fuse: negotiate per inode DAX in FUSE_INIT

Among the FUSE_INIT phase, client shall advertise per inode DAX if it's
mounted with "dax=inode". Then server is aware that client is in per
inode DAX mode, and will construct per-inode DAX attribute accordingly.

Server shall also advertise support for per inode DAX. If server doesn't
support it while client is mounted with "dax=inode", client will
silently fallback to "dax=never" since "dax=inode" is advisory only.
Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dax.c

@@ -1349,7 +1349,7 @@ static bool fuse_should_enable_dax(struct inode *inode, unsigned int flags)
 		return true;
 
 	/* dax_mode is FUSE_DAX_INODE* */
-	return flags & FUSE_ATTR_DAX;
+	return fc->inode_dax && (flags & FUSE_ATTR_DAX);
 }
 
 void fuse_dax_inode_init(struct inode *inode, unsigned int flags)

fs/fuse/fuse_i.h

@@ -780,6 +780,9 @@ struct fuse_conn {
 	/* Initialize security xattrs when creating a new inode */
 	unsigned int init_security:1;
 
+	/* Does the filesystem support per inode DAX? */
+	unsigned int inode_dax:1;
+
 	/** The number of requests waiting for completion */
 	atomic_t num_waiting;
 

fs/fuse/inode.c

@@ -1171,10 +1171,13 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 					min_t(unsigned int, fc->max_pages_limit,
 					max_t(unsigned int, arg->max_pages, 1));
 			}
-			if (IS_ENABLED(CONFIG_FUSE_DAX) &&
-			    flags & FUSE_MAP_ALIGNMENT &&
-			    !fuse_dax_check_alignment(fc, arg->map_alignment)) {
-				ok = false;
+			if (IS_ENABLED(CONFIG_FUSE_DAX)) {
+				if (flags & FUSE_MAP_ALIGNMENT &&
+				    !fuse_dax_check_alignment(fc, arg->map_alignment)) {
+					ok = false;
+				}
+				if (flags & FUSE_HAS_INODE_DAX)
+					fc->inode_dax = 1;
 			}
 			if (flags & FUSE_HANDLE_KILLPRIV_V2) {
 				fc->handle_killpriv_v2 = 1;
@@ -1233,6 +1236,8 @@ void fuse_send_init(struct fuse_mount *fm)
 #ifdef CONFIG_FUSE_DAX
 	if (fm->fc->dax)
 		flags |= FUSE_MAP_ALIGNMENT;
+	if (fuse_is_inode_dax_mode(fm->fc->dax_mode))
+		flags |= FUSE_HAS_INODE_DAX;
 #endif
 	if (fm->fc->auto_submounts)
 		flags |= FUSE_SUBMOUNTS;