mm: tiny-shmem fix lock ordering: mmap_sem vs i_mutex

tiny-shmem calls do_truncate in shmem_file_setup.  do_truncate takes
i_mutex, and shmem_file_setup is called with mmap_sem held.  However
i_mutex nests outside mmap_sem.

Copy the code in shmem.c to avoid this problem.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Nick Piggin <npiggin@suse.de>
Reported-and-tested-by: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/tiny-shmem.c

@@ -65,31 +65,25 @@ struct file *shmem_file_setup(char *name, loff_t size, unsigned long flags)
 	if (!dentry)
 		goto put_memory;
 
-	error = -ENOSPC;
-	inode = ramfs_get_inode(root->d_sb, S_IFREG | S_IRWXUGO, 0);
-	if (!inode)
-		goto put_dentry;
-
-	d_instantiate(dentry, inode);
 	error = -ENFILE;
-	file = alloc_file(shm_mnt, dentry, FMODE_WRITE | FMODE_READ,
-			&ramfs_file_operations);
+	file = get_empty_filp();
 	if (!file)
 		goto put_dentry;
 
-	inode->i_nlink = 0;	/* It is unlinked */
-
-	/* notify everyone as to the change of file size */
-	error = do_truncate(dentry, size, 0, file);
-	if (error < 0)
+	error = -ENOSPC;
+	inode = ramfs_get_inode(root->d_sb, S_IFREG | S_IRWXUGO, 0);
+	if (!inode)
 		goto close_file;
 
+	d_instantiate(dentry, inode);
+	inode->i_size = size;
+	inode->i_nlink = 0;	/* It is unlinked */
+	init_file(file, shm_mnt, dentry, FMODE_WRITE | FMODE_READ,
+			&ramfs_file_operations);
 	return file;
 
 close_file:
 	put_filp(file);
-	return ERR_PTR(error);
-
 put_dentry:
 	dput(dentry);
 put_memory: