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Date: Fri, 5 Jan 2007 12:42:08 -0600
From: Brian Ford <Brian DOT Ford AT FlightSafety DOT com>
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Subject: Re: 1.7.0 CVS mmap failure
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On Fri, 5 Jan 2007, Corinna Vinschen wrote:

> In the failing case this should still work, since 0x7fff7000 + 0x9000
> (36864 dec) == 0x80000000, so the mapping should fit into the usual 2
> Gig address space.  Why Windows fails to do it, I have no idea.  The
> error code 487 means invalid address which might mean "already taken"
> address, but that's not visible in the strace.  To figure that out would
> require to add a bit of VirtualQuery code to mmap and add appropriate
> debug output.

I'm not quite sure exactly what this means, but I stumbled onto it in gdb:

(gdb) info w32 selector
Selector $fs
0x03b: base=0x7fffe000 limit=0x00000fff 32-bit Data (Read/Write, Exp-up)
Priviledge level = 3. Byte granular.

So, it does indeed look taken.

> Actually this shows a problem in the mmap implementation with respect to
> MEM_TOP_DOWN.  I think, what mmap should actually do is to create a
> lightweight MAP_RESERVE anonymous mapping of the whole requested mapping
> size, then close it again and then reopen it with the address it got
> in this first try.  This would probably ensure that the subsequent two
> mapping will work.

I don't know what mmap magic Cygwin uses internally to do this, but can't
you just map the big region, then overmap the disk file without the map,
unmap, remap, remap?

-- 
Brian Ford
Lead Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
the best safety device in any aircraft is a well-trained crew...



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