scoutfs: add a stat_more ioctl

We have inode fields that we want to return to userspace with very low
overhead.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/ioctl.c

@@ -413,6 +413,24 @@ out:
 	return ret;
 }
 
+static long scoutfs_ioc_stat_more(struct file *file, unsigned long arg)
+{
+	struct inode *inode = file_inode(file);
+	struct scoutfs_ioctl_stat_more stm;
+
+	if (get_user(stm.valid_bytes, (__u64 __user *)arg))
+		return -EFAULT;
+
+	stm.valid_bytes = min_t(u64, stm.valid_bytes,
+				sizeof(struct scoutfs_ioctl_stat_more));
+	stm.data_version = scoutfs_inode_get_data_version(inode);
+
+	if (copy_to_user((void __user *)arg, &stm, stm.valid_bytes))
+		return -EFAULT;
+
+	return 0;
+}
+
 long scoutfs_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	switch (cmd) {
@@ -426,6 +444,8 @@ long scoutfs_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 		return scoutfs_ioc_release(file, arg);
 	case SCOUTFS_IOC_STAGE:
 		return scoutfs_ioc_stage(file, arg);
+	case SCOUTFS_IOC_STAT_MORE:
+		return scoutfs_ioc_stat_more(file, arg);
 	}
 
 	return -ENOTTY;

kmod/src/ioctl.h

@@ -130,4 +130,25 @@ struct scoutfs_ioctl_stage {
 #define SCOUTFS_IOC_STAGE _IOW(SCOUTFS_IOCTL_MAGIC, 6, \
 			       struct scoutfs_ioctl_stage)
 
+/*
+ * Give the user inode fields that are not otherwise visible.  statx()
+ * isn't always available and xattrs are relatively expensive.
+ *
+ * @valid_bytes stores the number of bytes that are valid in the
+ * structure.  The caller sets this to the size of the struct that they
+ * understand.  The kernel then fills and copies back the min of the
+ * size they and the user caller understand.  The user can tell if a
+ * field is set if all of its bytes are within the valid_bytes that the
+ * kernel set on return.
+ *
+ * New fields are only added to the end of the struct.
+ */
+struct scoutfs_ioctl_stat_more {
+	__u64 valid_bytes;
+	__u64 data_version;
+} __packed;
+
+#define SCOUTFS_IOC_STAT_MORE _IOW(SCOUTFS_IOCTL_MAGIC, 7, \
+				   struct scoutfs_ioctl_stat_more)
+
 #endif