linux-kernel - Re: [PATCH v2 3/3] fs/file.c: remove sanity_check from alloc

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Date: Tue, 25 Jun 2024 14:08:34 +0200
From: Jan Kara <jack@...e.cz>
To: Yu Ma <yu.ma@...el.com>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
	mjguzik@...il.com, edumazet@...gle.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	pan.deng@...el.com, tianyou.li@...el.com, tim.c.chen@...el.com,
	tim.c.chen@...ux.intel.com
Subject: Re: [PATCH v2 3/3] fs/file.c: remove sanity_check from alloc_fd()

On Sat 22-06-24 11:49:04, 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.
  ^^^ existing  ^^^ initialization

Well, since this is a sanity check, it is expected it never hits. Yet
searching the web shows it has hit a few times in the past :). So would
wrapping this with unlikely() give a similar performance gain while keeping
debugability? If unlikely() does not help, I agree we can remove this since
fd_install() actually has the same check:

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

and there we need the cacheline anyway so performance impact is minimal.
Now, this condition in alloc_fd() is nice that it does not take the kernel
down so perhaps we could change the BUG_ON to WARN() dumping similar kind
of info as alloc_fd()?

								Honza

> Combined with patch 1 and 2 in series, pts/blogbench-1.1.0 read improved by
> 32%, write improved by 17% on Intel ICX 160 cores configuration with v6.10-rc4.
> 
> Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
> Signed-off-by: Yu Ma <yu.ma@...el.com>
> ---
>  fs/file.c | 7 -------
>  1 file changed, 7 deletions(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index b4d25f6d4c19..1153b0b7ba3d 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -555,13 +555,6 @@ 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);
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR