Re: [PATCH v3 1/3] fs/file.c: remove sanity_check and add likely/unlikely in alloc_fd()

[Christian Brauner]

On Wed, Jul 03, 2024 at 10:33:09AM GMT, Yu Ma wrote:
> alloc_fd() has a sanity check inside to make sure the struct file mapping to the
> allocated fd is NULL. Remove this sanity check since it can be assured by
> exisitng zero initilization and NULL set when recycling fd. Meanwhile, add
> likely/unlikely and expand_file() call avoidance to reduce the work under
> file_lock.
> 
> Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
> ---
>  fs/file.c | 38 ++++++++++++++++----------------------
>  1 file changed, 16 insertions(+), 22 deletions(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index a3b72aa64f11..5178b246e54b 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -515,28 +515,29 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
>  	if (fd < files->next_fd)
>  		fd = files->next_fd;
>  
> -	if (fd < fdt->max_fds)
> +	if (likely(fd < fdt->max_fds))
>  		fd = find_next_fd(fdt, fd);
>  
> +	error = -EMFILE;
> +	if (unlikely(fd >= fdt->max_fds)) {
> +		error = expand_files(files, fd);
> +		if (error < 0)
> +			goto out;
> +		/*
> +		 * If we needed to expand the fs array we
> +		 * might have blocked - try again.
> +		 */
> +		if (error)
> +			goto repeat;
> +	}

So this ends up removing the expand_files() above the fd >= end check
which means that you can end up expanding the files_struct even though
the request fd is past the provided end. That seems odd. What's the
reason for that reordering?

> +
>  	/*
>  	 * N.B. For clone tasks sharing a files structure, this test
>  	 * will limit the total number of files that can be opened.
>  	 */
> -	error = -EMFILE;
> -	if (fd >= end)
> -		goto out;
> -
> -	error = expand_files(files, fd);
> -	if (error < 0)
> +	if (unlikely(fd >= end))
>  		goto out;
>  
> -	/*
> -	 * If we needed to expand the fs array we
> -	 * might have blocked - try again.
> -	 */
> -	if (error)
> -		goto repeat;
> -
>  	if (start <= files->next_fd)
>  		files->next_fd = fd + 1;
>  
> @@ -546,13 +547,6 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
>  	else
>  		__clear_close_on_exec(fd, fdt);
>  	error = fd;
> -#if 1
> -	/* Sanity check */
> -	if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
> -		printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
> -		rcu_assign_pointer(fdt->fd[fd], NULL);
> -	}
> -#endif
>  
>  out:
>  	spin_unlock(&files->file_lock);
> @@ -618,7 +612,7 @@ void fd_install(unsigned int fd, struct file *file)
>  		rcu_read_unlock_sched();
>  		spin_lock(&files->file_lock);
>  		fdt = files_fdtable(files);
> -		BUG_ON(fdt->fd[fd] != NULL);
> +		WARN_ON(fdt->fd[fd] != NULL);
>  		rcu_assign_pointer(fdt->fd[fd], file);
>  		spin_unlock(&files->file_lock);
>  		return;
> -- 
> 2.43.0
>