linux-kernel - Re: Xen-unstable Linux-6.1.0-rc5 BUG: unable to handle page fault for address: ffff8880083374d0

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Date:   Mon, 21 Nov 2022 08:10:10 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Sander Eikelenboom <linux@...elenboom.it>,
        Yu Zhao <yuzhao@...gle.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Xen-devel <xen-devel@...ts.xen.org>
Subject: Re: Xen-unstable Linux-6.1.0-rc5 BUG: unable to handle page fault for
 address: ffff8880083374d0

On 19.11.22 09:28, Sander Eikelenboom wrote:
> Hi Yu / Juergen,
> 
> This night I got a dom0 kernel crash on my new Ryzen box running Xen-unstable 
> and a Linux-6.1.0-rc5 kernel.
> I did enable the new and shiny MGLRU, could this be related ?

It might be related, but I think it could happen independently from it.

> Nov 19 06:30:11 serveerstertje kernel: [68959.647371] BUG: unable to handle page 
> fault for address: ffff8880083374d0
> Nov 19 06:30:11 serveerstertje kernel: [68959.663555] #PF: supervisor write 
> access in kernel mode
> Nov 19 06:30:11 serveerstertje kernel: [68959.677542] #PF: error_code(0x0003) - 
> permissions violation
> Nov 19 06:30:11 serveerstertje kernel: [68959.691181] PGD 3026067 P4D 3026067 
> PUD 3027067 PMD 7fee5067 PTE 8010000008337065
> Nov 19 06:30:11 serveerstertje kernel: [68959.705084] Oops: 0003 [#1] PREEMPT 
> SMP NOPTI
> Nov 19 06:30:11 serveerstertje kernel: [68959.718710] CPU: 7 PID: 158 Comm: 
> kswapd0 Not tainted 6.1.0-rc5-20221118-doflr-mac80211debug+ #1
> Nov 19 06:30:11 serveerstertje kernel: [68959.732457] Hardware name: To Be 
> Filled By O.E.M. To Be Filled By O.E.M./B450 Pro4 R2.0, BIOS P5.60 10/20/2022
> Nov 19 06:30:11 serveerstertje kernel: [68959.746391] RIP: 
> e030:pmdp_test_and_clear_young+0x25/0x40

The kernel tired to reset the "accessed" bit in the pmd entry.

It does so only since commit eed9a328aa1ae. Before that
pmdp_test_and_clear_young() could be called only for huge pages, which are
disabled in Xen PV guests.

pmdp_test_and_clear_young() does a test_and_clear_bit() of the pmd entry, which
is failing since the hypervisor is emulating pte entry modifications only (pmd
and pud entries can be set via hypercalls only).

Could you please test the attached patch whether it fixes the issue for you?


Juergen


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