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Date: Wed, 23 Mar 2011 07:08:26 -0400
From: Ken Brown <kbrown AT cornell DOT edu>
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To: cygwin AT cygwin DOT com
Subject: Re: Broken autoconf mmap test
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On 3/23/2011 5:19 AM, Corinna Vinschen wrote:
> On Mar 22 17:53, Ken Brown wrote:
>> On 3/22/2011 4:59 PM, Eric Blake wrote:
>>> On 03/20/2011 02:51 PM, Ken Brown wrote:
>>>> What's the status of the broken autoconf mmap test, which always fails
>>>> on Cygwin even though Cygwin has a working mmap?
>>>
>>> Autoconf 2.65 and newer do not have the bug.  But packages still exist
>>> where configure was generated with<   2.65, so these packages still have
>>> the bug unless you either rerun newer autoconf, or pre-seed the cache.
>>>
>>>> Was this ever resolved?  I see that cygport is still using the workaround
>>>>
>>>>    export ac_cv_func_mmap_fixed_mapped=yes
>>>
>>> This workaround is still necessary, as long as there are still packages
>>> that don't run autoreconf and where configure was generated with a
>>> too-old autoconf.
>>
>> I'm still seeing failures when building both emacs and texlive, even
>> after autoreconf with the current (2.68) version installed.
>> Attached is the relevant portion of config.log from an emacs build.
>> This is a build of the current emacs trunk, after running
>> 'autoreconf -I m4'.  The same thing happens if I build emacs-23.3.
>> If you want to try to reproduce it, here's the source:
>>
>>    wget http://ftp.gnu.org/pub/gnu/emacs/emacs-23.3.tar.gz
>>
>> In case it's relevant, my system is 64-bit Windows 7, with the most
>> recent cygwin snapshot:
>
> It is relevant, but it has nothing to do with Windows 7.  Actually, the
> autoconf mmap test is testing a scenario which does not work on 64 bit
> Windows systems.  Here's what happens in a nutshell.
>
> The actual pagesize on Windows is 4K.  But there's something called
> "allocation granularity" which is 64K.  If you request virtual memory on
> Windows, the memory address is always aligned to the allocation
> granularity boundary.  Even if you request 4K chunks, they are 64K
> aligned.
>
> If you mmap anonymous memory, this is no big problem.  Cygwin always
> requests memory in 64K chunks since Cygwin's pagesize is set to 64K.
>
> However, if you mmap a file, there's a problem.  The mapping always
> starts on a 64K boundary, but it end on the next 4K boundary beyond EOF.
> So, for a file of size 1 byte, the memory section will be 4K.  The other
> 60K following in the virtual address space are lost.  And they are not
> allocated.  And i you try to access them you get a SEGV.
>
> Fortunately, Cygwin uses the native NT calls in mmap.  The native calls
> support a flag called AT_ROUND_TO_PAGE, which allows to allocate memory
> on a 4K boundary.  So what Cygwin does is to mmap the file, and then to
> request virtual memory rounded to the next 4K page to fill the rest of
> the 64K allocation area.  Now we have the full 64K allocated.
>
> Unfortunately this doesn't work on 64 bit Windows.  The reason is that
> the WOW64 layer does not support the AT_ROUND_TO_PAGE flag.  If you try
> to use it, the OS call returns STTAUS_INVALID_PARAMETER.  So you can't
> allocate the remainder of the 64K allocation slot at all.
>
> Back to the autoconf mmap test.  What it does is to mmap a file of size
> 1 byte.  Then it tests each byte in the mapped page.  And since the
> pagesize in Cygwin is 64K it tests all of 64K.
>
> Given the above description, you can now imagine this works fine on 32
> bit Windows, but it crashes on 64 bit Windows.

Thanks for the explanation, Corinna.  I've reported this to the autoconf 
list.

Ken

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