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SubjectRe: [PATCH] r/o bind mounts: add vfsmount and superblock writer counts
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Date
On Tue, 2007-01-09 at 10:53 +0000, Christoph Hellwig wrote: 
> On Mon, Jan 08, 2007 at 10:34:45AM -0800, Dave Hansen wrote:
> > The other side-effect is that we can't have the bit in mnt_flags
> > to be a shortcut to the superblock's writeable state since we
> > don't have a way to go find the mounts and that bit when a fs
> > changes writeable state. This causes a potential cache miss
> > when we have to check the superblock directly during the
> > relatively common __mnt_is_readonly() function.
>
> Why? We _only_ need to check the vfsmount flag. vfsmount can
> become r/w if the superblock is marked r/o which means the
> underlying (block/network/etc) device is fundamentally not writeable.

It does seem a bit silly to go all the way down to the permissions
checks to finally find out that the sb is r/o when it is pretty easy to
catch here. But, I guess it is a common-case hit for a relatively
uncommon occurrence.

Here is an updated patch?

After this, do you think it is ready for -mm?

---

lxc-dave/fs/inode.c | 5 ++++
lxc-dave/fs/namespace.c | 47 ++++++++++++++++++++++++++++++++++++++---
lxc-dave/fs/super.c | 31 ++++++++++++++++++++++-----
lxc-dave/include/linux/fs.h | 5 ++++
lxc-dave/include/linux/mount.h | 20 ++++++++++-------
5 files changed, 92 insertions(+), 16 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 2007-01-10 10:26:33.000000000 -0800
+++ lxc-dave/fs/namespace.c 2007-01-10 10:26:34.000000000 -0800
@@ -56,6 +56,7 @@ struct vfsmount *alloc_vfsmnt(const char
struct vfsmount *mnt = kmem_cache_zalloc(mnt_cache, GFP_KERNEL);
if (mnt) {
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);
@@ -877,15 +878,52 @@ 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 (!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)
{
- if (__mnt_is_readonly(mnt))
- return -EROFS;
- return 0;
+ 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);
}

/*
@@ -986,6 +1024,7 @@ static int do_remount(struct nameidata *
security_sb_post_remount(nd->mnt, flags, data);
return err;
}
+EXPORT_SYMBOL_GPL(mnt_drop_write);

static inline int tree_contains_unbindable(struct vfsmount *mnt)
{
@@ -1410,6 +1449,8 @@ 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 2007-01-10 10:26:33.000000000 -0800
+++ lxc-dave/fs/super.c 2007-01-10 10:26:34.000000000 -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 +578,20 @@ 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);
+}
+
+static void sb_remount_rw(struct super_block *sb)
+{
+ /*
+ * No action, yet. This will soon
+ * notify the mounts that the sb
+ * has become writeable.
+ */
+}
+
/**
* do_remount_sb - asks filesystem to change mount options.
* @sb: superblock in question
@@ -587,7 +603,8 @@ 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))
@@ -600,13 +617,14 @@ 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);
@@ -614,6 +632,9 @@ 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;
}
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 2007-01-10 10:26:33.000000000 -0800
+++ lxc-dave/include/linux/fs.h 2007-01-10 10:26:34.000000000 -0800
@@ -969,6 +969,8 @@ 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;
@@ -1277,6 +1279,9 @@ static inline void check_nlink(struct in
return;

inode->i_state |= I_AWAITING_FINAL_IPUT;
+ spin_lock(&inode->i_sb->s_mnt_writers_lock);
+ inode->i_sb->s_writers--;
+ spin_unlock(&inode->i_sb->s_mnt_writers_lock);
}

/**
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 2007-01-10 10:26:34.000000000 -0800
+++ lxc-dave/include/linux/mount.h 2007-01-10 10:27:17.000000000 -0800
@@ -28,6 +28,7 @@ struct mnt_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

@@ -53,7 +54,7 @@ struct vfsmount {
struct list_head mnt_slave; /* slave list entry */
struct vfsmount *mnt_master; /* slave is on master->mnt_slave_list */
struct mnt_namespace *mnt_ns; /* containing namespace */
- atomic_t mnt_writers; /* nr files open for write */
+ int mnt_writers; /* nr files open for write */
/*
* We put mnt_count & mnt_expiry_mark at the end of struct vfsmount
* to let these frequently modified fields in a separate cache line
@@ -71,17 +72,20 @@ static inline struct vfsmount *mntget(st
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 0;
+ 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);
diff -L lxc-dave/include/linux/fs.h -puN /dev/null /dev/null
diff -puN fs/inode.c~03-24-add-vfsmount-writer-count fs/inode.c
--- lxc/fs/inode.c~03-24-add-vfsmount-writer-count 2007-01-10 10:26:34.000000000 -0800
+++ lxc-dave/fs/inode.c 2007-01-10 10:26:34.000000000 -0800
@@ -1101,6 +1101,11 @@ static inline void iput_final(struct ino
if (op && op->drop_inode)
drop = op->drop_inode;
drop(inode);
+ if (must_drop_sb_write) {
+ spin_lock(&sb->s_mnt_writers_lock);
+ sb->s_writers--;
+ spin_unlock(&sb->s_mnt_writers_lock);
+ }
}

/**
_

-- Dave

-
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