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

[Juergen Gross <jgross@xxxxxxxx>]

On 21.11.22 09:18, Yu Zhao wrote:
> On Mon, Nov 21, 2022 at 12:10 AM Juergen Gross <jgross@xxxxxxxx> wrote:
> > On 19.11.22 09:28, Sander Eikelenboom wrote:
> > > Hi Yu / Juergen,
> Hi Sander / Juergen,
>
> Thanks for the report and the analysis.
>
> > > 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.
> Yes, I think it's related.
>
> > > 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.
> Correct.
>
> > 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.
> Correct. After that commit, we also can clear the accessed bit in
> non-leaf PMD entries (pointing to PTE tables).
>
> > 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?
> There is a runtime kill switch for ARCH_HAS_NONLEAF_PMD_YOUNG, since I
> wasn't able to verify this capability on all x86 varieties. The following
> should do it:
>
>    # cat /sys/kernel/mm/lru_gen/enabled
>    0x0007
>    # echo 3 >/sys/kernel/mm/lru_gen/enabled
>
> Details are in Documentation/admin-guide/mm/multigen_lru.rst.
>
> Alternatively, we could make ARCH_HAS_NONLEAF_PMD_YOUNG a runtime
> check similar to arch_has_hw_pte_young() on arm64.
I like this idea.

The patch should be rather trivial. Let me have a try ...


Juergen

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