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Subject: Re: [RFC V3] mm: Generalize and rename notify_page_fault() as
 kprobe_page_fault()
To:     Christophe Leroy <christophe.leroy@c-s.fr>,
        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>
 <ec764ff4-f68a-fce5-ac1e-a4664e1123c7@c-s.fr>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <97e9c9b3-89c8-d378-4730-841a900e6800@arm.com>
Date:   Mon, 10 Jun 2019 08:09:11 +0530
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On 06/07/2019 09:01 PM, Christophe Leroy wrote:
> 
> 
> 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'

Which will make it a very lengthy condition check.

> 
>> +            ret = 1;
> 
> can do 'return true;' directly here
> 
>> +    }
>> +    return ret;
> 
> And 'return false' here.

Makes sense, will drop ret.