linux-kernel - Re: [PATCH v6] mm: Uninline copy

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Date:   Mon, 14 Feb 2022 15:24:03 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v6] mm: Uninline copy_overflow()



On 2/14/22 11:56 AM, Christophe Leroy wrote:
> While building a small config with CONFIG_CC_OPTIMISE_FOR_SIZE,
> I ended up with more than 50 times the following function in vmlinux
> because GCC doesn't honor the 'inline' keyword:
> 
> 	c00243bc <copy_overflow>:
> 	c00243bc:	94 21 ff f0 	stwu    r1,-16(r1)
> 	c00243c0:	7c 85 23 78 	mr      r5,r4
> 	c00243c4:	7c 64 1b 78 	mr      r4,r3
> 	c00243c8:	3c 60 c0 62 	lis     r3,-16286
> 	c00243cc:	7c 08 02 a6 	mflr    r0
> 	c00243d0:	38 63 5e e5 	addi    r3,r3,24293
> 	c00243d4:	90 01 00 14 	stw     r0,20(r1)
> 	c00243d8:	4b ff 82 45 	bl      c001c61c <__warn_printk>
> 	c00243dc:	0f e0 00 00 	twui    r0,0
> 	c00243e0:	80 01 00 14 	lwz     r0,20(r1)
> 	c00243e4:	38 21 00 10 	addi    r1,r1,16
> 	c00243e8:	7c 08 03 a6 	mtlr    r0
> 	c00243ec:	4e 80 00 20 	blr
> 
> With -Winline, GCC tells:
> 
> 	/include/linux/thread_info.h:212:20: warning: inlining failed in call to 'copy_overflow': call is unlikely and code size would grow [-Winline]
> 
> copy_overflow() is a non conditional warning called by
> check_copy_size() on an error path.
> 
> check_copy_size() have to remain inlined in order to benefit
> from constant folding, but copy_overflow() is not worth inlining.
> 
> Uninline the warning when CONFIG_BUG is selected.
> 
> When CONFIG_BUG is not selected, WARN() does nothing so skip it.
> 
> This reduces the size of vmlinux by almost 4kbytes.

Just wondering, is this the only such scenario which results in
an avoidable bloated vmlinux image ?

> 
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
> v6: I should have gone sleeping yesterday night instead of sending v5 out. Sorry for the noise. Fix EXPORT_SYMBOL()
> 
> v5: Change to EXPORT_SYMBOL() instead of EXPORT_SYMBOL_GPL()
> 
> v4: Make copy_overflow() a no-op when CONFIG_BUG is not selected
> 
> v3: Added missing ; after EXPORT_SYMBOL()
> 
> v2: Added missing EXPORT_SYMBOL() and enhanced commit message
> ---
>  include/linux/thread_info.h | 5 ++++-
>  mm/maccess.c                | 6 ++++++
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
> index 73a6f34b3847..9f392ec76f2b 100644
> --- a/include/linux/thread_info.h
> +++ b/include/linux/thread_info.h
> @@ -209,9 +209,12 @@ __bad_copy_from(void);
>  extern void __compiletime_error("copy destination size is too small")
>  __bad_copy_to(void);
>  
> +void __copy_overflow(int size, unsigned long count);
> +
>  static inline void copy_overflow(int size, unsigned long count)
>  {
> -	WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
> +	if (IS_ENABLED(CONFIG_BUG))
> +		__copy_overflow(size, count);
>  }
>  
>  static __always_inline __must_check bool
> diff --git a/mm/maccess.c b/mm/maccess.c
> index d3f1a1f0b1c1..3fed2b876539 100644
> --- a/mm/maccess.c
> +++ b/mm/maccess.c
> @@ -335,3 +335,9 @@ long strnlen_user_nofault(const void __user *unsafe_addr, long count)
>  
>  	return ret;
>  }
> +
> +void __copy_overflow(int size, unsigned long count)
> +{
> +	WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
> +}
> +EXPORT_SYMBOL(__copy_overflow);
>