On Thu, 2011-01-20 at 04:41 +0800, Andrew Morton wrote:
> On Wed, 19 Jan 2011 09:15:18 +0800
> Shaohua Li <shaohua.li@xxxxxxxxx> wrote:
>
> > Subject: add metadata_incore ioctl in vfs
> >
> > Add an ioctl to dump filesystem's metadata in memory in vfs. Userspace collects
> > such info and uses it to do metadata readahead.
> > Filesystem can hook to super_operations.metadata_incore to get metadata in
> > specific approach. Next patch will give an example how to implement
> > .metadata_incore in btrfs.
> >
> > ...
> >
> > /*
> > + * Copy info about metadata in memory to userspace
> > + * Returns:
> > + * = 1, one metadata range copied to userspace
> > + * = 0, no more metadata
> > + * < 0, error
> > + */
> > +static int ioctl_metadata_incore(struct file *filp, void __user *argp)
> > +{
> > + struct super_block *sb = filp->f_path.dentry->d_inode->i_sb;
> > + struct metadata_incore_args args;
> > + loff_t offset;
> > + ssize_t size;
> > +
> > + if (!sb->s_op->metadata_incore)
> > + return -EINVAL;
> > +
> > + if (copy_from_user(&args, argp, sizeof(args)))
> > + return -EFAULT;
> > +
> > + /* we check metadata info in page unit */
> > + if (args.offset & ~PAGE_CACHE_MASK)
> > + return -EINVAL;
> > +
> > + offset = args.offset;
> > +
> > + if (sb->s_op->metadata_incore(sb, &offset, &size) < 0)
> > + return 0;
> > +
> > + args.address = offset;
> > + args.size = size;
> > + args.unused = 0;
> > +
> > + if (copy_to_user(argp, &args, sizeof(args)))
> > + return -EFAULT;
> > +
> > + return 1;
> > +}
>
> So userspace opens any file on the fs and runs this ioctl against it?
>
> That's a pretty awkward interface - we're doing an fs-wide operation
> but the fs is identified by a single file which happens to live on that
> fs. For example, this precludes a future extension whereby userspace
> can query the incore metadata for a particular file. The statfs
> syscall sucks in the same manner.
>
> I don't know if this is worth addressing. Perhaps require that the
> filp refers to the root of the fs?
I didn't see why this is needed, but I can limit the fip to the root of
the fs.
> Also, is this a privileged operation? If not, then that might be a
> problem - could it be used by unprivileged users to work out which
> files have been opened recently or something like that?
it's harmless even a unprivileged user uses it. I don't think
unprivileged user can decode the data returned from the ioctl.
Thanks,
Shaohua
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