X-Recipient: archive-cygwin AT delorie DOT com
X-Spam-Check-By: sourceware.org
Date: Wed, 23 Mar 2011 14:26:22 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Broken autoconf mmap test
Message-ID: <20110323132622.GM31220@calimero.vinschen.de>
Reply-To: cygwin AT cygwin DOT com
Mail-Followup-To: cygwin AT cygwin DOT com
References: <4D8668C6 DOT 8020500 AT cornell DOT edu> <4D890D95 DOT 6090302 AT redhat DOT com> <4D891A55 DOT 6050601 AT cornell DOT edu> <20110323091921 DOT GB10657 AT calimero DOT vinschen DOT de> <4D89D4AA DOT 6070903 AT cornell DOT edu>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <4D89D4AA.6070903@cornell.edu>
User-Agent: Mutt/1.5.21 (2010-09-15)
Mailing-List: contact cygwin-help AT cygwin DOT com; run by ezmlm
Precedence: bulk
List-Id: <cygwin.cygwin.com>
List-Unsubscribe: <mailto:cygwin-unsubscribe-archive-cygwin=delorie DOT com AT cygwin DOT com>
List-Subscribe: <mailto:cygwin-subscribe AT cygwin DOT com>
List-Archive: <http://sourceware.org/ml/cygwin/>
List-Post: <mailto:cygwin AT cygwin DOT com>
List-Help: <mailto:cygwin-help AT cygwin DOT com>, <http://sourceware.org/ml/#faqs>
Sender: cygwin-owner AT cygwin DOT com
Mail-Followup-To: cygwin AT cygwin DOT com
Delivered-To: mailing list cygwin AT cygwin DOT com

On Mar 23 07:08, Ken Brown wrote:
> On 3/23/2011 5:19 AM, Corinna Vinschen wrote:
> >On Mar 22 17:53, Ken Brown wrote:
> >>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.

Btw., I just experimented with another flag which has not been exposed
to the Win32 API, and which is not officially documented.  It's called
AT_EXTENDABLE_FILE and allows to specify a view which is bigger than
the filesize.  I tried this on W7-64, and it works.  Just not exactly
as we need it.  The trailing pages are not commited but only reserved,
so accessing them still results in a SEGV.  And while commiting them
allows to run the autoconf testcase, it also changes the filesize of
the file on disk.  Oh well.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

--
Problem reports:       http://cygwin.com/problems.html
FAQ:                   http://cygwin.com/faq/
Documentation:         http://cygwin.com/docs.html
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple