linux-kernel - Re: mmap syscall problem

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Date:	Mon, 06 Jul 2009 16:43:50 +0200
From:	Michal Simek <michal.simek@...alogix.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Paul Mundt <lethal@...ux-sh.org>,
	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

Arnd Bergmann wrote:
> On Monday 06 July 2009, Michal Simek wrote:
>   
>>> 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.
>>     
>
> There may still be a problem with data being queued in some write
> buffers that don't get flushed before reading back from another
> address.
>
> What happens in a simple user space program that mmaps the same
> page to two addresses? Something like
>
> #include <sys/mman.h>
> #include <fcntl.h>
> int main(void)
> {
> 	int fd = open("existing-4k-file", O_RDWR);
> 	char *p1 = mmap(0, 4096, PROT_READ | PROT_WRITE,
> 				MAP_SHARED, fd, 0);
> 	char *p2 = mmap(p1 + 4096, 4096, PROT_READ | PROT_WRITE,
> 				MAP_SHARED | MAP_FIXED, fd, 0);
>
> 	*p1 = 0xaa; *p2 = 0x55;
>   
I closed fd too.
    close(fd);
> 	return *p1; /* returns 0xaa if broken, 0x55 if correct */
> }
>   
# ls -la existing-4k-file
-rw-rw-r--    1 monstr   monstr       4096 Jul  6  2009 existing-4k-file

# ./test-arnd
# echo $?
85
# dd if=existing-4k-file of=/dev/console count=1 2>/dev/null
U#

in file is first char U (0x55) which is IMO correct.

Michal



> 	Arnd <><
>   


-- 
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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