fuse: fuse: add time_gran to INIT_OUT

Allow userspace fs to specify time granularity.

This is needed because with writeback_cache mode the kernel is responsible
for generating mtime and ctime, but if the underlying filesystem doesn't
support nanosecond granularity then the cache will contain a different
value from the one stored on the filesystem resulting in a change of times
after a cache flush.

Make the default granularity 1s.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/inode.c

@@ -891,6 +891,11 @@ static void process_init_reply(struct fuse_conn *fc, struct fuse_req *req)
 				fc->async_dio = 1;
 			if (arg->flags & FUSE_WRITEBACK_CACHE)
 				fc->writeback_cache = 1;
+			if (arg->time_gran && arg->time_gran <= 1000000000)
+				fc->sb->s_time_gran = arg->time_gran;
+			else
+				fc->sb->s_time_gran = 1000000000;
+
 		} else {
 			ra_pages = fc->max_read / PAGE_CACHE_SIZE;
 			fc->no_lock = 1;

include/uapi/linux/fuse.h

@@ -96,6 +96,8 @@
  *
  * 7.23
  *  - add FUSE_WRITEBACK_CACHE
+ *  - add time_gran to fuse_init_out
+ *  - add reserved space to fuse_init_out
  */
 
 #ifndef _LINUX_FUSE_H
@@ -559,6 +561,9 @@ struct fuse_init_in {
 	uint32_t	flags;
 };
 
+#define FUSE_COMPAT_INIT_OUT_SIZE 8
+#define FUSE_COMPAT_22_INIT_OUT_SIZE 24
+
 struct fuse_init_out {
 	uint32_t	major;
 	uint32_t	minor;
@@ -567,6 +572,8 @@ struct fuse_init_out {
 	uint16_t	max_background;
 	uint16_t	congestion_threshold;
 	uint32_t	max_write;
+	uint32_t	time_gran;
+	uint32_t	unused[9];
 };
 
 #define CUSE_INIT_INFO_MAX 4096