cache the value of file_inode() in struct file

Note that this thing does *not* contribute to inode refcount;
it's pinned down by dentry.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/file_table.c

@@ -176,6 +176,7 @@ struct file *alloc_file(struct path *path, fmode_t mode,
 		return file;
 
 	file->f_path = *path;
+	file->f_inode = path->dentry->d_inode;
 	file->f_mapping = path->dentry->d_inode->i_mapping;
 	file->f_mode = mode;
 	file->f_op = fop;
@@ -258,6 +259,7 @@ static void __fput(struct file *file)
 		drop_file_write_access(file);
 	file->f_path.dentry = NULL;
 	file->f_path.mnt = NULL;
+	file->f_inode = NULL;
 	file_free(file);
 	dput(dentry);
 	mntput(mnt);

fs/open.c

@@ -689,7 +689,7 @@ static int do_dentry_open(struct file *f,
 		f->f_mode = FMODE_PATH;
 
 	path_get(&f->f_path);
-	inode = file_inode(f);
+	inode = f->f_inode = f->f_path.dentry->d_inode;
 	if (f->f_mode & FMODE_WRITE) {
 		error = __get_file_write_access(inode, f->f_path.mnt);
 		if (error)
@@ -752,6 +752,7 @@ static int do_dentry_open(struct file *f,
 	path_put(&f->f_path);
 	f->f_path.mnt = NULL;
 	f->f_path.dentry = NULL;
+	f->f_inode = NULL;
 	return error;
 }
 

include/linux/fs.h

@@ -769,6 +769,7 @@ struct file {
 	} f_u;
 	struct path		f_path;
 #define f_dentry	f_path.dentry
+	struct inode		*f_inode;	/* cached value */
 	const struct file_operations	*f_op;
 
 	/*
@@ -2217,7 +2218,7 @@ static inline bool execute_ok(struct inode *inode)
 
 static inline struct inode *file_inode(struct file *f)
 {
-	return f->f_path.dentry->d_inode;
+	return f->f_inode;
 }
 
 /*