Re: [PATCH v3 1/5] add metadata_incore ioctl in vfs

On Thu, 2011-01-20 at 04:41 +0800, Andrew Morton wrote:

On Wed, 19 Jan 2011 09:15:18 +0800
Shaohua Li [off-list ref] wrote:

quoted

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