linux-kernel - Re: [PATCH] mm: cma: fix CMA aligned offset calculation

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Message-ID: <xa1twq37ow1n.fsf@mina86.com>
Date:	Tue, 24 Feb 2015 22:10:28 +0100
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Danesh Petigara <dpetigara@...adcom.com>, akpm@...ux-foundation.org
Cc:	m.szyprowski@...sung.com, iamjoonsoo.kim@....com,
	aneesh.kumar@...ux.vnet.ibm.com,
	laurent.pinchart+renesas@...asonboard.com, gregory.0xf0@...il.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Danesh Petigara <dpetigara@...adcom.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH] mm: cma: fix CMA aligned offset calculation

On Tue, Feb 24 2015, Danesh Petigara <dpetigara@...adcom.com> wrote:
> The CMA aligned offset calculation is incorrect for
> non-zero order_per_bit values.
>
> For example, if cma->order_per_bit=1, cma->base_pfn=
> 0x2f800000 and align_order=12, the function returns
> a value of 0x17c00 instead of 0x400.
>
> This patch fixes the CMA aligned offset calculation.
>
> Cc: stable@...r.kernel.org
> Signed-off-by: Danesh Petigara <dpetigara@...adcom.com>
> Reviewed-by: Gregory Fong <gregory.0xf0@...il.com>

Acked-by: Michal Nazarewicz <mina86@...a86.com>

> ---
>  mm/cma.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/mm/cma.c b/mm/cma.c
> index 75016fd..58f37bd 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -70,9 +70,13 @@ static unsigned long cma_bitmap_aligned_offset(struct cma *cma, int align_order)
>  
>  	if (align_order <= cma->order_per_bit)
>  		return 0;
> -	alignment = 1UL << (align_order - cma->order_per_bit);
> -	return ALIGN(cma->base_pfn, alignment) -
> -		(cma->base_pfn >> cma->order_per_bit);
> +
> +	/*
> +	 * Find a PFN aligned to the specified order and return
> +	 * an offset represented in order_per_bits.
> +	 */

It probably makes sense to move this comment outside of the function as
function documentation.

> +	return (ALIGN(cma->base_pfn, (1UL << align_order))
> +		- cma->base_pfn) >> cma->order_per_bit;
>  }
>  
>  static unsigned long cma_bitmap_maxno(struct cma *cma)
> -- 
> 1.9.1
>

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
ooo +--<mpn@...gle.com>--<xmpp:mina86@...ber.org>--ooO--(_)--Ooo--
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