[PATCH 55/58] vfs: add a FALLOC_FL_UNSHARE mode to fallocate to unshare a range of blocks

["Darrick J. Wong" <darrick.wong@oracle.com>]

Add a new mode to fallocate that unshares a range of blocks.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 fs/open.c                   |    5 +++++
 include/linux/falloc.h      |    3 ++-
 include/uapi/linux/falloc.h |   14 ++++++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)


diff --git a/fs/open.c b/fs/open.c
index b6f1e96..58e2b61 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -256,6 +256,11 @@ int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
 	    (mode & ~FALLOC_FL_INSERT_RANGE))
 		return -EINVAL;
 
+	/* Unshare range should only be used exclusively. */
+	if ((mode & FALLOC_FL_UNSHARE_RANGE) &&
+	    (mode & ~FALLOC_FL_UNSHARE_RANGE))
+		return -EINVAL;
+
 	if (!(file->f_mode & FMODE_WRITE))
 		return -EBADF;
 
diff --git a/include/linux/falloc.h b/include/linux/falloc.h
index 9961110..7494dc6 100644
--- a/include/linux/falloc.h
+++ b/include/linux/falloc.h
@@ -25,6 +25,7 @@ struct space_resv {
 					 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_ */
diff --git a/include/uapi/linux/falloc.h b/include/uapi/linux/falloc.h
index 3e445a7..a1c293b 100644
--- a/include/uapi/linux/falloc.h
+++ b/include/uapi/linux/falloc.h
@@ -58,4 +58,18 @@
  */
 #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.
+ */
+#define FALLOC_FL_UNSHARE_RANGE		0x40
+
 #endif /* _UAPI_FALLOC_H_ */