linux-kernel - Re: md-raid 6.11.8 page fault oops

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Message-ID: <Zz23Z3RK/AHSXY1I@li-bb2b2a4c-3307-11b2-a85c-8fa5c3a69313.ibm.com>
Date: Wed, 20 Nov 2024 15:48:15 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: Genes Lists <lists@...ience.com>
Cc: song@...nel.org, yukuai3@...wei.com, linux-raid@...r.kernel.org,
        linux-kernel@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
        linux@...mhuis.info
Subject: Re: md-raid  6.11.8 page fault oops

On Tue, Nov 19, 2024 at 08:31:02AM -0500, Genes Lists wrote:
> On Tue, 2024-11-19 at 17:04 +0530, Ojaswin Mujoo wrote:
> > > 
> ...
> 
> > >  (gdb) list *(rb_first+0x13)
> > >   0xffffffff81de1af3 is in rb_first (lib/rbtree.c:473).
> > >   468       struct rb_node  *n;
> > >   469
> > >   470       n = root->rb_node;
> > >   471       if (!n)
> > >   472           return NULL;
> > >   473       while (n->rb_left)
> > 
> > Now this looks strange, we already make sure n is not NULL and then
> > somehow this line ends up in
> > 
> >  BUG: unable to handle page fault for address: 0000000000200010
> > 
> > Now, decoding the code with an x86 vmlinux, I see the fauling opcode
> > faulting:
> > 
> > Code starting with the faulting instruction
> > ===========================================
> >    0:   0f 1f 80 00 00 00 00    nopl   0x0(%rax)
> >    7:   90                      nop
> >    8:   90                      nop
> >    9:   90                      nop
> >    a:   90                      nop
> >    b:   90                      nop
> >    c:   90                      nop
> >    d:   90                      nop
> >    e:   90                      nop
> >    f:   90                      nop
> > 
> > Now RAX is 0x200000 but I don't think the nopl instruction should
> > have resulted
> > in a mem access AFA my limited understanding of x86 ISA goes.
> > 
> > I also don't see nopl in my vmlinux in rb_first, my binary being
> > compiled with
> > gcc 8.5. Are you by chance using clang or higher version or higher
> > optimization in gcc.
> > 
> > Regards,
> > ojaswin
> 
> I am using Arch toolchain with 
> 
>    gcc 14.2.1+r134+gab884fffe3fc-1
> 
> I do not set CFLAGS_KERNEL  so compile options are the default.

Got it, I'm still not sure what might be causing this oops. Would you
happen to a have a reproducer that I can play around with on my system?

Regards,
ojaswin
> 
> thanks
> 
> gene
> 
>