linux-kernel - Re: mmap syscall problem

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Message-ID: <4A51F3F3.6040501@petalogix.com>
Date:	Mon, 06 Jul 2009 14:54:11 +0200
From:	Michal Simek <michal.simek@...alogix.com>
To:	Paul Mundt <lethal@...ux-sh.org>,
	Michal Simek <michal.simek@...alogix.com>,
	Arnd Bergmann <arnd@...db.de>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	LTP <ltp-list@...ts.sourceforge.net>,
	John Williams <john.williams@...alogix.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	subrata@...ux.vnet.ibm.com
Subject: Re: mmap syscall problem

Paul Mundt wrote:
> On Mon, Jul 06, 2009 at 02:07:06PM +0200, Michal Simek wrote:
>   
>> Arnd Bergmann wrote:
>>     
>>> On Monday 06 July 2009, Michal Simek wrote:
>>>   
>>>       
>>>>> Does this happen on microblaze-mmu or microblaze-nommu, or both?
>>>>> The mmap code for the two is very different.
>>>>>   
>>>>>       
>>>>>           
>>>> For MMU code.
>>>>     
>>>>         
>>> Could this be a cache-aliasing problem? If your cache is 'virtually-indexed'
>>> (most architectures are 'physically-indexed'), the kernel may have written
>>> into different parts of the D-cache than what the user space is reading
>>> from. If you have a write-through cache, that can explain why you only
>>> see the stale data at the beginning of the page -- the cache controller
>>> is still busy writing back the data when you start reading it from
>>> DRAM through the cache alias.
>>>   
>>>       
>> I don't think so because we run that test on Microblaze without caches
>> and test failed too.
>> I think that this is sufficient test to tell that the problem is not
>> relate with caches.
>>
>>     
> Not necessarily, even on platforms that manage aliases in hardware
> mappings that violate the aliasing constraints can still result in
> undefined behaviour, this really depends more on your cache controller
> and MMU than anything else. I notice that microblaze sets SHMLBA to
> PAGE_SIZE, you may want to see if this test still breaks after bumping it
> up to something like PAGE_SIZE * 4.
>   
Yes, test still break - behavior is the same. I don't have accurate
information about MMU unit
but I will ask a question about. We are able to turn off cache
controller directly in HW.
> This is unfortunately one of the areas where what POSIX says is possible
> and what hardware can support are at odds (you can look through
> arch/sh/mm/mmap.c for a better idea).
>   

Thanks,
Michal


-- 
Michal Simek, Ing. (M.Eng)
PetaLogix - Linux Solutions for a Reconfigurable World
w: www.petalogix.com p: +61-7-30090663,+42-0-721842854 f: +61-7-30090663

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