linux-kernel - Re: [PATCH] mm/vmalloc: fix align value calculation error

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Date:	Fri, 5 Aug 2016 10:27:25 +0800
From:	zijun_hu <zijun_hu@...o.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	tj@...nel.org, hannes@...xchg.org, mhocko@...nel.org,
	minchan@...nel.org, zijun_hu@....com, rientjes@...gle.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/vmalloc: fix align value calculation error

On 08/05/2016 05:24 AM, Andrew Morton wrote:
>>
>> it causes double align requirement for __get_vm_area_node() if parameter
>> size is power of 2 and VM_IOREMAP is set in parameter flags
>>
>> it is fixed by handling the specail case manually due to lack of
>> get_count_order() for long parameter
>>
>> ...
>>
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -1357,11 +1357,16 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
>>  {
>>  	struct vmap_area *va;
>>  	struct vm_struct *area;
>> +	int ioremap_size_order;
>>  
>>  	BUG_ON(in_interrupt());
>> -	if (flags & VM_IOREMAP)
>> -		align = 1ul << clamp_t(int, fls_long(size),
>> -				       PAGE_SHIFT, IOREMAP_MAX_ORDER);
>> +	if (flags & VM_IOREMAP) {
>> +		ioremap_size_order = fls_long(size);
>> +		if (is_power_of_2(size) && size != 1)
>> +			ioremap_size_order--;
>> +		align = 1ul << clamp_t(int, ioremap_size_order, PAGE_SHIFT,
>> +				IOREMAP_MAX_ORDER);
>> +	}
>>  
>>  	size = PAGE_ALIGN(size);
>>  	if (unlikely(!size))
> 
> I'm having trouble with this, and a more complete description would
> have helped!
> 
> As far as I can tell, the current code will decide the following:
> 
> size=0x10000: alignment=0x10000
> size=0x0f000: alignment=0x8000
>
no, the current code doesn't achieve the above results as shown below
size=0x10000 -> fls_long(0x10000)=17 -> alignment=0x20000
size=0x0f000 -> fls_long(0x0f000)=16 -> alignment=0x10000
it is wrong for power of 2 value such as size=0x10000
 
> And your patch will change it so that
> 
> size=0x10000: alignment=0x8000
> size=0x0f000: alignment=0x8000
> 
> Correct?
>
no, my patch will results in the following calculations
size=0x10000: alignment=0x10000
size=0x0f000: alignment=0x10000

> If so, I'm struggling to see the sense in this.  Shouldn't we be
> changing things so that
> 
> size=0x10000: alignment=0x10000
> size=0x0f000: alignment=0x10000
> 
> ?
okay, it is the aim of my patch as explained above
> 
>