[PATCH v4] powerpc: Avoid nmi_enter/nmi_exit in real mode interrupt.

[Michael Ellerman]

Hi Mahesh,

Mahesh Salgaonkar <mahesh at linux.ibm.com> writes:
> nmi_enter()/nmi_exit() touches per cpu variables which can lead to kernel
> crash when invoked during real mode interrupt handling (e.g. early HMI/MCE
> interrupt handler) if percpu allocation comes from vmalloc area.
>
> Early HMI/MCE handlers are called through DEFINE_INTERRUPT_HANDLER_NMI()
> wrapper which invokes nmi_enter/nmi_exit calls. We don't see any issue when
> percpu allocation is from the embedded first chunk. However with
> CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK enabled there are chances where percpu
> allocation can come from the vmalloc area.
>
> With kernel command line "percpu_alloc=page" we can force percpu allocation
> to come from vmalloc area and can see kernel crash in machine_check_early:
>
> [    1.215714] NIP [c000000000e49eb4] rcu_nmi_enter+0x24/0x110
> [    1.215717] LR [c0000000000461a0] machine_check_early+0xf0/0x2c0
> [    1.215719] --- interrupt: 200
> [    1.215720] [c000000fffd73180] [0000000000000000] 0x0 (unreliable)
> [    1.215722] [c000000fffd731b0] [0000000000000000] 0x0
> [    1.215724] [c000000fffd73210] [c000000000008364] machine_check_early_common+0x134/0x1f8
>
> Fix this by avoiding use of nmi_enter()/nmi_exit() in real mode if percpu
> first chunk is not embedded.

My system (powernv) doesn't even boot with percpu_alloc=page.

AFAIK the only reason we added support for it was to handle 4K kernels
with HPT. See commit eb553f16973a ("powerpc/64/mm: implement page
mapping percpu first chunk allocator").

So I wonder if we should change the Kconfig to only offer it as an
option in that case, and change the logic in setup_per_cpu_areas() to
only use it as a last resort.

I guess we probably still need this commit though, even if just for 4K HPT.


> Signed-off-by: Mahesh Salgaonkar <mahesh at linux.ibm.com>
> ---
> Changes in v4:
> - Fix coding style issues.
>
> Changes in v3:
> - Address comments from Christophe Leroy to avoid using #ifdefs in the
>   code
> - v2 at https://lore.kernel.org/linuxppc-dev/20240205053647.1763446-1-mahesh@linux.ibm.com/
>
> Changes in v2:
> - Rebase to upstream master
> - Use jump_labels, if CONFIG_JUMP_LABEL is enabled, to avoid redoing the
>   embed first chunk test at each interrupt entry.
> - v1 is at https://lore.kernel.org/linuxppc-dev/164578465828.74956.6065296024817333750.stgit@jupiter/
> ---
>  arch/powerpc/include/asm/interrupt.h | 10 ++++++++++
>  arch/powerpc/include/asm/percpu.h    | 10 ++++++++++
>  arch/powerpc/kernel/setup_64.c       |  3 +++
>  3 files changed, 23 insertions(+)
>
> diff --git a/arch/powerpc/include/asm/interrupt.h b/arch/powerpc/include/asm/interrupt.h
> index a4196ab1d0167..0b96464ff0339 100644
> --- a/arch/powerpc/include/asm/interrupt.h
> +++ b/arch/powerpc/include/asm/interrupt.h
> @@ -336,6 +336,14 @@ static inline void interrupt_nmi_enter_prepare(struct pt_regs *regs, struct inte
>  	if (IS_ENABLED(CONFIG_KASAN))
>  		return;
>  
> +	/*
> +	 * Likewise, do not use it in real mode if percpu first chunk is not
> +	 * embedded. With CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK enabled there
> +	 * are chances where percpu allocation can come from vmalloc area.
> +	 */
> +	if (IS_ENABLED(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK) && !is_embed_first_chunk)

I think this would be clearer if it was inverted, eg:

        if (percpu_first_chunk_is_paged)
               return;

That way you shouldn't need to check CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK here.
Instead it can be part of the ifdef in the header.

> @@ -351,6 +359,8 @@ static inline void interrupt_nmi_exit_prepare(struct pt_regs *regs, struct inter
>  		// no nmi_exit for a pseries hash guest taking a real mode exception
>  	} else if (IS_ENABLED(CONFIG_KASAN)) {
>  		// no nmi_exit for KASAN in real mode
> +	} else if (IS_ENABLED(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK) && !is_embed_first_chunk) {
> +		// no nmi_exit if percpu first chunk is not embedded
>  	} else {
>  		nmi_exit();
>  	}
> diff --git a/arch/powerpc/include/asm/percpu.h b/arch/powerpc/include/asm/percpu.h
> index 8e5b7d0b851c6..e24063eb0b33b 100644
> --- a/arch/powerpc/include/asm/percpu.h
> +++ b/arch/powerpc/include/asm/percpu.h
> @@ -15,6 +15,16 @@
>  #endif /* CONFIG_SMP */
>  #endif /* __powerpc64__ */
>  
> +#ifdef CONFIG_PPC64
> +#include <linux/jump_label.h>
> +DECLARE_STATIC_KEY_FALSE(__percpu_embed_first_chunk);
> +
> +#define is_embed_first_chunk	\
> +		(static_key_enabled(&__percpu_embed_first_chunk.key))
> +#else
> +#define is_embed_first_chunk	true
> +#endif /* CONFIG_PPC64 */
> +

Something like:

#ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK
#include <linux/jump_label.h>
DECLARE_STATIC_KEY_FALSE(__percpu_first_chunk_is_paged);

#define percpu_first_chunk_is_paged	\
		(static_key_enabled(&__percpu_first_chunk_is_paged.key))
#else
#define percpu_first_chunk_is_paged	false
#endif /* CONFIG_PPC64 */

> diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
> index 2f19d5e944852..e04f0ff69d4b6 100644
> --- a/arch/powerpc/kernel/setup_64.c
> +++ b/arch/powerpc/kernel/setup_64.c
> @@ -834,6 +834,7 @@ static __init int pcpu_cpu_to_node(int cpu)
>  
>  unsigned long __per_cpu_offset[NR_CPUS] __read_mostly;
>  EXPORT_SYMBOL(__per_cpu_offset);
> +DEFINE_STATIC_KEY_FALSE(__percpu_embed_first_chunk);
>  
>  void __init setup_per_cpu_areas(void)
>  {
> @@ -869,6 +870,8 @@ void __init setup_per_cpu_areas(void)
>  			pr_warn("PERCPU: %s allocator failed (%d), "
>  				"falling back to page size\n",
>  				pcpu_fc_names[pcpu_chosen_fc], rc);
> +		else
> +			static_key_enable(&__percpu_embed_first_chunk.key);
>  	}
>  
>  	if (rc < 0)
 
Finally, the current patch breaks the microwatt build:

  $ make microwatt_defconfig ; make -s -j (nproc)
  make[1]: Entering directory '/home/michael/linux/.build'
    GEN     Makefile
  #
  # configuration written to .config
  #
  make[1]: Leaving directory '/home/michael/linux/.build'
  ld: arch/powerpc/kernel/traps.o:(.toc+0x0): undefined reference to `__percpu_embed_first_chunk'
  ld: arch/powerpc/kernel/mce.o:(.toc+0x0): undefined reference to `__percpu_embed_first_chunk'
  make[3]: *** [../scripts/Makefile.vmlinux:37: vmlinux] Error 1

I guess because it has CONFIG_JUMP_LABEL=n?

cheers