Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!columbia!rutgers!sri-spam!ames!ptsfa!hoptoad!academ!killer!jfh
From: jfh@killer.UUCP (John Haugh)
Newsgroups: comp.unix.wizards
Subject: Re: Why "u." and not "u->" ?  (I'm reading Bach book)
Message-ID: <885@killer.UUCP>
Date: Thu, 14-May-87 18:32:36 EDT
Article-I.D.: killer.885
Posted: Thu May 14 18:32:36 1987
Date-Received: Sat, 23-May-87 04:23:58 EDT
References: <626@vixie.UUCP> <3568@spool.WISC.EDU>
Organization: The Unix(tm) Connection, Dallas, Texas
Lines: 33
Summary: U page may move in physical memory

In article <3568@spool.WISC.EDU>, lm@cottage.WISC.EDU (Larry McVoy) writes:
> In article <626@vixie.UUCP> paul@vixie.UUCP (Paul Vixie Esq) writes:
> >re-map the page, and is the trade-off worth it?  Just how much is 'u.' used
> >between context switches, anyway?
> 
> I daresay that it used every single time you enter the kernel for a
> system call (note: system call not system entry - drivers shouldn't
> pook about in the u struct 'cause it's probably someone elses).
> 
> Larry McVoy 	        lm@cottage.wisc.edu  or  uwvax!mcvoy

I'm not sure this is the best reason for keeping the U page fixed in
kernel virtual memory - but it seems to be a _very_ good one.  If you
get swapped in the middle of a system call, such as a terminal read,
when you are swapped back in your U page may reside at a different
physical address.  On PDP-11's the U page was fixed at 0140000 in Kernel
D space.  I forget how they convinced the linker to give that address
to _u when unix was linked.  Also, I don't think it is possible to make
_u a pointer to the struct user as you could get swapped after the
pointer value was fetched, but before it was used.

(Like this)
	movl	_u,r0		; get address of U page from pointer
- interupt - swap out and wait for a while - swap in somewheres else -
	movl	u_error(r0),r0	; r0 now has wrong address ...

I don't know if all of the schedulers are capable of this kind of mess,
but it seems that something like this could happen.

- John.		(jfh@killer.UUCP)

Disclaimer -
	No disclaimer.  Whatcha gonna do, sue me?