netdev - Re: [PATCH] be2net: fix adapter->big_page

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Date:   Fri, 12 Jul 2019 20:27:09 -0400
From:   Qian Cai <cai@....pw>
To:     David Miller <davem@...emloft.net>
Cc:     sathya.perla@...adcom.com, ajit.khaparde@...adcom.com,
        sriharsha.basavapatna@...adcom.com, somnath.kotur@...adcom.com,
        arnd@...db.de, dhowells@...hat.com, hpa@...or.com,
        netdev@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] be2net: fix adapter->big_page_size miscaculation



> On Jul 12, 2019, at 6:46 PM, David Miller <davem@...emloft.net> wrote:
> 
> From: Qian Cai <cai@....pw>
> Date: Fri, 12 Jul 2019 15:23:21 -0400
> 
>> The commit d66acc39c7ce ("bitops: Optimise get_order()") introduced a
>> problem for the be2net driver as "rx_frag_size" could be a module
>> parameter that can be changed while loading the module.
> 
> Why is this a problem?

Well, for example, if rx_frag_size was set to 8096 when loading the module, the kernel has already used the default value 2048 during compilation time.

> 
>> That commit checks __builtin_constant_p() first in get_order() which
>> cause "adapter->big_page_size" to be assigned a value based on the
>> the default "rx_frag_size" value at the compilation time. It also
>> generate a compilation warning,
> 
> rx_frag_size is not a constant, therefore the __builtin_constant_p()
> test should not pass.
> 
> This explanation doesn't seem valid.

Actually, GCC would consider it a const with -O2 optimized level because it found that it was never modified and it does not understand it is a module parameter. Considering the following code.

# cat const.c 
#include <stdio.h>

static int a = 1;

int main(void)
{
	if (__builtin_constant_p(a))
		printf("a is a const.\n");

	return 0;
}

# gcc -O2 const.c -o const

# ./const 
a is a const.