABLE 0x100
+#define MNT_IMBALANCED_WRITE_COUNT 0x200 /* just for debugging */

#define MNT_SHARED 0x1000 /* if the vfsmount is a shared mount */
#define MNT_UNBINDABLE 0x2000 /* if the vfsmount is a unbindable mount */
@@ -61,6 +63,11 @@ struct vfsmount {
atomic_t mnt_count;
int mnt_expiry_mark; /* true if marked for expiry */
int mnt_pinned;
+ /*
+ * This value is not stable unless all of the mnt_writers[] spinlocks
+ * are held, and all mnt_writer[]s on this mount have 0 as their ->count
+ */
+ atomic_t __mnt_writers;
};

static inline struct vfsmount *mntget(struct vfsmount *mnt)
_