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

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Date:   Mon, 14 Feb 2022 13:21:22 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     David Laight <David.Laight@...LAB.COM>,
        'Anshuman Khandual' <anshuman.khandual@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v6] mm: Uninline copy_overflow()



Le 14/02/2022 à 12:31, David Laight a écrit :
> From: Anshuman Khandual
>> Sent: 14 February 2022 09:54
> ...
>>> 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.
>>
> 
>>> +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);
>>>   }
> 
>> Just wondering, is this the only such scenario which results in
>> an avoidable bloated vmlinux image ?
> 
> The more interesting question is whether the call to __copy_overflow()
> is actually significantly smaller than the one to WARN()?
> And if so why.
> 
unsigned long tst_copy_to_user(void __user *to, unsigned long n)
{
	return copy_to_user(to, &jiffies_64, n);
}

With the patch:

00003c78 <tst_copy_to_user>:
     3c78:	28 04 00 08 	cmplwi  r4,8
     3c7c:	7c 85 23 78 	mr      r5,r4
     3c80:	41 81 00 10 	bgt     3c90 <tst_copy_to_user+0x18>
     3c84:	3c 80 00 00 	lis     r4,0
			3c86: R_PPC_ADDR16_HA	jiffies_64
     3c88:	38 84 00 00 	addi    r4,r4,0
			3c8a: R_PPC_ADDR16_LO	jiffies_64
     3c8c:	48 00 00 00 	b       3c8c <tst_copy_to_user+0x14>
			3c8c: R_PPC_REL24	_copy_to_user

     3c90:	94 21 ff f0 	stwu    r1,-16(r1)
     3c94:	7c 08 02 a6 	mflr    r0
     3c98:	38 60 00 08 	li      r3,8
     3c9c:	90 01 00 14 	stw     r0,20(r1)
     3ca0:	90 81 00 08 	stw     r4,8(r1)
     3ca4:	48 00 00 01 	bl      3ca4 <tst_copy_to_user+0x2c>
			3ca4: R_PPC_REL24	__copy_overflow
     3ca8:	80 a1 00 08 	lwz     r5,8(r1)
     3cac:	80 01 00 14 	lwz     r0,20(r1)
     3cb0:	7c a3 2b 78 	mr      r3,r5
     3cb4:	7c 08 03 a6 	mtlr    r0
     3cb8:	38 21 00 10 	addi    r1,r1,16
     3cbc:	4e 80 00 20 	blr


Without the patch:

00003c88 <tst_copy_to_user>:
     3c88:	28 04 00 08 	cmplwi  r4,8
     3c8c:	7c 85 23 78 	mr      r5,r4
     3c90:	41 81 00 10 	bgt     3ca0 <tst_copy_to_user+0x18>
     3c94:	3c 80 00 00 	lis     r4,0
			3c96: R_PPC_ADDR16_HA	jiffies_64
     3c98:	38 84 00 00 	addi    r4,r4,0
			3c9a: R_PPC_ADDR16_LO	jiffies_64
     3c9c:	48 00 00 00 	b       3c9c <tst_copy_to_user+0x14>
			3c9c: R_PPC_REL24	_copy_to_user

     3ca0:	94 21 ff f0 	stwu    r1,-16(r1)
     3ca4:	3c 60 00 00 	lis     r3,0
			3ca6: R_PPC_ADDR16_HA	.rodata.str1.4+0x30
     3ca8:	90 81 00 08 	stw     r4,8(r1)
     3cac:	7c 08 02 a6 	mflr    r0
     3cb0:	38 63 00 00 	addi    r3,r3,0
			3cb2: R_PPC_ADDR16_LO	.rodata.str1.4+0x30
     3cb4:	38 80 00 08 	li      r4,8
     3cb8:	90 01 00 14 	stw     r0,20(r1)
     3cbc:	48 00 00 01 	bl      3cbc <tst_copy_to_user+0x34>
			3cbc: R_PPC_REL24	__warn_printk
     3cc0:	80 a1 00 08 	lwz     r5,8(r1)
     3cc4:	0f e0 00 00 	twui    r0,0
     3cc8:	80 01 00 14 	lwz     r0,20(r1)
     3ccc:	7c a3 2b 78 	mr      r3,r5
     3cd0:	7c 08 03 a6 	mtlr    r0
     3cd4:	38 21 00 10 	addi    r1,r1,16
     3cd8:	4e 80 00 20 	blr