Re: [PATCH 02/22] r/o bind mounts: add vfsmount writer counts

Dave Hansen
2007-02-10T00:10:16Z

On Sat, 2007-02-10 at 00:41 +0100, Eric Dumazet wrote:
> Dave, please read again this comment in struct vfsmount definition.
>
> If I understand your infrastructure, mnt=5Fwriters is going to be frequently
> modified, so it should be placed at the end of struct vfsmount, in the same
> cache line than mnt_count.

That's an excellent point, thanks for catching it. Here's an updated
patch.

-- Dave

This patch actually adds the mount and superblock writer
counts, and the mnt_want/drop_write() functions that use
them.

Before these can become useful, we must first cover each
place in the VFS where writes are performed with a
want/drop pair. When that is complete, we can actually
introduce code that will safely check the counts before
allowing r/w<->r/o transitions to occur.

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

lxc-dave/fs/namespace.c | 53 +++++++++++++++++++++++++++++++++++++++++
lxc-dave/fs/super.c | 18 ++++++++++---
lxc-dave/include/linux/fs.h | 2 +
lxc-dave/include/linux/mount.h | 28 +++++++++++++++++++--
4 files changed, 94 insertions(+), 7 deletions(-)

diff -puN fs/namespace.c~03-24-add-vfsmount-writer-count fs/namespace.c
--- lxc/fs/namespace.c~03-24-add-vfsmount-writer-count 2007-02-09 16:04:40.000000000 -0800
+++ lxc-dave/fs/namespace.c 2007-02-09 16:04:40.000000000 -0800
@@ -58,6 +58,7 @@ struct vfsmount *alloc_vfsmnt(const char
if (mnt) {
mnt->mnt_user_ns = get_user_ns(current->nsproxy->user_ns);
atomic_set(&mnt->mnt_count, 1);
+ mnt->mnt_writers = 0;
INIT_LIST_HEAD(&mnt->mnt_hash);
INIT_LIST_HEAD(&mnt->mnt_child);
INIT_LIST_HEAD(&mnt->mnt_mounts);
@@ -78,6 +79,56 @@ struct vfsmount *alloc_vfsmnt(const char
return mnt;
}

+int mnt_make_readonly(struct vfsmount *mnt)
+{
+ int ret = 0;
+
+ WARN_ON(__mnt_is_readonly(mnt));
+
+ /*
+ * This flag set is actually redundant with what
+ * happens in do_remount(), but since we do this
+ * under the lock, anyone attempting to get a write
+ * on it after this will fail.
+ */
+ spin_lock(&mnt->mnt_sb->s_mnt_writers_lock);
+ if (!mnt->mnt_writers)
+ mnt->mnt_flags |= MNT_READONLY;
+ else
+ ret = -EBUSY;
+ spin_unlock(&mnt->mnt_sb->s_mnt_writers_lock);
+ return ret;
+}
+
+int mnt_want_write(struct vfsmount *mnt)
+{
+ int ret = 0;
+
+ spin_lock(&mnt->mnt_sb->s_mnt_writers_lock);
+ if (mnt->mnt_writers)
+ goto out;
+
+ if (__mnt_is_readonly(mnt)) {
+ ret = -EROFS;
+ goto out;
+ }
+ mnt->mnt_sb->s_writers++;
+ mnt->mnt_writers++;
+out:
+ spin_unlock(&mnt->mnt_sb->s_mnt_writers_lock);
+ return ret;
+}
+EXPORT_SYMBOL_GPL(mnt_want_write);
+
+void mnt_drop_write(struct vfsmount *mnt)
+{
+ spin_lock(&mnt->mnt_sb->s_mnt_writers_lock);
+ mnt->mnt_sb->s_writers--;
+ mnt->mnt_writers--;
+ spin_unlock(&mnt->mnt_sb->s_mnt_writers_lock);
+}
+EXPORT_SYMBOL_GPL(mnt_drop_write);
+
int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)
{
mnt->mnt_sb = sb;
@@ -1415,6 +1466,8 @@ long do_mount(char *dev_name, char *dir_
((char *)data_page)[PAGE_SIZE - 1] = 0;

