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

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Date:	Thu, 29 Dec 2011 14:44:19 +0100
From:	Michal Simek <monstr@...str.eu>
To:	unlisted-recipients:; (no To-header on input)
CC:	Tejun Heo <tj@...nel.org>,
	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

Michal Simek wrote:
> 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.

Tejun and Andrew: any other comment?

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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