Date: Sun, 19 Nov 2000 14:56:47 -0800 (PST) From: Richard Hodges <rh@matriplex.com> To: Mike Smith <msmith@FreeBSD.ORG> Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: page fault question Message-ID: <Pine.BSF.4.10.10011191443280.52841-100000@mail.matriplex.com> In-Reply-To: <200011151129.eAFBToF02993@mass.osd.bsdi.com>
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On Wed, 15 Nov 2000, Mike Smith wrote: > > I have been having a great time :-) debugging a device driver, > > and have run into a really fun way to panic. With one type > > of traffic, [something] happens and the kernel drops into > > DDB, just the way I want. [snip panic info] > This is pretty normal; ddb is a little fragile sometimes. You want to go > back and look at the very first trap; it will probably be different and > will be the *real* trap. All the rest are just ddb exploding. Yep. Unfortunately, the original trap led me on a wild goose chase, trying to figure out why system memory was being overwritten by received device data. I really suspected something funny in the DMA... It turns out that the network stack gets really unhappy when you trim an mbuf chain and leave the last mbuf with a negative length :-( > > Now looking back at the panic message, it looks like the stack has > > pushed into the "frame pointer". Is this an actual problem, or > > just some side effect of the page fault? > The frame pointer is a pointer into the stack, so no, it's not a problem. Of course (doh!) I realized that shortly after posting. > Typically stack overruns lead to double faults (because there's no stack > on which to handle the fault) and a spontaneous reboot. This just sounds > like there's something about your first trap that kills DDB (eg. an > invalid instruction pointer, etc.) I did check the SP, and it looks like the kernel stack stays in the "temporary" 8k stack set up in i386/i386/locore.s Does that sound right? > Hope this helps; let us know if the first trap isn't any more > illuminating. You might also try using remote gdb instead of ddb. Thanks. I also had to dig out a couple bugs involving word alignment when doing DMA transfers, and learned NOT to mess with the data inside mbufs with external data ;-) I guess I've left enough offerings at the altar of stupidity, so maybe Loki will leave me alone now. All the best, -Richard ------------------------------------------- Richard Hodges | Matriplex, inc. <title> | 769 Basque Way rh@matriplex.com | Carson City, NV 89706 775-886-6477 | www.matriplex.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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