Mail Archives: cygwin/2011/03/23/09:30:20
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
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