linux-kernel - Re: [PATCH 7/7] Remove expensive WARN_ON in pagefault_disabled

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Date:   Wed, 15 Mar 2017 14:49:04 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 7/7] Remove expensive WARN_ON in pagefault_disabled_dec

On Tue, 14 Mar 2017 19:14:31 -0700 Andi Kleen <andi@...stfloor.org> wrote:

> From: Andi Kleen <ak@...ux.intel.com>
> 
> pagefault_disabled_dec is frequently used inline, and it has a WARN_ON
> for underflow that expands to about 6.5k of extra code. The warning
> doesn't seem to be that useful and worth so much code so remove it.
> 
> If it was needed could make it depending on some debug kernel option.
> 
> Saves ~6.5k in my kernel
> 
>    text    data     bss     dec     hex filename
> 9039417 5367568 11116544        25523529        1857549 vmlinux-before-pf
> 9032805 5367568 11116544        25516917        1855b75 vmlinux-pf
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> ---
>  include/linux/uaccess.h | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index f30c187ed785..b691aad918fb 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -12,7 +12,6 @@ static __always_inline void pagefault_disabled_inc(void)
>  static __always_inline void pagefault_disabled_dec(void)
>  {
>  	current->pagefault_disabled--;
> -	WARN_ON(current->pagefault_disabled < 0);
>  }

Fair enough.  We could switch to VM_WARN_ON but apparently even that is now
being enabled in some production systems, which somewhat defeats its
intent...