Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!boulder!sunybcs!ugkamins
From: ugkamins@sunybcs.uucp (John Kaminski)
Newsgroups: comp.os.minix
Subject: Re: Swapping (was Re: Blitter with fork)
Message-ID: <5647@cs.Buffalo.EDU>
Date: 3 May 89 05:59:59 GMT
References: <1164@uvm-gen.UUCP>
Sender: nobody@cs.Buffalo.EDU
Reply-To: ugkamins@sunybcs.UUCP (John Kaminski)
Organization: SUNY/Buffalo Computer Science
Lines: 33

In article <1164@uvm-gen.UUCP> ackerman@kira.uvm.edu (Steve Ackerman) writes:
=>and the memory demands invoke the swapper, elle, will be chosen for
=>swap out.  If, during the time that elle is swapped out, a user hits a
=>key, the TTY driver will attempt to copy the typed character directly
=>to the user address space; therefore, encountering the same problem as
=>before.  Again, a work around scheme has been developed (a kludge if
=>you will).  
=>
=>There are more instances of this type of occurance.  For example, I
=>believe the Printer driver will read directly from the process' data
=>space.  Other problems include a few race conditions that need to be
=>hammered out.  Also, I'm not sure what will happen when a process is
=>blocked on a pipe, but I suspect the same type of problem as the TTY
=>driver. 
=>
=>This brings me to my question - Does anyone have a better solution
=>than to go through the code, find all the places where this type of
=>problem occurs, and provide a work around?  I think that if the
=>swapper was operating in a protected mode environment, a write/read
=>from a swapped out segment would generate an exception that could be
=>caught, but I'm not sure (don't have my 286/386 book with me, so I
=>can't look it up!).  But the problem still remains in Real Mode.  I
=>think the problem would exist on the Atari version as well.  Anyone
=>have a suggestion?

how 'bout tweaking the code for phys_copy, send_rec, etc. instead of worrying
about individual instances?  Does any of the MINIX code *NOT* use these sys-
tem routines?  I think it highly unlikely that it would.  I would think that
swap information is a new part of the process table, and therefore you could
access the swap info when doing a send_rec() for instance.

Hey, I know this might sound obvious, but sometimes the obvious can seem
soooooooooo concealed until a fresh set of eyes looks at the problem.