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

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Date:	Tue, 20 Dec 2011 10:19:18 +0100
From:	Michal Simek <monstr@...str.eu>
To:	Tejun Heo <tj@...nel.org>
CC:	Michal Simek <monstr@...str.eu>,
	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

Hi Tejun,

> 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?

The main problem is with PFN_DOWN/UP macros and it is in __init section.
The result will be definitely u32 type (for 32bit archs) anyway and seems to me
better solution than ignoring the last page.

Is there any internal kernel test code to test all pages - try to allocate/use/test it?
It will be especially good to do so on the last page to see if there is any problem or not.

That two conditions in memblock should be ok.

Thanks,
Michal

-- 
Michal Simek, Ing. (M.Eng)
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel 2.6 Microblaze Linux - http://www.monstr.eu/fdt/
Microblaze U-BOOT custodian
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