Unverified Commit 9eda581b authored by Mateusz Guzik's avatar Mateusz Guzik Committed by Christian Brauner
Browse files

fs: move fd_install() slowpath into a dedicated routine and provide commentary 



On stock kernel gcc 14 emits avoidable register spillage:
	endbr64
	call   ffffffff81374630 <__fentry__>
	push   %r13
	push   %r12
	push   %rbx
	sub    $0x8,%rsp
	[snip]

Total fast path is 99 bytes.

Moving the slowpath out avoids it and shortens the fast path to 74
bytes.

Signed-off-by: default avatarMateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251110095634.1433061-1-mjguzik@gmail.com


Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 21b561da

fs/file.c

+29 −6
Original line number Diff line number Diff line
@@ -641,6 +641,34 @@ void put_unused_fd(unsigned int fd) 


EXPORT_SYMBOL(put_unused_fd);



/*

 * Install a file pointer in the fd array while it is being resized.

 *

 * We need to make sure our update to the array does not get lost as the resizing

 * thread can be copying the content as we modify it.

 *

 * We have two ways to do it:

 * - go off CPU waiting for resize_in_progress to clear

 * - take the spin lock

 *

 * The latter is trivial to implement and saves us from having to might_sleep()

 * for debugging purposes.

 *

 * This is moved out of line from fd_install() to convince gcc to optimize that

 * routine better.

 */

static void noinline fd_install_slowpath(unsigned int fd, struct file *file)

{

	struct files_struct *files = current->files;

	struct fdtable *fdt;



	spin_lock(&files->file_lock);

	fdt = files_fdtable(files);

	VFS_BUG_ON(rcu_access_pointer(fdt->fd[fd]) != NULL);

	rcu_assign_pointer(fdt->fd[fd], file);

	spin_unlock(&files->file_lock);

}



/**

 * fd_install - install a file pointer in the fd array

 * @fd: file descriptor to install the file in
@@ -658,14 +686,9 @@ void fd_install(unsigned int fd, struct file *file) 
		return;



	rcu_read_lock_sched();



	if (unlikely(files->resize_in_progress)) {

		rcu_read_unlock_sched();

		spin_lock(&files->file_lock);

		fdt = files_fdtable(files);

		VFS_BUG_ON(rcu_access_pointer(fdt->fd[fd]) != NULL);

		rcu_assign_pointer(fdt->fd[fd], file);

		spin_unlock(&files->file_lock);

		fd_install_slowpath(fd, file);

		return;

	}

	/* coupled with smp_wmb() in expand_fdtable() */