linux-kernel - Re: memblock and bootmem problems if start + size = 4GB

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Message-ID: <20111219162835.GA24519@google.com>
Date:	Mon, 19 Dec 2011 08:28:35 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Michal Simek <monstr@...str.eu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Yinghai Lu <yinghai@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Sam Ravnborg <sam@...nborg.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: memblock and bootmem problems if start + size = 4GB

Hello, Michal.

On Mon, Dec 19, 2011 at 02:58:13PM +0100, Michal Simek wrote:
> I have reached some problems with memblock and bootmem code for some configurations.
> We can completely setup the whole system and all addresses in it.
> The problem happens if we place main memory to the end of address space when
> mem_start + size reach 4GB limit.
> 
> For example:
> mem_start      0xF000 0000
> mem_size       0x1000 0000 (or better lowmem size)
> mem_end        0xFFFF FFFF
> start + size 0x1 0000 0000 (u32 limit reached).
> 
> I have done some patches which completely remove start + size values from architecture specific
> code but I have found some problem in generic code too.
> 
> For example in bootmem code where are three places where physaddr + size is used.
> I would prefer to retype it to u64 because baseaddr and size don't need to be 2^n.
> 
> Is it correct solution? If yes, I will create proper patch.

Yeah, that's an inherent problem in using [) ranges but I think
chopping off the last page probably is simpler and more robust
solution.  Currently, memblock_add_region() would simply ignore if
address range overflows but making it just ignore the last page is
several lines of addition.  Wouldn't that be effective enough while
staying very simple?

Thanks.

-- 
tejun
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