linux-kernel - Re: [RFC V2] mm: Generalize notify_page

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Message-ID: <87sgsomg91.fsf@concordia.ellerman.id.au>
Date:   Wed, 05 Jun 2019 21:19:22 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        x86@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [RFC V2] mm: Generalize notify_page_fault()

Anshuman Khandual <anshuman.khandual@....com> writes:
> 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()
> need not be wrapped again within CONFIG_KPROBES. Trap number argument can
> now contain upto an 'unsigned int' accommodating all possible platforms.
...
> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> index 58f69fa..1bc3b18 100644
> --- a/arch/arm/mm/fault.c
> +++ b/arch/arm/mm/fault.c
> @@ -30,28 +30,6 @@
>  
>  #ifdef CONFIG_MMU
>  
> -#ifdef CONFIG_KPROBES
> -static inline int notify_page_fault(struct pt_regs *regs, unsigned int fsr)
> -{
> -	int ret = 0;
> -
> -	if (!user_mode(regs)) {
> -		/* kprobe_running() needs smp_processor_id() */
> -		preempt_disable();
> -		if (kprobe_running() && kprobe_fault_handler(regs, fsr))
> -			ret = 1;
> -		preempt_enable();
> -	}
> -
> -	return ret;
> -}
> -#else

You've changed several of the architectures from something like above,
where it disables preemption around the call into the below:

> +int __kprobes notify_page_fault(struct pt_regs *regs, unsigned int trap)
> +{
> +	int ret = 0;
> +
> +	/*
> +	 * To be potentially processing a kprobe fault and to be allowed
> +	 * to call kprobe_running(), we have to be non-preemptible.
> +	 */
> +	if (kprobes_built_in() && !preemptible() && !user_mode(regs)) {
> +		if (kprobe_running() && kprobe_fault_handler(regs, trap))
> +			ret = 1;
> +	}
> +	return ret;
> +}

Which skips everything if we're preemptible. Is that an equivalent
change? If so can you please explain why in more detail.

Also why not have it return bool?

cheers