Re: [PATCH] arm/arm64: xen: Fix to convert percpu address to gfn correctly

Masami Hiramatsu
2020-10-06T02:41:05Z

On Mon, 5 Oct 2020 18:13:22 -0700 (PDT)
Stefano Stabellini <sstabellini@kernel.org> wrote:

> On Mon, 5 Oct 2020, Julien Grall wrote:
> > Hi Masami,
> >
> > On 05/10/2020 14:39, Masami Hiramatsu wrote:
> > > Use per_cpu_ptr_to_phys() instead of virt_to_phys() for per-cpu
> > > address conversion.
> > >
> > > In xen_starting_cpu(), per-cpu xen_vcpu_info address is converted
> > > to gfn by virt_to_gfn() macro. However, since the virt_to_gfn(v)
> > > assumes the given virtual address is in contiguous kernel memory
> > > area, it can not convert the per-cpu memory if it is allocated on
> > > vmalloc area (depends on CONFIG_SMP).
> >
> > Are you sure about this? I have a .config with CONFIG_SMP=y where the per-cpu
> > region for CPU0 is allocated outside of vmalloc area.
> >
> > However, I was able to trigger the bug as soon as CONFIG_NUMA_BALANCING was
> > enabled.
>
> I cannot reproduce the issue with defconfig, but I can with Masami's
> kconfig.
>
> If I disable just CONFIG_NUMA_BALANCING from Masami's kconfig, the
> problem still appears.
>
> If I disable CONFIG_NUMA from Masami's kconfig, it works, which is
> strange because CONFIG_NUMA is enabled in defconfig, and defconfig
> works.

Hmm, strange, because when I disabled CONFIG_NUMA_BALANCING, the issue
disappeared.

--- config-5.9.0-rc4+ 2020-10-06 11:36:20.620107129 +0900
+++ config-5.9.0-rc4+.buggy 2020-10-05 21:04:40.369936461 +0900
@@ -131,7 +131,8 @@
CONFIG_ARCH_SUPPORTS_NUMA_BALANCING=y
CONFIG_CC_HAS_INT128=y
CONFIG_ARCH_SUPPORTS_INT128=y
-# CONFIG_NUMA_BALANCING is not set
+CONFIG_NUMA_BALANCING=y
+CONFIG_NUMA_BALANCING_DEFAULT_ENABLED=y
CONFIG_CGROUPS=y
CONFIG_PAGE_COUNTER=y
CONFIG_MEMCG=y

So buggy config just enabled NUMA_BALANCING (and default enabled)

> > [...]
> >
> > > Fixes: 250c9af3d831 ("arm/xen: Add support for 64KB page granularity")
> >
> > FWIW, I think the bug was already present before 250c9af3d831.
>
> Yeah, I bet 250c9af3d831 is not what introduced the issue. Whatever
> caused virt_to_phys to stop working on vmalloc'ed addresses is the cause
> of the problem. It is something that went in 5.9 (5.8 works) but I don't
> know what for sure.

OK.

>
>
> > > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> > > ---
> > > arch/arm/xen/enlighten.c | 2 +-
> > > include/xen/arm/page.h | 3 +++
> > > 2 files changed, 4 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
> > > index e93145d72c26..a6ab3689b2f4 100644
> > > --- a/arch/arm/xen/enlighten.c
> > > +++ b/arch/arm/xen/enlighten.c
> > > @@ -150,7 +150,7 @@ static int xen_starting_cpu(unsigned int cpu)
> > > pr_info("Xen: initializing cpu%d\n", cpu);
> > > vcpup = per_cpu_ptr(xen_vcpu_info, cpu);
> > > - info.mfn = virt_to_gfn(vcpup);
> > > + info.mfn = percpu_to_gfn(vcpup);
> > > info.offset = xen_offset_in_page(vcpup);
> > > err = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_info, xen_vcpu_nr(cpu),
> > > diff --git a/include/xen/arm/page.h b/include/xen/arm/page.h
> > > index 39df751d0dc4..ac1b65470563 100644
> > > --- a/include/xen/arm/page.h
> > > +++ b/include/xen/arm/page.h
> > > @@ -83,6 +83,9 @@ static inline unsigned long bfn_to_pfn(unsigned long bfn)
> > > })
> > > #define gfn_to_virt(m) (__va(gfn_to_pfn(m) <<
> > > XEN_PAGE_SHIFT))
> > > +#define percpu_to_gfn(v) \
> > > + (pfn_to_gfn(per_cpu_ptr_to_phys(v) >> XEN_PAGE_SHIFT))
> > > +
> > > /* Only used in PV code. But ARM guests are always HVM. */
> > > static inline xmaddr_t arbitrary_virt_to_machine(void *vaddr)
> > > {
>
>
> The fix is fine for me. I tested it and it works. We need to remove the
> "Fixes:" line from the commit message. Ideally, replacing it with a
> reference to what is the source of the problem.

OK, as I said, it seems commit 9a9ab3cc00dc ("xen/arm: SMP support") has
introduced the per-cpu code. So note it instead of Fixes tag.

>
> Aside from that:
>
> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

Thank you!