Re: parisc: fix mmap(MAP_FIXED|MAP_SHARED) to already mmapped address

From: John David Anglin <dave.anglin@bell.net>
To: Helge Deller <deller@gmx.de>, Carlos O'Donell <carlos@systemhalted.org>
Cc: Aaro Koskinen <aaro.koskinen@iki.fi>,
	Mike Frysinger <vapier@gentoo.org>,
	linux-parisc <linux-parisc@vger.kernel.org>
Subject: Re: parisc: fix mmap(MAP_FIXED|MAP_SHARED) to already mmapped address
Date: Thu, 3 Apr 2014 16:03:14 -0400	[thread overview]
Message-ID: <BLU0-SMTP7204937E7AF88D8736354E976C0@phx.gbl> (raw)
In-Reply-To: <533DB961.9010607@gmx.de>

On 4/3/2014 3:41 PM, Helge Deller wrote:
> On 04/02/2014 11:41 PM, John David Anglin wrote:
>> On 4/2/2014 5:09 PM, Helge Deller wrote:
>>> On 04/02/2014 09:09 PM, Carlos O'Donell wrote:
>>>> On Tue, Apr 1, 2014 at 2:49 PM, Helge Deller <deller@gmx.de> wrote:
>>>>> Yes.
>>>>> But it's not a kernel bug. Kernel 3.14 and previous stable releases are OK.
>>>>>
>>>>> I did proposed a glibc change in my previous mail (http://www.spinics.net/lists/linux-parisc/msg05384.html).
>>>>> Debian bug report with patch is here:
>>>>> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=741243
>>>>>
>>>>> And this is what I proposed:
>>>>>
>>>>> A trivial FIX/workaround would be to change libc-mmap.h like this:
>>>>> #ifdef __hppa__
>>>>> #define MAP_FIXED_ALIGNMENT 4096
>>>>> #else
>>>>> #define MAP_FIXED_ALIGNMENT SHMLBA
>>>>> #endif
>>>>>
>>>>> That works because then the new aligned address is then the same as the original
>>>>> (the mmap call returns 4k aligned addresses, so it stays unchanged), but I'm not sure
>>>>> if such a patch would be acceptable.
>>>>> Do you have another idea/proposal?
>>>> The responsibility for fixing this falls to me, but I've been busy.
>>> No problem.
>>>
>>>> If someone wants to propose a patch for glibc please email
>>>> libc-alpha@sourceware.org, TO me, and I'll review and commit the patch
>>>> granted that you show you've done the appropriate testing.
>>>>
>>>> Otherwise I'll get to this at some point in the next couple of weeks :-(
>>> Hi Carlos,
>>>
>>> I'm not really sure if my patch is the best way to go. Technically it's correct
>>> and it's tested since all our debian buildservers currently run with this patch.
>>> But all other options would probably involve more code changes.
>>>
>>> So, I think I'm happy if you can look at it at some point when you find time.
>>> Your input would be very valuable here.
>> I'm wondering if kernel value for SHMLBA shouldn't change to PAGE_SIZE to better
>> reflect that attach addresses are page aligned.  The color alignment for shared maps
>> seems a separate issue which maybe userspace doesn't need to worry about.
> I think this is a very interesting idea and it should be pretty simple!
>
> The attached patch for eglibc should resolve it.
> And the attached patch for kernel isn't necessary, but makes it clear that the colouring is important.
>   
> I did tested the kernel patch - and it seems to work without problems.
>
> I'm not sure if this might introduce userspace compile problems though (although unlikely).
Very nice!  In our current Debian eglibc build, SHMLBA is set to 4096.
So, it should work just fine with the kernel patch.  The buildds have 
been running for some time
and I'm not aware of any mmap issues aside from the pthread_create 
ENOMEM errors.

Do you think this helps the allocation of small maps (perl locale test bug)?

Dave

-- 
John David Anglin    dave.anglin@bell.net