Re: [PATCH] powerpc/smp: Use GFP_ATOMIC while allocating tmp mask

[Michael Ellerman]

Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx> writes:

> Qian Cai reported a regression where CPU Hotplug fails with the latest
> powerpc/next
>
> BUG: sleeping function called from invalid context at mm/slab.h:494
> in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 0, name: swapper/88
> no locks held by swapper/88/0.
> irq event stamp: 18074448
> hardirqs last  enabled at (18074447): [<c0000000001a2a7c>] tick_nohz_idle_enter+0x9c/0x110
> hardirqs last disabled at (18074448): [<c000000000106798>] do_idle+0x138/0x3b0
> do_idle at kernel/sched/idle.c:253 (discriminator 1)
> softirqs last  enabled at (18074440): [<c0000000000bbec4>] irq_enter_rcu+0x94/0xa0
> softirqs last disabled at (18074439): [<c0000000000bbea0>] irq_enter_rcu+0x70/0xa0
> CPU: 88 PID: 0 Comm: swapper/88 Tainted: G        W         5.9.0-rc8-next-20201007 #1
> Call Trace:
> [c00020000a4bfcf0] [c000000000649e98] dump_stack+0xec/0x144 (unreliable)
> [c00020000a4bfd30] [c0000000000f6c34] ___might_sleep+0x2f4/0x310
> [c00020000a4bfdb0] [c000000000354f94] slab_pre_alloc_hook.constprop.82+0x124/0x190
> [c00020000a4bfe00] [c00000000035e9e8] __kmalloc_node+0x88/0x3a0
> slab_alloc_node at mm/slub.c:2817
> (inlined by) __kmalloc_node at mm/slub.c:4013
> [c00020000a4bfe80] [c0000000006494d8] alloc_cpumask_var_node+0x38/0x80
> kmalloc_node at include/linux/slab.h:577
> (inlined by) alloc_cpumask_var_node at lib/cpumask.c:116
> [c00020000a4bfef0] [c00000000003eedc] start_secondary+0x27c/0x800
> update_mask_by_l2 at arch/powerpc/kernel/smp.c:1267
> (inlined by) add_cpu_to_masks at arch/powerpc/kernel/smp.c:1387
> (inlined by) start_secondary at arch/powerpc/kernel/smp.c:1420
> [c00020000a4bff90] [c00000000000c468] start_secondary_resume+0x10/0x14
>
> Allocating a temporary mask while performing a CPU Hotplug operation
> with CONFIG_CPUMASK_OFFSTACK enabled, leads to calling a sleepable
> function from a atomic context. Fix this by allocating the temporary
> mask with GFP_ATOMIC flag.
>
> If there is a failure to allocate a mask, scheduler is going to observe
> that this CPU's topology is broken. Instead of having to speculate why
> the topology is broken, add a WARN_ON_ONCE.
>
> Fixes: 70a94089d7f7 ("powerpc/smp: Optimize update_coregroup_mask")
> Fixes: 3ab33d6dc3e9 ("powerpc/smp: Optimize update_mask_by_l2")
> Reported-by: Qian Cai <cai@xxxxxxxxxx>
> Suggested-by: Qian Cai <cai@xxxxxxxxxx>
> Signed-off-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> Cc: linuxppc-dev <linuxppc-dev@xxxxxxxxxxxxxxxx>
> Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Cc: Nathan Lynch <nathanl@xxxxxxxxxxxxx>
> Cc: Gautham R Shenoy <ego@xxxxxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
> Cc: Qian Cai <cai@xxxxxxxxxx>
> ---
>  arch/powerpc/kernel/smp.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 0dc1b85..1268558 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1264,7 +1264,8 @@ static bool update_mask_by_l2(int cpu)
>  		return false;
>  	}
>  
> -	alloc_cpumask_var_node(&mask, GFP_KERNEL, cpu_to_node(cpu));
> +	/* In CPU-hotplug path, hence use GFP_ATOMIC */
> +	WARN_ON_ONCE(!alloc_cpumask_var_node(&mask, GFP_ATOMIC, cpu_to_node(cpu)));

A failed memory allocation is not something that should trigger a WARN,
a pr_warn() maybe.

But ...

>  	cpumask_and(mask, cpu_online_mask, cpu_cpu_mask(cpu));

If the allocation failed this will oops (mask will be NULL).

cheers