Re: [RFC V3] mm: Generalize and rename notify_page_fault() as kprobe_page_fault()

[Anshuman Khandual]

On 06/07/2019 09:01 PM, Christophe Leroy wrote:
> 
> 
> Le 07/06/2019 Ã 12:34, Anshuman Khandual a ÃcritÂ:
>> 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@xxxxxxxxxxxxxxxxxxx
>> Cc: linux-ia64@xxxxxxxxxxxxxxx
>> Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
>> Cc: linux-s390@xxxxxxxxxxxxxxx
>> Cc: linux-sh@xxxxxxxxxxxxxxx
>> Cc: sparclinux@xxxxxxxxxxxxxxx
>> Cc: x86@xxxxxxxxxx
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>> Cc: Mark Rutland <mark.rutland@xxxxxxx>
>> Cc: Christophe Leroy <christophe.leroy@xxxxxx>
>> Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
>> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
>> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
>> Cc: Paul Mackerras <paulus@xxxxxxxxx>
>> Cc: Russell King <linux@xxxxxxxxxxxxxxx>
>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>> Cc: Will Deacon <will.deacon@xxxxxxx>
>> Cc: Tony Luck <tony.luck@xxxxxxxxx>
>> Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
>> Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
>> Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
>> Cc: Yoshinori Sato <ysato@xxxxxxxxxxxxxxxxxxxx>
>> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
>> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>> ---
>> 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/)
>>
>> Â arch/arm/mm/fault.cÂÂÂÂÂ | 24 +-----------------------
>> Â arch/arm64/mm/fault.cÂÂÂ | 24 +-----------------------
>> Â arch/ia64/mm/fault.cÂÂÂÂ | 24 +-----------------------
>>  arch/powerpc/mm/fault.c | 23 ++---------------------
>> Â arch/s390/mm/fault.cÂÂÂÂ | 16 +---------------
>> Â arch/sh/mm/fault.cÂÂÂÂÂÂ | 18 ++----------------
>> Â arch/sparc/mm/fault_64.c | 16 +---------------
>> Â arch/x86/mm/fault.cÂÂÂÂÂ | 21 ++-------------------
>>  include/linux/kprobes.h | 16 ++++++++++++++++
>> Â 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)
>> Â }
>> Â #endif
>> Â +static nokprobe_inline bool kprobe_page_fault(struct pt_regs *regs,
>> +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unsigned int trap)
>> +{
>> +ÂÂÂ int ret = 0;
> 
> ret is pointless.
> 
>> +
>> +ÂÂÂ /*
>> +ÂÂÂÂ * To be potentially processing a kprobe fault and to be allowed
>> +ÂÂÂÂ * to call kprobe_running(), we have to be non-preemptible.
>> +ÂÂÂÂ */
>> +ÂÂÂ if (kprobes_built_in() && !preemptible() && !user_mode(regs)) {
>> +ÂÂÂÂÂÂÂ 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.

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

Makes sense, will drop ret.