linux-kernel - Re: some questions about block and scatterlist

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Message-ID: <CABi1daFSV6ByHQ59zwbPjEeg5Mood9XUw=N4LeSD+bFGU4+_4Q@mail.gmail.com>
Date:	Mon, 7 Nov 2011 09:08:03 -0800
From:	Dave Hylands <dhylands@...il.com>
To:	loody <miloody@...il.com>
Cc:	Jonathan Corbet <corbet@....net>,
	LKML <linux-kernel@...r.kernel.org>,
	kernelnewbies@...nelnewbies.org
Subject: Re: some questions about block and scatterlist

Hi loody,

On Mon, Nov 7, 2011 at 5:16 AM, loody <miloody@...il.com> wrote:
...snip...
>>> 1. Are all struct page 32-bit aligned?
>>
>> Certainly, I would expect any structures found in the system memory map to
>> be so aligned, yes.  If you've created an arbitrary one elsewhere I'm only
>> 99% sure it would be.
>>
>>> If so, where is this part of code for handleing struct page be 32-bits aligned?
>>
>> Note that the size of struct page is carefully considered, and that they
>> are packed as densely as possible, both externally and internally.  No
>> lesser alignment would make sense.
> I found kernel stealing LSB of page_link to put additional
> information, such as sg_is_chain and sg_is_last.
>
> That means the address of all pages have to be at least 4byte alignment.
> But I didn't see any additional attribute added at the end of struct
> or any precaution to take care this.

This is done automatically by the compiler.
http://en.wikipedia.org/wiki/Data_structure_alignment

-- 
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com
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