Re: [PATCH 1/3] Implement generic freeze feature

["Takashi Sato" <t-sato@xxxxxxxxxxxxx>]

Hi,

Christoph Hellwig wrote:
> On Mon, Aug 18, 2008 at 09:28:19PM +0900, Takashi Sato wrote:
> > +    down(&bdev->bd_freeze_sem);
> > +    bdev->bd_freeze_count++;
> > +    if (bdev->bd_freeze_count > 1) {
> > +            sb = get_super(bdev);
> > +            drop_super(sb);
> > +            up(&bdev->bd_freeze_sem);
> > +            return sb;
> > +    }
> > +
> >      down(&bdev->bd_mount_sem);
>
> Now you have a reference counter of freezes which actually is pretty
> sensible, but also needs some documentation.  What I don't understand
> here at all is why you do the get_super/drop_super in the already frozen
> case.

Even if the filesystem has already been frozen, the superblock
should be returned. Because a caller should recognize the success of
freeze_bdev() and call thaw_bdev() to decrease the reference count. 
But I will remove drop_super() as it should be called in thaw_bdev().

> Now that the freeze_count has replaced one of the uses of bd_mount_sem
> you should also replace the other use in the unmount path by simply
> checking for the freez_count and abort if it's set.  To do so you'll
> need to hold the bd_mount_sem over the whole unmount operation to
> prevent new frezes from coming in.

In the original implementation,
unmount is protected by s_umount(semaphore),
not bd_mount_sem.  So, unmount task waits for unfreeze.
I think this original behavior shouldn't be changed, 
so the existing s_umount lock is better.

> As others noted it should be a mutex and not a semaphore.

As you said, we should use the mutex.
I will replace it.

> >  /*
> > + * ioctl_freeze - Freeze the filesystem.
> > + *
> > + * @filp:   target file
> > + *
> > + * Call freeze_bdev() to freeze the filesystem.
> > + */
> > +static int ioctl_freeze(struct file *filp)
>
> This is not quite kerneldcoc format, which would ne a /** as commnt
> start.  But I don't think the comment is actually needed, it's a pretty
> obvious file scope function. (Same commnt also applies to ioctl_thaw)

I will remove these comments.

> > +    struct super_block *sb = filp->f_path.dentry->d_inode->i_sb;
> > +
> > +    if (!capable(CAP_SYS_ADMIN))
> > +            return -EPERM;
> > +
> > +    /* If filesystem doesn't support freeze feature, return. */
> > +    if (sb->s_op->write_super_lockfs == NULL)
> > +            return -EOPNOTSUPP;
> > +
> > +    /* If a regular file or a directory isn't specified, return. */
> > +    if (sb->s_bdev == NULL)
> > +            return -EINVAL;
>
> I don't understand this commnt.  What you are checking is that the
> filesystem has a non-NULL s_bdev, which implies a not blockdevice-backed
> filesystem.

I will fix the comment as :
" If a blockdevice-backed filesystem isn't specified, return."

Cheers, Takashi