vfs: add a FALLOC_FL_UNSHARE mode to fallocate to unshare a range of blocks

Add a new fallocate mode flag that explicitly unshares blocks on
filesystems that support such features. The new flag can only
be used with an allocate-mode fallocate call.

Signed-off-by: Darrick J. Wong <[e-mail]>

fs/open.c

···
 	    (mode & ~FALLOC_FL_INSERT_RANGE))
 		return -EINVAL;
 
+	/* Unshare range should only be used with allocate mode. */
+	if ((mode & FALLOC_FL_UNSHARE_RANGE) &&
+	    (mode & ~(FALLOC_FL_UNSHARE_RANGE | FALLOC_FL_KEEP_SIZE)))
+		return -EINVAL;
+
 	if (!(file->f_mode & FMODE_WRITE))
 		return -EBADF;
 

include/linux/falloc.h

···
 					 FALLOC_FL_PUNCH_HOLE |		\
 					 FALLOC_FL_COLLAPSE_RANGE |	\
 					 FALLOC_FL_ZERO_RANGE |		\
-					 FALLOC_FL_INSERT_RANGE)
+					 FALLOC_FL_INSERT_RANGE |	\
+					 FALLOC_FL_UNSHARE_RANGE)
 
 #endif /* _FALLOC_H_ */

include/uapi/linux/falloc.h

···
  */
 #define FALLOC_FL_INSERT_RANGE		0x20
 
+/*
+ * FALLOC_FL_UNSHARE_RANGE is used to unshare shared blocks within the
+ * file size without overwriting any existing data. The purpose of this
+ * call is to preemptively reallocate any blocks that are subject to
+ * copy-on-write.
+ *
+ * Different filesystems may implement different limitations on the
+ * granularity of the operation. Most will limit operations to filesystem
+ * block size boundaries, but this boundary may be larger or smaller
+ * depending on the filesystem and/or the configuration of the filesystem
+ * or file.
+ *
+ * This flag can only be used with allocate-mode fallocate, which is
+ * to say that it cannot be used with the punch, zero, collapse, or
+ * insert range modes.
+ */
+#define FALLOC_FL_UNSHARE_RANGE		0x40
+
 #endif /* _UAPI_FALLOC_H_ */