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From: David Dawes <dawes@rf900.physics.usyd.edu.au>
Message-Id: <199603161007.VAA09725@rf900.physics.usyd.edu.au>
Subject: Re: Try this vm_mmap.c -- please
To: sos@FreeBSD.org
Date: Sat, 16 Mar 1996 21:07:31 +1100 (EST)
Cc: dyson@FreeBSD.org, current@FreeBSD.org, mmead@glock.com
In-Reply-To: <199603160903.KAA00185@DeepCore.dk> from "sos@FreeBSD.org" at Mar 16, 96 10:03:46 am
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>> This vm_mmap.c contains what I believe some fixes to the mmap code.  Note
>> that there were some severe bogons in the code, and one that I cannot
>> easily get rid of.  Firstly we allow mmaps that don't have either MAP_PRIVATE or
>> MAP_SHARED set.  This is wrong because logically one is necessary.  The
>> solution is that I default to MAP_PRIVATE for vnodes and MAP_SHARED for
>> devices (video memory.)  Also, the old code did not allow for shared
>> anonymous mappings,  this is very bogus.  Eventually, we should enforce
>> a MAP_PRIVATE or MAP_SHARED bit being set, but there is code out there
>> like XFree that doesn't specify either bit, and since it was originally
>> not "wrong", it is a PROBLEM :-(.  PLEASE PLEASE give me some feedback on
>> this, as my entire life for this weekend is dedicated to fixing this
>> problem (not really, but you know what I mean.) :-).
>
>Sorry, but it still breaks my accelx server :(
>
>I guess you are in for a looonngg weekend :) 

Does accelx by any chance use MAP_PRIVATE?

The 1.38 version has:

        if ((flags & (MAP_ANON|MAP_SHARED)) == 0 && (type != OBJT_DEVICE)) { 
                docow = MAP_COPY_ON_WRITE;

whereas the new version has:

        if ((type == OBJT_DEVICE) &&
                (flags & (MAP_PRIVATE|MAP_SHARED)) == 0)
                flags |= MAP_SHARED;

        if (flags & MAP_PRIVATE) {
                docow = MAP_COPY_ON_WRITE;


With the 1.38 version MAP_PRIVATE won't cause COW for a device, but it
will cause COW for a device with the latest version.

David