fuse: invalidate inode mapping if mtime changes

We currently invalidate the inode address space mapping
if the file size changes unexpectedly. In the case of a
fuse network filesystem, a portion of a file could be
overwritten remotely without changing the file size.
Compare the old mtime as well to detect this condition
and invalidate the mapping if the file has been updated.

The original logic (to ignore changes in mtime) is
preserved unless the client specifies FUSE_AUTO_INVAL_DATA
on init.

Signed-off-by: Brian Foster <[e-mail]>
Signed-off-by: Miklos Szeredi <[e-mail]>

fs/fuse/inode.c

···
 	struct fuse_conn *fc = get_fuse_conn(inode);
 	struct fuse_inode *fi = get_fuse_inode(inode);
 	loff_t oldsize;
+	struct timespec old_mtime;
 
 	spin_lock(&fc->lock);
 	if (attr_version != 0 && fi->attr_version > attr_version) {
···
 		return;
 	}
 
+	old_mtime = inode->i_mtime;
 	fuse_change_attributes_common(inode, attr, attr_valid);
 
 	oldsize = inode->i_size;
 	i_size_write(inode, attr->size);
 	spin_unlock(&fc->lock);
 
-	if (S_ISREG(inode->i_mode) && oldsize != attr->size) {
-		truncate_pagecache(inode, oldsize, attr->size);
-		invalidate_inode_pages2(inode->i_mapping);
+	if (S_ISREG(inode->i_mode)) {
+		bool inval = false;
+
+		if (oldsize != attr->size) {
+			truncate_pagecache(inode, oldsize, attr->size);
+			inval = true;
+		} else if (fc->auto_inval_data) {
+			struct timespec new_mtime = {
+				.tv_sec = attr->mtime,
+				.tv_nsec = attr->mtimensec,
+			};
+
+			/*
+			 * Auto inval mode also checks and invalidates if mtime
+			 * has changed.
+			 */
+			if (!timespec_equal(&old_mtime, &new_mtime))
+				inval = true;
+		}
+
+		if (inval)
+			invalidate_inode_pages2(inode->i_mapping);
 	}
 }