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?
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?
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