[RFC][PATCH 0/4] kill open files traverse on remount ro

[RFC][PATCH 0/4] kill open files traverse on remount ro

[Dave Hansen]

This is just RFC for now.  I'm tracking down a wee bit of
list corruption.  But, I wanted to send out so you could
compare to the last approach.

[RFC][PATCH 1/4] use helper to set mnt_sb

[Dave Hansen]

We need to make sure that all mounts get into the
sb list.  So, require that new setters of mnt->mnt_sb
use simple_set_mnt() instead of doing it themselves.

NFS does the same thing a few times in a row, so give
it a nice little helper.

---

 linux-2.6.git-dave/fs/namespace.c |    3 +--
 linux-2.6.git-dave/fs/nfs/super.c |   31 ++++++++++++++++---------------
 2 files changed, 17 insertions(+), 17 deletions(-)

diff -puN fs/namespace.c~make-mnt_sb-off-limits fs/namespace.c
--- linux-2.6.git/fs/namespace.c~make-mnt_sb-off-limits	2008-01-10 10:36:37.000000000 -0800
+++ linux-2.6.git-dave/fs/namespace.c	2008-01-10 10:36:37.000000000 -0800
@@ -486,8 +486,7 @@ static struct vfsmount *clone_mnt(struct
 	if (mnt) {
 		mnt->mnt_flags = old->mnt_flags;
 		atomic_inc(&sb->s_active);
-		mnt->mnt_sb = sb;
-		mnt->mnt_root = dget(root);
+		simple_set_mnt(mnt, sb);
 		mnt->mnt_mountpoint = mnt->mnt_root;
 		mnt->mnt_parent = mnt;
 
diff -puN fs/nfs/super.c~make-mnt_sb-off-limits fs/nfs/super.c
--- linux-2.6.git/fs/nfs/super.c~make-mnt_sb-off-limits	2008-01-10 10:36:37.000000000 -0800
+++ linux-2.6.git-dave/fs/nfs/super.c	2008-01-10 10:36:37.000000000 -0800
@@ -1364,6 +1364,17 @@ static int nfs_compare_super(struct supe
 	return nfs_compare_mount_options(sb, server, mntflags);
 }
 
+static void nfs_set_mnt(struct vfsmount *mnt, struct super_block *s)
+{
+	s->s_flags |= MS_ACTIVE;
+	simple_set_mnt(mnt, s);
+	/*
+	 * simple_set_mnt() does a dput, which we already did
+	 * in nfs_get_root().
+	 */
+	dput(s->s_root);
+}
+
 static int nfs_get_sb(struct file_system_type *fs_type,
 	int flags, const char *dev_name, void *raw_data, struct vfsmount *mnt)
 {
@@ -1417,9 +1428,7 @@ static int nfs_get_sb(struct file_system
 		goto error_splat_super;
 	}
 
-	s->s_flags |= MS_ACTIVE;
-	mnt->mnt_sb = s;
-	mnt->mnt_root = mntroot;
+	nfs_set_mnt(mnt, s);
 	error = 0;
 
 out:
@@ -1505,9 +1514,7 @@ static int nfs_xdev_get_sb(struct file_s
 		goto error_splat_super;
 	}
 
-	s->s_flags |= MS_ACTIVE;
-	mnt->mnt_sb = s;
-	mnt->mnt_root = mntroot;
+	nfs_set_mnt(mnt, s);
 
 	dprintk("<-- nfs_xdev_get_sb() = 0\n");
 	return 0;
@@ -1766,9 +1773,7 @@ static int nfs4_get_sb(struct file_syste
 		goto error_splat_super;
 	}
 
-	s->s_flags |= MS_ACTIVE;
-	mnt->mnt_sb = s;
-	mnt->mnt_root = mntroot;
+	nfs_set_mnt(mnt, s);
 	error = 0;
 
 out:
@@ -1851,9 +1856,7 @@ static int nfs4_xdev_get_sb(struct file_
 		goto error_splat_super;
 	}
 
-	s->s_flags |= MS_ACTIVE;
-	mnt->mnt_sb = s;
-	mnt->mnt_root = mntroot;
+	nfs_set_mnt(mnt, s);
 
 	dprintk("<-- nfs4_xdev_get_sb() = 0\n");
 	return 0;
@@ -1925,9 +1928,7 @@ static int nfs4_referral_get_sb(struct f
 		goto error_splat_super;
 	}
 
-	s->s_flags |= MS_ACTIVE;
-	mnt->mnt_sb = s;
-	mnt->mnt_root = mntroot;
+	nfs_set_mnt(mnt, s);
 
 	dprintk("<-- nfs4_referral_get_sb() = 0\n");
 	return 0;
_

[RFC][PATCH 2/4] change mnt_writers underflow protection logic

[Dave Hansen]

The comment tells most of the story.  I want to make the
spinlock in this case into a mutex, and the current
underflow protection mechanism uses preempt disabling from
put/get_cpu_Var(). I can't use that with a mutex.

