Re: [RFC] mm: Generalize notify_page_fault()

[Matthew Wilcox]

On Thu, May 30, 2019 at 11:25:13AM +0530, Anshuman Khandual wrote:
> Similar notify_page_fault() definitions are being used by architectures
> duplicating much of the same code. This attempts to unify them into a
> single implementation, generalize it and then move it to a common place.
> kprobes_built_in() can detect CONFIG_KPROBES, hence notify_page_fault()
> must not be wrapped again within CONFIG_KPROBES. Trap number argument can

This is a funny quirk of the English language.  "must not" means "is not
allowed to be", not "does not have to be".

> @@ -141,6 +142,19 @@ static int __init init_zero_pfn(void)
>  core_initcall(init_zero_pfn);
>  
>  
> +int __kprobes notify_page_fault(struct pt_regs *regs, unsigned int trap)
> +{
> +	int ret = 0;
> +
> +	if (kprobes_built_in() && !user_mode(regs)) {
> +		preempt_disable();
> +		if (kprobe_running() && kprobe_fault_handler(regs, trap))
> +			ret = 1;
> +		preempt_enable();
> +	}
> +	return ret;
> +}
> +
>  #if defined(SPLIT_RSS_COUNTING)

Comparing this to the canonical implementation (ie x86), it looks similar.

static nokprobe_inline int kprobes_fault(struct pt_regs *regs)
{
        if (!kprobes_built_in())
                return 0;
        if (user_mode(regs))
                return 0;
        /*
         * To be potentially processing a kprobe fault and to be allowed to call
         * kprobe_running(), we have to be non-preemptible.
         */
        if (preemptible())
                return 0;
        if (!kprobe_running())
                return 0;
        return kprobe_fault_handler(regs, X86_TRAP_PF);
}

The two handle preemption differently.  Why is x86 wrong and this one
correct?