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From: thomson@uthub.UUCP (Brian Thomson)
Newsgroups: net.unix-wizards
Subject: Re: A problem with kernel buffers (struct buf).
Message-ID: <224@uthub.UUCP>
Date: Wed, 28-Aug-85 21:59:17 EDT
Article-I.D.: uthub.224
Posted: Wed Aug 28 21:59:17 1985
Date-Received: Thu, 29-Aug-85 01:08:02 EDT
References: <318@ucsbcsl.UUCP>
Organization: CSRG, University of Toronto
Lines: 20

The file /sys/h/param.h defines the parameter MAXBSIZE, with
the comment

    MAXBSIZE primarily determines the size of buffers in the buffer
    pool.

Your system, like most, probably has MAXBSIZE set to 8192.
Although geteblk(n) doesn't check to see if you've exceeded that
limit, it is an error to do so.

This happens because the system allocates MAXBSIZE bytes of system
virtual space per buffer at startup, then slaps in as many real pages
per buffer as you need when you need them.  By asking for a 10k buffer,
you slop over into the virtual space reserved for the next buffer,
and those extra pages may be freed when that next buffer shrinks.
The symptom would be a protection trap panic accessing an address
more than 8k from the start of the buffer (bp->b_un.b_addr).
-- 
		    Brian Thomson,	    CSRI Univ. of Toronto
		    {linus,ihnp4,uw-beaver,floyd,utzoo}!utcsrgv!uthub!thomson