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

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Message-Id: <20160804142421.576426492d629f0839298f9a@linux-foundation.org>
Date:	Thu, 4 Aug 2016 14:24:21 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	zijun_hu <zijun_hu@...o.com>
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

> 
> 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

And your patch will change it so that

size=0x10000: alignment=0x8000
size=0x0f000: alignment=0x8000

Correct?

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

?