libfuse: add atomic write support

Add the single flag that we need to turn on atomic write support in
fuse.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
diff --git a/include/fuse_common.h b/include/fuse_common.h
index 8e585cc..1977026 100644
--- a/include/fuse_common.h
+++ b/include/fuse_common.h
@@ -548,6 +548,8 @@
  * FUSE_IOMAP_SUPPORT_FILEIO: basic file I/O functionality through iomap
  */
 #define FUSE_IOMAP_SUPPORT_FILEIO	(1ULL << 0)
+/* untorn writes through iomap */
+#define FUSE_IOMAP_SUPPORT_ATOMIC	(1ULL << 1)
 
 /**
  * Connection information, passed to the ->init() method
@@ -1237,6 +1239,8 @@
 #define FUSE_IFLAG_DAX			(1U << 0)
 /* use iomap for this inode */
 #define FUSE_IFLAG_IOMAP		(1U << 1)
+/* enable untorn writes */
+#define FUSE_IFLAG_ATOMIC		(1U << 2)
 
 /* Which fields are set in fuse_iomap_config_out? */
 #define FUSE_IOMAP_CONFIG_SID		(1 << 0ULL)
diff --git a/include/fuse_kernel.h b/include/fuse_kernel.h
index 1470b59..fcf02c9 100644
--- a/include/fuse_kernel.h
+++ b/include/fuse_kernel.h
@@ -242,6 +242,7 @@
  *  - add FUSE_ATTR_IOMAP to enable iomap for specific inodes
  *  - add FUSE_IOMAP_CONFIG so the fuse server can configure more fs geometry
  *  - add FUSE_NOTIFY_IOMAP_DEV_INVAL to invalidate iomap bdev ranges
+ *  - add FUSE_ATTR_ATOMIC for single-fsblock atomic write support
  */
 
 #ifndef _LINUX_FUSE_H
@@ -584,10 +585,12 @@
  * FUSE_ATTR_SUBMOUNT: Object is a submount root
  * FUSE_ATTR_DAX: Enable DAX for this file in per inode DAX mode
  * FUSE_ATTR_IOMAP: Use iomap for this inode
+ * FUSE_ATTR_ATOMIC: Enable untorn writes
  */
 #define FUSE_ATTR_SUBMOUNT      (1 << 0)
 #define FUSE_ATTR_DAX		(1 << 1)
 #define FUSE_ATTR_IOMAP		(1 << 2)
+#define FUSE_ATTR_ATOMIC	(1 << 3)
 
 /**
  * Open flags
diff --git a/lib/fuse_lowlevel.c b/lib/fuse_lowlevel.c
index f730a7f..ee73de4 100644
--- a/lib/fuse_lowlevel.c
+++ b/lib/fuse_lowlevel.c
@@ -126,6 +126,8 @@
 		attr->flags |= FUSE_ATTR_DAX;
 	if (iflags & FUSE_IFLAG_IOMAP)
 		attr->flags |= FUSE_ATTR_IOMAP;
+	if (iflags & FUSE_IFLAG_ATOMIC)
+		attr->flags |= FUSE_ATTR_ATOMIC;
 }
 
 static void convert_attr(const struct fuse_setattr_in *attr, struct stat *stbuf)