[PATCH 07/28] Add vfsmount writer count

Dave Hansen
2006-08-01T23:59:41Z

This allows a vfsmount to keep track of how many instances
of files open for write there are at a given time.

A mount can refuse to allow writers. This can be because
it is a read-only bind mount, or for other functionality
in the future. A mount can also now refuse to make a
transition from r/w to r/o whenever it has currently active
writers.

When a mount gets its first writer, it tells the superblock
about it by incrementing sb->s_mnt_writers. When the last
writer goes away, this count is bumped back down. This makes
the question of whether or not anything is writing to the sb
a much quicker, easier operation than scanning the open files
list.

The WARN_ON()s can go away before this is merged into mainline
provided it has had some time in -mm or equivalent.

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

lxc-dave/fs/namespace.c | 61 +++++++++++++++++++++++++++++++++++++
lxc-dave/fs/super.c | 66 +++++++++++++++++++++++++++++++++++++----
lxc-dave/include/linux/fs.h | 2 +
lxc-dave/include/linux/mount.h | 18 +++++++++++
4 files changed, 142 insertions(+), 5 deletions(-)

diff -puN fs/namespace.c~C-add-vfsmount-writer_count fs/namespace.c
--- lxc/fs/namespace.c~C-add-vfsmount-writer_count 2006-08-01 16:35:18.000000000 -0700
+++ lxc-dave/fs/namespace.c 2006-08-01 16:35:19.000000000 -0700
@@ -58,6 +58,7 @@ struct vfsmount *alloc_vfsmnt(const char
if (mnt) {
memset(mnt, 0, sizeof(struct vfsmount));
atomic_set(&mnt->mnt_count, 1);
+ atomic_set(&mnt->mnt_writers, 0);
INIT_LIST_HEAD(&mnt->mnt_hash);
INIT_LIST_HEAD(&mnt->mnt_child);
INIT_LIST_HEAD(&mnt->mnt_mounts);
@@ -886,6 +887,60 @@ out_unlock:
return err;
}

+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 (!atomic_read(&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;
+repeat:
+ if (atomic_add_unless(&mnt->mnt_writers, 1, 0))
+ return 0;
+
+ spin_lock(&mnt->mnt_sb->s_mnt_writers_lock);
+ if (__mnt_is_readonly(mnt)) {
+ ret = -EROFS;
+ goto out;
+ }
+ if (atomic_add_return(1, &mnt->mnt_writers) != 1) {
+ atomic_dec(&mnt->mnt_writers);
+ spin_unlock(&mnt->mnt_sb->s_mnt_writers_lock);
+ goto repeat;
+ }
+ atomic_inc(&mnt->mnt_sb->s_mnt_writers);
+out:
+ spin_unlock(&mnt->mnt_sb->s_mnt_writers_lock);
+ return ret;
+}
+
+void mnt_drop_write(struct vfsmount *mnt)
+{
+ if (!atomic_dec_and_lock(&mnt->mnt_writers,
+ &mnt->mnt_sb->s_mnt_writers_lock))
+ return;
+
+ atomic_dec(&mnt->mnt_sb->s_mnt_writers);
+ spin_unlock(&mnt->mnt_sb->s_mnt_writers_lock);
+}
+
/*
* recursively change the type of the mountpoint.
*/
@@ -977,6 +1032,8 @@ static int do_remount(struct nameidata *

down_write(&sb->s_umount);
err = do_remount_sb(sb, flags, data, 0);
+ if (!(sb->s_flags & MS_RDONLY))
+ mnt_flags |= MNT_SB_WRITABLE;
if (!err)
nd->mnt->mnt_flags = mnt_flags;
up_write(&sb->s_umount);
@@ -1117,6 +1174,8 @@ int do_add_mount(struct vfsmount *newmnt
if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
goto unlock;

+ if (!(newmnt->mnt_sb->s_flags & MS_RDONLY))
+ mnt_flags |= MNT_SB_WRITABLE;
newmnt->mnt_flags = mnt_flags;
if ((err = graft_tree(newmnt, nd)))
goto unlock;
@@ -1408,6 +1467,8 @@ 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~C-add-vfsmount-writer_count fs/super.c
--- lxc/fs/super.c~C-add-vfsmount-writer_count 2006-08-01 16:35:18.000000000 -0700
+++ lxc-dave/fs/super.c 2006-08-01 16:35:19.000000000 -0700
@@ -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;
+ atomic_set(&s->s_mnt_writers, 0);
+ spin_lock_init(&s->s_mnt_writers_lock);
}
out:
return s;
@@ -527,6 +529,53 @@ static void mark_files_ro(struct super_b
file_list_unlock();
}