Without the preempt disabling, there is no limit to the
number of cpus that might get to:

        use_cpu_writer_for_mount(cpu_writer, mnt);
        if (cpu_writer->count > 0) {
                cpu_writer->count--;
        } else {
                atomic_dec(&mnt->__mnt_writers);
        }
        spin_unlock(&cpu_writer->lock);
---->HERE
        if (must_check_underflow)
                handle_write_count_underflow(mnt);

because they get preempted once the spinlock is unlocked.

So, there's no limit on how many times __mnt_writers may
be decremented.  (I know the limit is still the number of
tasks on the system, but that's a heck of a lot higher
than the number of cpus.)  Doing the simple check in this
patch before the decrement and under a lock removes the
possibility that this can happen.

Since there are only NR_CPUS mnt_writer[]s, we can only
have NR_CPUS lock holders in the critical section at a
time, __mnt_writers can only underflow by
MNT_WRITER_UNDERFLOW_LIMIT+NR_CPUS.

Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
---

 linux-2.6.git-dave/fs/namespace.c |   28 +++++++++++++++++-----------
 1 file changed, 17 insertions(+), 11 deletions(-)

diff -puN fs/namespace.c~change-underflow-protection-logic fs/namespace.c
--- linux-2.6.git/fs/namespace.c~change-underflow-protection-logic	2008-01-10 10:36:37.000000000 -0800
+++ linux-2.6.git-dave/fs/namespace.c	2008-01-10 10:36:37.000000000 -0800
@@ -267,15 +267,30 @@ void mnt_drop_write(struct vfsmount *mnt
 	int must_check_underflow = 0;
 	struct mnt_writer *cpu_writer;
 
-	cpu_writer = &get_cpu_var(mnt_writers);
+retry:
+	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;
+		/* Without this check, it is theoretically
+		 * possible to underflow __mnt_writers.
+		 * An unlimited number of processes could
+		 * all do this decrement, unlock, and then
+		 * stall before the underflow handling.
+		 * Doing this check limits the underflow
+		 * to the number of cpu_writer->lock
+		 * holders (NR_CPUS).
+		 */
+		if (atomic_read(&mnt->__mnt_writers) <
+		    MNT_WRITER_UNDERFLOW_LIMIT) {
+			spin_unlock(&cpu_writer->lock);
+			goto retry;
+		}
 		atomic_dec(&mnt->__mnt_writers);
+		must_check_underflow = 1;
 	}
 
 	spin_unlock(&cpu_writer->lock);
@@ -286,15 +301,6 @@ void mnt_drop_write(struct vfsmount *mnt
 	 */
 	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);
 
_

[RFC][PATCH 3/4] change mnt_writers[] spinlock to mutex

[Dave Hansen]

---

 linux-2.6.git-dave/fs/namespace.c |   21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff -puN fs/namespace.c~change-spinlock-to-mutex fs/namespace.c
--- linux-2.6.git/fs/namespace.c~change-spinlock-to-mutex	2008-01-10 10:45:47.000000000 -0800
+++ linux-2.6.git-dave/fs/namespace.c	2008-01-10 10:46:01.000000000 -0800
@@ -118,7 +118,7 @@ struct mnt_writer {
 	 * If holding multiple instances of this lock, they
 	 * must be ordered by cpu number.
 	 */
-	spinlock_t lock;
+	struct mutex lock;
 	struct lock_class_key lock_class; /* compiles out with !lockdep */
 	unsigned long count;
 	struct vfsmount *mnt;
@@ -130,7 +130,7 @@ 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);
+		mutex_init(&writer->lock);
 		lockdep_set_class(&writer->lock, &writer->lock_class);
 		writer->count = 0;
 	}
@@ -145,7 +145,7 @@ static void mnt_unlock_cpus(void)
 
 	for_each_possible_cpu(cpu) {
 		cpu_writer = &per_cpu(mnt_writers, cpu);
-		spin_unlock(&cpu_writer->lock);
+		mutex_unlock(&cpu_writer->lock);
 	}
 }
 
