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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: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251110095634.1433061-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Mateusz Guzik 2025-11-10 10:56:34 +01:00 committed by Christian Brauner
parent 21b561dab1
commit 9eda581bfe
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GPG Key ID: 91C61BC06578DCA2
1 changed files with 29 additions and 6 deletions
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35
fs/file.c
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@ -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() */