Re: [PATCH v4 08/14] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0

[Quentin Perret <qperret@google.com>]

On Tuesday 24 Nov 2020 at 15:50:33 (+0000), Will Deacon wrote:
> When exec'ing a 32-bit task on a system with mismatched support for
> 32-bit EL0, try to ensure that it starts life on a CPU that can actually
> run it.
> 
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/kernel/process.c | 42 ++++++++++++++++++++++++++++++++++++-
>  1 file changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 1540ab0fbf23..72116b0c7c73 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -31,6 +31,7 @@
>  #include <linux/interrupt.h>
>  #include <linux/init.h>
>  #include <linux/cpu.h>
> +#include <linux/cpuset.h>
>  #include <linux/elfcore.h>
>  #include <linux/pm.h>
>  #include <linux/tick.h>
> @@ -625,6 +626,45 @@ unsigned long arch_align_stack(unsigned long sp)
>  	return sp & ~0xf;
>  }
>  
> +static void adjust_compat_task_affinity(struct task_struct *p)
> +{
> +	cpumask_var_t cpuset_mask;
> +	const struct cpumask *possible_mask = system_32bit_el0_cpumask();
> +	const struct cpumask *newmask = possible_mask;
> +
> +	/*
> +	 * Restrict the CPU affinity mask for a 32-bit task so that it contains
> +	 * only the 32-bit-capable subset of its original CPU mask. If this is
> +	 * empty, then try again with the cpuset allowed mask. If that fails,
> +	 * forcefully override it with the set of all 32-bit-capable CPUs that
> +	 * we know about.
> +	 *
> +	 * From the perspective of the task, this looks similar to what would
> +	 * happen if the 64-bit-only CPUs were hot-unplugged at the point of
> +	 * execve().
> +	 */
> +	if (!restrict_cpus_allowed_ptr(p, possible_mask))
> +		goto out;
> +
> +	if (alloc_cpumask_var(&cpuset_mask, GFP_KERNEL)) {
> +		cpuset_cpus_allowed(p, cpuset_mask);
> +		if (cpumask_and(cpuset_mask, cpuset_mask, possible_mask)) {
> +			newmask = cpuset_mask;
> +			goto out_set_mask;
> +		}
> +	}
> +
> +	if (printk_ratelimit()) {
> +		printk_deferred("Overriding affinity for 32-bit process %d (%s) to CPUs %*pbl\n",
> +				task_pid_nr(p), p->comm, cpumask_pr_args(newmask));
> +	}
> +out_set_mask:
> +	set_cpus_allowed_ptr(p, newmask);
> +	free_cpumask_var(cpuset_mask);
> +out:
> +	set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
> +}

This starts to look an awful lot like select_fallback_rq(), but I
suppose we shouldn't bother factoring out that code yet as we probably
don't want this pattern to be re-used all over, so:

Reviewed-by: Quentin Perret <qperret@google.com>

Thanks,
Quentin