Re: [PATCH] dynamically allocate struct cpumask

[Vitaly Kuznetsov]

Nick Desaulniers <nick.desaulniers@xxxxxxxxx> writes:

> This helps avoid avoid a potentially large stack allocation.
>
> When building with:
> $ make CC=clang arch/x86/ CFLAGS=-Wframe-larger-than=1000
> The following warning is observed:
> arch/x86/kernel/kvm.c:494:13: warning: stack frame size of 1064 bytes in
> function 'kvm_send_ipi_mask_allbutself' [-Wframe-larger-than=]
> static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int
> vector)
>             ^
> Debugging with:
> https://github.com/ClangBuiltLinux/frame-larger-than
> via:
> $ python3 frame_larger_than.py arch/x86/kernel/kvm.o \
>   kvm_send_ipi_mask_allbutself
> points to the stack allocated `struct cpumask newmask` in
> `kvm_send_ipi_mask_allbutself`. The size of a `struct cpumask` is
> potentially large, as it's CONFIG_NR_CPUS divided by BITS_PER_LONG for
> the target architecture. CONFIG_NR_CPUS for X86_64 can be as high as
> 8192, making a single instance of a `struct cpumask` 1024 B.
>
> Signed-off-by: Nick Desaulniers <nick.desaulniers@xxxxxxxxx>
> ---
>  arch/x86/kernel/kvm.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 32ef1ee733b7..d41c0a0d62a2 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -494,13 +494,15 @@ static void kvm_send_ipi_mask(const struct cpumask *mask, int vector)
>  static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int vector)
>  {
>  	unsigned int this_cpu = smp_processor_id();
> -	struct cpumask new_mask;
> +	struct cpumask *new_mask;
>  	const struct cpumask *local_mask;
>  
> -	cpumask_copy(&new_mask, mask);
> -	cpumask_clear_cpu(this_cpu, &new_mask);
> -	local_mask = &new_mask;
> +	new_mask = kmalloc(sizeof(*new_mask), GFP_KERNEL);

You could've used alloc_cpumask_var() instead, however, I think that
memory allocation on this path is undesireable. We can always
pre-allocate 1 cpumask variable per cpu and use it every time, e.g. we
do this for Hyper-V.

> +	cpumask_copy(new_mask, mask);
> +	cpumask_clear_cpu(this_cpu, new_mask);
> +	local_mask = new_mask;
>  	__send_ipi_mask(local_mask, vector);
> +	kfree(new_mask);
>  }
>  
>  /*

-- 
Vitaly