@@ -191,8 +191,8 @@ int mnt_want_write(struct vfsmount *mnt)
 	int ret = 0;
 	struct mnt_writer *cpu_writer;
 
-	cpu_writer = &get_cpu_var(mnt_writers);
-	spin_lock(&cpu_writer->lock);
+	cpu_writer = &__get_cpu_var(mnt_writers);
+	mutex_lock(&cpu_writer->lock);
 	if (__mnt_is_readonly(mnt)) {
 		ret = -EROFS;
 		goto out;
@@ -200,8 +200,7 @@ int mnt_want_write(struct vfsmount *mnt)
 	use_cpu_writer_for_mount(cpu_writer, mnt);
 	cpu_writer->count++;
 out:
-	spin_unlock(&cpu_writer->lock);
-	put_cpu_var(mnt_writers);
+	mutex_unlock(&cpu_writer->lock);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(mnt_want_write);
@@ -213,7 +212,7 @@ static void lock_and_coalesce_cpu_mnt_wr
 
 	for_each_possible_cpu(cpu) {
 		cpu_writer = &per_cpu(mnt_writers, cpu);
-		spin_lock(&cpu_writer->lock);
+		mutex_lock(&cpu_writer->lock);
 		__clear_mnt_count(cpu_writer);
 		cpu_writer->mnt = NULL;
 	}
@@ -269,7 +268,7 @@ void mnt_drop_write(struct vfsmount *mnt
 
 retry:
 	cpu_writer = &__get_cpu_var(mnt_writers);
-	spin_lock(&cpu_writer->lock);
+	mutex_lock(&cpu_writer->lock);
 
 	use_cpu_writer_for_mount(cpu_writer, mnt);
 	if (cpu_writer->count > 0) {
@@ -286,14 +285,14 @@ retry:
 		 */
 		if (atomic_read(&mnt->__mnt_writers) <
 		    MNT_WRITER_UNDERFLOW_LIMIT) {
-			spin_unlock(&cpu_writer->lock);
+			mutex_unlock(&cpu_writer->lock);
 			goto retry;
 		}
 		atomic_dec(&mnt->__mnt_writers);
 		must_check_underflow = 1;
 	}
 
-	spin_unlock(&cpu_writer->lock);
+	mutex_unlock(&cpu_writer->lock);
 	/*
 	 * Logically, we could call this each time,
 	 * but the __mnt_writers cacheline tends to
_

Re: [RFC][PATCH 3/4] change mnt_writers[] spinlock to mutex

[Dave Hansen]

Missed the description on that one.  Here it is:

We're shortly going to need to be able to block new
mnt_writers for long periods of time during a
superblock remount operation.  Since this operation
can sleep, we can not use a spinlock.  We opt for
a mutex instead.

This are very, very rarely contented, mostly because
they are per-cpu.  So, this should be very close to
as fast as the spinlocks just with the added benefit
that we can sleep while holding them.

We also need to change the get_cpu_var() to use
__get_cpu_var() so that we don't disable preemption.
Otherwise, we'll be in_atomic() when we try to lock
the (sleepable) mutex.  We only use the per-cpu data
for cache benefits and its per-cpuness is not part
of locking logic, so this is OK.
_
-- Dave

[RFC][PATCH 4/4] check mount writers at superblock remount

[Dave Hansen]

We're moving a big chunk of the burden of keeping people from
writing to r/o filesystems from the superblock into the
vfsmount.  This requires that we keep track somehow of the
mounts that might allow writes to a superblock.

You could track this directly by keeping a count of such
mounts in the superblock, but I think this is much simpler.

Introduce a list of vfsmounts hanging off the superblock.
On remount rw->ro operations, consult each mount of the
superblock and examine its __mnt_writers.  Lock out any
new writers to the mount until the remount operation is
complete.

Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
---

 linux-2.6.git-dave/fs/namespace.c        |   28 ++++++++++++++
 linux-2.6.git-dave/fs/super.c            |   58 ++++++++++++++++++++++++++++---
 linux-2.6.git-dave/include/linux/fs.h    |    1 
 linux-2.6.git-dave/include/linux/mount.h |    3 +
 4 files changed, 85 insertions(+), 5 deletions(-)

diff -puN fs/namespace.c~25-24-reintroduce-list-of-vfsmounts-over-superblock fs/namespace.c
--- linux-2.6.git/fs/namespace.c~25-24-reintroduce-list-of-vfsmounts-over-superblock	2008-01-10 10:46:08.000000000 -0800
+++ linux-2.6.git-dave/fs/namespace.c	2008-01-10 10:46:08.000000000 -0800
@@ -219,6 +219,21 @@ static void lock_and_coalesce_cpu_mnt_wr
 }
 