/* Separate the per-mountpoint flags */
+ if (flags & MS_RDONLY)
+ mnt_flags |= MNT_READONLY;
if (flags & MS_NOSUID)
mnt_flags |= MNT_NOSUID;
if (flags & MS_NODEV)
diff -puN fs/super.c~03-24-add-vfsmount-writer-count fs/super.c
--- lxc/fs/super.c~03-24-add-vfsmount-writer-count 2007-02-09 16:04:40.000000000 -0800
+++ lxc-dave/fs/super.c 2007-02-09 16:04:40.000000000 -0800
@@ -93,6 +93,8 @@ static struct super_block *alloc_super(s
s->s_qcop = sb_quotactl_ops;
s->s_op = &default_op;
s->s_time_gran = 1000000000;
+ s->s_writers = 0;
+ spin_lock_init(&s->s_mnt_writers_lock);
}
out:
return s;
@@ -576,6 +578,11 @@ static void mark_files_ro(struct super_b
file_list_unlock();
}

+static int sb_remount_ro(struct super_block *sb)
+{
+ return fs_may_remount_ro(sb);
+}
+
/**
* do_remount_sb - asks filesystem to change mount options.
* @sb: superblock in question
@@ -587,7 +594,8 @@ static void mark_files_ro(struct super_b
*/
int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
{
- int retval;
+ int retval = 0;
+ int sb_started_ro = (sb->s_flags & MS_RDONLY);

#ifdef CONFIG_BLOCK
if (!(flags & MS_RDONLY) && bdev_read_only(sb->s_bdev))
@@ -600,11 +608,13 @@ 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 ((flags & MS_RDONLY) && !sb_started_ro) {
if (force)
mark_files_ro(sb);
- else if (!fs_may_remount_ro(sb))
- return -EBUSY;
+ else
+ retval = sb_remount_ro(sb);
+ if (retval)
+ return retval;
}

if (sb->s_op->remount_fs) {
diff -puN include/linux/fs.h~03-24-add-vfsmount-writer-count include/linux/fs.h
--- lxc/include/linux/fs.h~03-24-add-vfsmount-writer-count 2007-02-09 16:04:40.000000000 -0800
+++ lxc-dave/include/linux/fs.h 2007-02-09 16:04:40.000000000 -0800
@@ -972,6 +972,8 @@ struct super_block {
struct list_head s_io; /* parked for writeback */
struct hlist_head s_anon; /* anonymous dentries for (nfs) exporting */
struct list_head s_files;
+ int s_writers; /* number of files open for write */
+ spinlock_t s_mnt_writers_lock; /* taken when mounts change rw state */

struct block_device *s_bdev;
struct list_head s_instances;
diff -puN include/linux/mount.h~03-24-add-vfsmount-writer-count include/linux/mount.h
--- lxc/include/linux/mount.h~03-24-add-vfsmount-writer-count 2007-02-09 16:04:40.000000000 -0800
+++ lxc-dave/include/linux/mount.h 2007-02-09 16:07:28.000000000 -0800
@@ -29,6 +29,7 @@ struct user_namespace;
#define MNT_NOATIME 0x08
#define MNT_NODIRATIME 0x10
#define MNT_RELATIME 0x20
+#define MNT_READONLY 0x40 /* does the user want this to be r/o? */

#define MNT_SHRINKABLE 0x100

@@ -57,12 +58,14 @@ struct vfsmount {
struct mnt_namespace *mnt_ns; /* containing namespace */
struct user_namespace *mnt_user_ns; /* namespace for uid interpretation */
/*
- * 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)
+ * We put mnt_count, mnt_expiry_mark, and mnt_writers 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_writers; /* nr files open for write */
int mnt_pinned;
};

@@ -72,7 +75,26 @@ static inline struct vfsmount *mntget(st
atomic_inc(&mnt->mnt_count);
return mnt;
}
+/*
+ * This is temporary for now. We also don't want to check
+ * the SB in because it is already checked in other
+ * code paths. We'll have a better way to do this in
+ * the end of this series
+ */
+static inline int __mnt_is_readonly(struct vfsmount *mnt)
+{
+ return mnt->mnt_flags & MNT_READONLY;
+}
+
+static inline void __mnt_unmake_readonly(struct vfsmount *mnt)
+{
+ WARN_ON(!__mnt_is_readonly(mnt));
+ mnt->mnt_flags &= ~MNT_READONLY;
+}

+extern int mnt_make_readonly(struct vfsmount *mnt);
+extern int mnt_want_write(struct vfsmount *mnt);
+extern void mnt_drop_write(struct vfsmount *mnt);
extern void mntput_no_expire(struct vfsmount *mnt);
extern void mnt_pin(struct vfsmount *mnt);
extern void mnt_unpin(struct vfsmount *mnt);
_