+static void __sb_mounts_mod_flag(struct super_block *sb, int set, int flag)
+{
+ struct list_head *p;
+
+ spin_lock(&vfsmount_lock);
+ list_for_each(p, &sb->s_vfsmounts) {
+ struct vfsmount *mnt =
+ list_entry(p, struct vfsmount, mnt_sb_list);
+ if (set)
+ mnt->mnt_flags |= flag;
+ else
+ mnt->mnt_flags &= ~flag;
+ }
+ spin_unlock(&vfsmount_lock);
+}
+
+static void sb_mounts_set_flag(struct super_block *sb, int flag)
+{
+ __sb_mounts_mod_flag(sb, 1, flag);
+}
+static void sb_mounts_clear_flag(struct super_block *sb, int flag)
+{
+ __sb_mounts_mod_flag(sb, 0, flag);
+}
+
+static int sb_remount_ro(struct super_block *sb)
+{
+ return fs_may_remount_ro(sb);
+ spin_lock(&sb->s_mnt_writers_lock);
+ if (atomic_read(&sb->s_mnt_writers) > 0) {
+ spin_unlock(&sb->s_mnt_writers_lock);
+ return -EBUSY;
+ }
+
+ sb_mounts_clear_flag(sb, MNT_SB_WRITABLE);
+ spin_unlock(&sb->s_mnt_writers_lock);
+
+ return 0;
+}
+
+static void sb_remount_rw(struct super_block *sb)
+{
+ spin_lock(&sb->s_mnt_writers_lock);
+ sb_mounts_set_flag(sb, MNT_SB_WRITABLE);
+ spin_unlock(&sb->s_mnt_writers_lock);
+}
+
/**
* do_remount_sb - asks filesystem to change mount options.
* @sb: superblock in question
@@ -538,7 +587,8 @@ 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);

if (!(flags & MS_RDONLY) && bdev_read_only(sb->s_bdev))
return -EACCES;
@@ -549,13 +599,14 @@ 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) {
lock_super(sb);
retval = sb->s_op->remount_fs(sb, &flags, data);
@@ -563,6 +614,9 @@ int do_remount_sb(struct super_block *sb
if (retval)
return retval;
}
+ if (!(flags & MS_RDONLY) && sb_started_ro)
+ sb_remount_rw(sb);
+
sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (flags & MS_RMT_MASK);
return 0;
}
@@ -849,6 +903,8 @@ vfs_kern_mount(struct file_system_type *

mnt->mnt_mountpoint = mnt->mnt_root;
mnt->mnt_parent = mnt;
+ if (!(mnt->mnt_sb->s_flags & MS_RDONLY))
+ mnt->mnt_flags |= MNT_SB_WRITABLE;
up_write(&mnt->mnt_sb->s_umount);
free_secdata(secdata);
return mnt;
diff -puN include/linux/fs.h~C-add-vfsmount-writer_count include/linux/fs.h
--- lxc/include/linux/fs.h~C-add-vfsmount-writer_count 2006-08-01 16:35:18.000000000 -0700
+++ lxc-dave/include/linux/fs.h 2006-08-01 16:35:19.000000000 -0700
@@ -963,6 +963,8 @@ 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_vfsmounts;
+ atomic_t s_mnt_writers; /* vfsmounts with active writers */
+ spinlock_t s_mnt_writers_lock; /* taken when mounts change rw state */
struct list_head s_files;

struct block_device *s_bdev;
diff -puN include/linux/mount.h~C-add-vfsmount-writer_count include/linux/mount.h
--- lxc/include/linux/mount.h~C-add-vfsmount-writer_count 2006-08-01 16:35:18.000000000 -0700
+++ lxc-dave/include/linux/mount.h 2006-08-01 16:35:19.000000000 -0700
@@ -27,6 +27,8 @@ struct namespace;
#define MNT_NOEXEC 0x04
#define MNT_NOATIME 0x08
#define MNT_NODIRATIME 0x10
+#define MNT_READONLY 0x20 /* does the user want this to be r/o? */
+#define MNT_SB_WRITABLE 0x40 /* does the SB currently allow writes? */

#define MNT_SHRINKABLE 0x100

@@ -44,6 +46,7 @@ struct vfsmount {
struct list_head mnt_mounts; /* list of children, anchored here */
struct list_head mnt_child; /* and going through their mnt_child */
atomic_t mnt_count;
+ atomic_t mnt_writers;
int mnt_flags;
int mnt_expiry_mark; /* true if marked for expiry */
char *mnt_devname; /* Name of device e.g. /dev/dsk/hda1 */
@@ -64,6 +67,21 @@ static inline struct vfsmount *mntget(st
return mnt;
}

+static inline int __mnt_is_readonly(struct vfsmount *mnt)
+{
+ return (mnt->mnt_flags & MNT_READONLY) ||
+ !(mnt->mnt_flags & MNT_SB_WRITABLE);
+}
+
+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);
_