 /*
+ * These are just some shorter names for
+ * external users.
+ *
+ * Note that this nests outside lock_super().
+ */
+void lock_mnt_writers(void)
+{
+	lock_and_coalesce_cpu_mnt_writer_counts();
+}
+void unlock_mnt_writers(void)
+{
+	mnt_unlock_cpus();
+}
+
+/*
  * 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
@@ -331,10 +346,18 @@ static void __mnt_unmake_readonly(struct
 	mnt->mnt_flags &= ~MNT_READONLY;
 }
 
+void add_mount_to_sb_list(struct vfsmount *mnt, struct super_block *sb)
+{
+	spin_lock(&vfsmount_lock);
+	list_add(&mnt->mnt_sb_list, &sb->s_vfsmounts);
+	spin_unlock(&vfsmount_lock);
+}
+
 int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)
 {
 	mnt->mnt_sb = sb;
 	mnt->mnt_root = dget(sb->s_root);
+	add_mount_to_sb_list(mnt, sb);
 	return 0;
 }
 
@@ -342,6 +365,10 @@ EXPORT_SYMBOL(simple_set_mnt);
 
 void free_vfsmnt(struct vfsmount *mnt)
 {
+	spin_lock(&vfsmount_lock);
+	if (mnt->mnt_sb)
+		list_del(&mnt->mnt_sb_list);
+	spin_unlock(&vfsmount_lock);
 	kfree(mnt->mnt_devname);
 	kmem_cache_free(mnt_cache, mnt);
 }
@@ -494,6 +521,7 @@ static struct vfsmount *clone_mnt(struct
 		simple_set_mnt(mnt, sb);
 		mnt->mnt_mountpoint = mnt->mnt_root;
 		mnt->mnt_parent = mnt;
+		add_mount_to_sb_list(mnt, sb);
 
 		if (flag & CL_SLAVE) {
 			list_add(&mnt->mnt_slave, &old->mnt_slave_list);
diff -puN fs/super.c~25-24-reintroduce-list-of-vfsmounts-over-superblock fs/super.c
--- linux-2.6.git/fs/super.c~25-24-reintroduce-list-of-vfsmounts-over-superblock	2008-01-10 10:46:08.000000000 -0800
+++ linux-2.6.git-dave/fs/super.c	2008-01-10 10:46:08.000000000 -0800
@@ -36,6 +36,7 @@
 #include <linux/writeback.h>		/* for the emergency remount stuff */
 #include <linux/idr.h>
 #include <linux/kobject.h>
+#include <linux/list.h>
 #include <linux/mutex.h>
 #include <asm/uaccess.h>
 
@@ -65,6 +66,7 @@ static struct super_block *alloc_super(s
 		INIT_LIST_HEAD(&s->s_io);
 		INIT_LIST_HEAD(&s->s_more_io);
 		INIT_LIST_HEAD(&s->s_files);
+		INIT_LIST_HEAD(&s->s_vfsmounts);
 		INIT_LIST_HEAD(&s->s_instances);
 		INIT_HLIST_HEAD(&s->s_anon);
 		INIT_LIST_HEAD(&s->s_inodes);
@@ -575,6 +577,34 @@ static void mark_files_ro(struct super_b
 }
 
 /**
+ *	__sb_can_remount_ro - check a superblock for active writers
+ *	@sb: superblock in question
+ *
+ *	Must ensure that no new writers to the superblock can come
+ *	in (must hold lock_mnt_writers()) and that the s_vfsmounts
+ *	list can not change (must acquire lock_super()).
+ *
+ *	Returns 0 on success.
+ */
+static int __sb_can_remount_ro(struct super_block *sb)
+{
+	struct list_head *entry;
+	int ret = 0;
+
+	lock_super(sb);
+	list_for_each(entry, &sb->s_vfsmounts) {
+		struct vfsmount *mnt;
+		mnt = list_entry(entry, struct vfsmount, mnt_sb_list);
+		if (!atomic_read(&mnt->__mnt_writers))
+			continue;
+		ret = -EBUSY;
+		break;
+	}
+	unlock_super(sb);
+	return ret;
+}
+
+/**
  *	do_remount_sb - asks filesystem to change mount options.
  *	@sb:	superblock in question
  *	@flags:	numeric part of options
@@ -585,6 +615,7 @@ static void mark_files_ro(struct super_b
  */
 int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
 {
+	int mnt_writers_locked = 0;
 	int retval;
 	
