linux-kernel - Re: [PATCH 2/3] writeback: fix incorrect return value type of bdi_dirty

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Date:	Sun, 19 Aug 2012 12:23:15 +0200
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Fengguang Wu <fengguang.wu@...el.com>
CC:	Namjae Jeon <linkinjeon@...il.com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/3] writeback: fix incorrect return value type of bdi_dirty_limit

Il 19/08/2012 04:17, Fengguang Wu ha scritto:
> On Sat, Aug 18, 2012 at 05:49:21AM -0400, Namjae Jeon wrote:
>> unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
>>
>> Above function return type is unsigned long, but return value is
>> kept in "u64 bdi_dirty".
>> It can return incorrect value by type casting.
>
> I don't see how the cast can return wrong value.
> The u64 is necessary for the intermediate calculations.
>
> Thanks,
> Fengguang
>

We are sure u64 is 8byte long, but unsigned long is 4byte long on 32-bit 
machine, so if bdi_dirty can assume values bigger than 2^32 there is an 
implicit truncate of its value, isn't it?

Marco
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