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

[Masami Hiramatsu <mhiramat@kernel.org>]

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!

-- 
Masami Hiramatsu <mhiramat@kernel.org>