 #ifdef CONFIG_BLOCK
@@ -599,10 +630,19 @@ int do_remount_sb(struct super_block *sb
 	/* If we are remounting RDONLY and current sb is read/write,
 	   make sure there are no rw files opened */
 	if ((flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY)) {
-		if (force)
+		if (force) {
 			mark_files_ro(sb);
-		else if (!fs_may_remount_ro(sb))
-			return -EBUSY;
+		} else {
+			/*
+			 * keeps new mnt writers out until
+			 * we can set the sb r/o flag
+			 */
+			lock_mnt_writers();
+			mnt_writers_locked = 1;
+			retval = __sb_can_remount_ro(sb);
+			if (retval)
+				goto out;
+		}
 	}
 
 	if (sb->s_op->remount_fs) {
@@ -610,10 +650,18 @@ int do_remount_sb(struct super_block *sb
 		retval = sb->s_op->remount_fs(sb, &flags, data);
 		unlock_super(sb);
 		if (retval)
-			return retval;
+			goto out;
 	}
 	sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (flags & MS_RMT_MASK);
-	return 0;
+out:
+	/*
+	 * This must stay locked until after the s_flags are
+	 * set because the flag will also lock out any new
+	 * writers.
+	 */
+	if (mnt_writers_locked)
+		unlock_mnt_writers();
+	return retval;
 }
 
 static void do_emergency_remount(unsigned long foo)
diff -puN include/linux/fs.h~25-24-reintroduce-list-of-vfsmounts-over-superblock include/linux/fs.h
--- linux-2.6.git/include/linux/fs.h~25-24-reintroduce-list-of-vfsmounts-over-superblock	2008-01-10 10:46:08.000000000 -0800
+++ linux-2.6.git-dave/include/linux/fs.h	2008-01-10 10:46:08.000000000 -0800
@@ -1012,6 +1012,7 @@ struct super_block {
 	struct list_head	s_io;		/* parked for writeback */
 	struct list_head	s_more_io;	/* parked for more writeback */
 	struct hlist_head	s_anon;		/* anonymous dentries for (nfs) exporting */
+	struct list_head	s_vfsmounts;
 	struct list_head	s_files;
 
 	struct block_device	*s_bdev;
diff -puN include/linux/mount.h~25-24-reintroduce-list-of-vfsmounts-over-superblock include/linux/mount.h
--- linux-2.6.git/include/linux/mount.h~25-24-reintroduce-list-of-vfsmounts-over-superblock	2008-01-10 10:46:08.000000000 -0800
+++ linux-2.6.git-dave/include/linux/mount.h	2008-01-10 10:46:08.000000000 -0800
@@ -44,6 +44,7 @@ struct vfsmount {
 	struct dentry *mnt_mountpoint;	/* dentry of mountpoint */
 	struct dentry *mnt_root;	/* root of the mounted tree */
 	struct super_block *mnt_sb;	/* pointer to superblock */
+	struct list_head mnt_sb_list;	/* list of all mounts on same sb */
 	struct list_head mnt_mounts;	/* list of children, anchored here */
 	struct list_head mnt_child;	/* and going through their mnt_child */
 	int mnt_flags;
@@ -80,6 +81,8 @@ static inline struct vfsmount *mntget(st
 
 extern int mnt_want_write(struct vfsmount *mnt);
 extern void mnt_drop_write(struct vfsmount *mnt);
+extern void lock_mnt_writers(void);
+extern void unlock_mnt_writers(void);
 extern void mntput_no_expire(struct vfsmount *mnt);
 extern void mnt_pin(struct vfsmount *mnt);
 extern void mnt_unpin(struct vfsmount *mnt);
diff -puN fs/xfs/xfs_inode.c~25-24-reintroduce-list-of-vfsmounts-over-superblock fs/xfs/xfs_inode.c
diff -puN fs/dquot.c~25-24-reintroduce-list-of-vfsmounts-over-superblock fs/dquot.c
_

Re: [RFC][PATCH 0/4] kill open files traverse on remount ro

[Serge E. Hallyn]

Quoting Dave Hansen (haveblue@us.ibm.com):
> This is just RFC for now.  I'm tracking down a wee bit of
> list corruption.  But, I wanted to send out so you could
> compare to the last approach.

Looks reasonable to me, and quite readable.

So after this set, you'd be able to remove s_files altogether?

-serge