linux-kernel - Re: [PATCH] fs: predict not reaching the limit in alloc_empty

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Message-ID: <5d6wy7gdbsk5tvusuwx3kkoc3474gjxpwq4hoc2c25jihuvlup@zb5mwj2rhnjd>
Date: Wed, 19 Mar 2025 17:23:08 +0100
From: Jan Kara <jack@...e.cz>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: predict not reaching the limit in alloc_empty_file()

On Wed 19-03-25 13:49:23, Mateusz Guzik wrote:
> Eliminates a jump over a call to capable() in the common case.
> 
> By default the limit is not even set, in which case the check can't even
> fail to begin with. It remains unset at least on Debian and Ubuntu.
> For this cases this can probably become a static branch instead.
> 
> In the meantime tidy it up.
> 
> I note the check separate from the bump makes the entire thing racy.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/file_table.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/file_table.c b/fs/file_table.c
> index 5b4cc9da1344..c04ed94cdc4b 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -221,7 +221,8 @@ struct file *alloc_empty_file(int flags, const struct cred *cred)
>  	/*
>  	 * Privileged users can go above max_files
>  	 */
> -	if (get_nr_files() >= files_stat.max_files && !capable(CAP_SYS_ADMIN)) {
> +	if (unlikely(get_nr_files() >= files_stat.max_files) &&
> +	    !capable(CAP_SYS_ADMIN)) {
>  		/*
>  		 * percpu_counters are inaccurate.  Do an expensive check before
>  		 * we go and fail.
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR