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

[Helge Deller <deller@gmx.de>]

On 04/03/2014 10:03 PM, John David Anglin wrote:
> 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.

*No*, it's not!
In current eglibc it's set to 0x00400000
That's what my eglibc-patch changes...
I'm currently building a eglibc on hpviz with SHMLBA set to 4096 (__getpagesize()).

> So, it should work just fine with the kernel patch. 

No, the kernel patch isn't necessary. Only the glibc 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.

True - we need to find the cause.
I just suspected the arch_get_unmapped_area() kernel functions, but they seem correct.
 
> Do you think this helps the allocation of small maps (perl locale test bug)?

No. Only my other (unfinished) patches will resolve this.

Helge