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Subject: Re: [RFC V3] mm: Generalize and rename notify_page_fault() as
 kprobe_page_fault()
To:     Anshuman Khandual <anshuman.khandual@arm.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Cc:     linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
        linux-sh@vger.kernel.org, sparclinux@vger.kernel.org,
        x86@kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Andrey Konovalov <andreyknvl@google.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Paul Mackerras <paulus@samba.org>,
        Russell King <linux@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Tony Luck <tony.luck@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Yoshinori Sato <ysato@users.sourceforge.jp>,
        "David S. Miller" <davem@davemloft.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>
References: <1559903655-5609-1-git-send-email-anshuman.khandual@arm.com>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <ec764ff4-f68a-fce5-ac1e-a4664e1123c7@c-s.fr>
Date:   Fri, 7 Jun 2019 17:31:01 +0200
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Le 07/06/2019 à 12:34, Anshuman Khandual a écrit :
> Very similar definitions for notify_page_fault() are being used by multiple
> architectures duplicating much of the same code. This attempts to unify all
> of them into a generic implementation, rename it as kprobe_page_fault() and
> then move it to a common header.
> 
> kprobes_built_in() can detect CONFIG_KPROBES, hence new kprobe_page_fault()
> need not be wrapped again within CONFIG_KPROBES. Trap number argument can
> now contain upto an 'unsigned int' accommodating all possible platforms.
> 
> kprobe_page_fault() goes the x86 way while dealing with preemption context.
> As explained in these following commits the invoking context in itself must
> be non-preemptible for kprobes processing context irrespective of whether
> kprobe_running() or perhaps smp_processor_id() is safe or not. It does not
> make much sense to continue when original context is preemptible. Instead
> just bail out earlier.
> 
> commit a980c0ef9f6d
> ("x86/kprobes: Refactor kprobes_fault() like kprobe_exceptions_notify()")
> 
> commit b506a9d08bae ("x86: code clarification patch to Kprobes arch code")
> 
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-ia64@vger.kernel.org
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: linux-s390@vger.kernel.org
> Cc: linux-sh@vger.kernel.org
> Cc: sparclinux@vger.kernel.org
> Cc: x86@kernel.org
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Christophe Leroy <christophe.leroy@c-s.fr>
> Cc: Stephen Rothwell <sfr@canb.auug.org.au>
> Cc: Andrey Konovalov <andreyknvl@google.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Tony Luck <tony.luck@intel.com>
> Cc: Fenghua Yu <fenghua.yu@intel.com>
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> 
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> Testing:
> 
> - Build and boot tested on arm64 and x86
> - Build tested on some other archs (arm, sparc64, alpha, powerpc etc)
> 
> Changes in RFC V3:
> 
> - Updated the commit message with an explaination for new preemption behaviour
> - Moved notify_page_fault() to kprobes.h with 'static nokprobe_inline' per Matthew
> - Changed notify_page_fault() return type from int to bool per Michael Ellerman
> - Renamed notify_page_fault() as kprobe_page_fault() per Peterz
> 
> Changes in RFC V2: (https://patchwork.kernel.org/patch/10974221/)
> 
> - Changed generic notify_page_fault() per Mathew Wilcox
> - Changed x86 to use new generic notify_page_fault()
> - s/must not/need not/ in commit message per Matthew Wilcox
> 
> Changes in RFC V1: (https://patchwork.kernel.org/patch/10968273/)
> 
>   arch/arm/mm/fault.c      | 24 +-----------------------
>   arch/arm64/mm/fault.c    | 24 +-----------------------
>   arch/ia64/mm/fault.c     | 24 +-----------------------
>   arch/powerpc/mm/fault.c  | 23 ++---------------------
>   arch/s390/mm/fault.c     | 16 +---------------
>   arch/sh/mm/fault.c       | 18 ++----------------
>   arch/sparc/mm/fault_64.c | 16 +---------------
>   arch/x86/mm/fault.c      | 21 ++-------------------
>   include/linux/kprobes.h  | 16 ++++++++++++++++
>   9 files changed, 27 insertions(+), 155 deletions(-)
> 

[...]

> diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> index 443d980..064dd15 100644
> --- a/include/linux/kprobes.h
> +++ b/include/linux/kprobes.h
> @@ -458,4 +458,20 @@ static inline bool is_kprobe_optinsn_slot(unsigned long addr)
>   }
>   #endif
>   
> +static nokprobe_inline bool kprobe_page_fault(struct pt_regs *regs,
> +					      unsigned int trap)
> +{
> +	int ret = 0;

ret is pointless.

> +
> +	/*
> +	 * 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))

don't need an 'if A if B', can do 'if A && B'

> +			ret = 1;

can do 'return true;' directly here

> +	}
> +	return ret;

And 'return false' here.

Christophe

> +}
> +
>   #endif /* _LINUX_KPROBES_H */
>