Path: utzoo!utgpu!watserv1!watmath!att!att!bu.edu!rpi!zaphod.mps.ohio-state.edu!usc!rutgers!bellcore!decvax.dec.com!crater!paradis
From: paradis@crater.zk3.dec.com (Jim Paradis)
Newsgroups: comp.os.minix
Subject: Help needed with MINIX scheduler
Message-ID: <454@decvax.decvax.dec.com.UUCP>
Date: 28 Nov 90 17:44:04 GMT
Sender: news@decvax.dec.com.UUCP
Reply-To: paradis@decvax.dec.com (Jim Paradis)
Organization: DEC ULTRIX, Nashua, NH
Lines: 63


I'm currently running MINIX 1.5.10 with Bruce Evans' 32-bit patches.
I'm in the process of implementing demand-paged virtual memory, and
I'm running into a problem.  Here's a summary of the situation:

	- For simplicity's sake, I have a single page table
	  for the entire system (much as Bruce does now).
	  This way, MM and friends can operate as they do
	  now, assuming a single system-wide linear memory
	  address space.  Protection et al is handled by
	  the segmentation mechanism.

	- I have a task called PAGER, which handles pageins
	  and pageouts.

	- When MM allocates a range of memory, it asks the
	  kernel to set up a range of PTEs as demand-zero
	  pages.  These aren't backed with physical memory
	  until they are actually referenced, at which time
	  the PAGER allocates a page from the free pool, maps
	  it in, and zeros it.

	- When I take a page fault, the page fault handler
	  finds the faulting virtual (linear) address,
	  fires off a message to the PAGER, and waits for
	  a reply to come back before continuing.

At least that's how it's SUPPOSED to happen.  I've been tearing my hair
out for the past week actually trying to MAKE it happen.  I don't have
paging to/from disk implemented yet, but I am taking page faults 
to resolve demand-zero pages.  Things seem to work just fine UNTIL a
process takes a page fault while in the middle of a system call.
What happens then is that for some reason the pager doesn't get scheduled
and that either the faulting process or the IDLE task gets re-scheduled.

One note on my page fault handler:  I modeled it after the SYSCALL
handler such that it saves the process registers on the stack, switches
DS and SS to the kernel data segment, gets the fault address, and calls 
the C handler routine.

I've tried two different mechanisms for jolting the pager task:  one
using the interrupt() routine, and one using lock_mini_send() to send
a message.  Both work just fine when it's a TASK taking the page fault,
but both fail when it's a user process inside a syscall.

So:  is there anyone out there with any insight into the MINIX
scheduler who might be able to give me some suggestions?  At this
point I'm stuck... I've tried experimenting with this and that, all
to no avail.  I've been glued to my computer during every free moment
I have, determined to track this one down... my wife and cats are
starting not to recognize me anymore 8-)

Oh, one other question:  Is there any reason (other than the
implementation of brk() and a few other things) why DS and SS can't
be in separate segments?  It would be really nice to be able to make
two separate, expandable segments and not have to bother with "chmem"
anymore...

Thanks,


Jim Paradis, working at but not employed by DEC.	(603)881-1221
paradis@decvax.dec.com     			"All I got was a rock!"