linux-kernel - Re: [PATCH v2 2/3] arm64: hwpoison: add VM_FAULT_HWPOISON[

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Message-ID: <877f0n1yhy.fsf@e105922-lin.cambridge.arm.com>
Date:   Wed, 07 Jun 2017 18:47:05 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     catalin.marinas@....com,
        "Jonathan \(Zhixiong\) Zhang" <zjzhang@...eaurora.org>,
        tbaicar@...eaurora.org, steve.capper@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        manoj.iyer@...onical.com
Subject: Re: [PATCH v2 2/3] arm64: hwpoison: add VM_FAULT_HWPOISON[_LARGE] handling

Will Deacon <will.deacon@....com> writes:

> On Wed, May 17, 2017 at 04:23:35PM +0100, Punit Agrawal wrote:
>> From: "Jonathan (Zhixiong) Zhang" <zjzhang@...eaurora.org>
>> 
>> Add VM_FAULT_HWPOISON[_LARGE] handling to the arm64 page fault
>> handler. Handling of VM_FAULT_HWPOISON[_LARGE] is very similar
>> to VM_FAULT_OOM, the only difference is that a different si_code
>> (BUS_MCEERR_AR) is passed to user space and si_addr_lsb field is
>> initialized.
>> 
>> Signed-off-by: Jonathan (Zhixiong) Zhang <zjzhang@...eaurora.org>
>> Signed-off-by: Tyler Baicar <tbaicar@...eaurora.org>
>> (fix new __do_user_fault call-site)
>> Signed-off-by: Punit Agrawal <punit.agrawal@....com>
>> Acked-by: Steve Capper <steve.capper@....com>
>> ---
>>  arch/arm64/mm/fault.c | 22 +++++++++++++++++++---
>>  1 file changed, 19 insertions(+), 3 deletions(-)
>> 
>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
>> index 37b95dff0b07..a85b44343ac6 100644
>> --- a/arch/arm64/mm/fault.c
>> +++ b/arch/arm64/mm/fault.c
>> @@ -31,6 +31,7 @@
>>  #include <linux/highmem.h>
>>  #include <linux/perf_event.h>
>>  #include <linux/preempt.h>
>> +#include <linux/hugetlb.h>
>>  
>>  #include <asm/bug.h>
>>  #include <asm/cpufeature.h>
>> @@ -239,10 +240,11 @@ static void __do_kernel_fault(struct mm_struct *mm, unsigned long addr,
>>   */
>>  static void __do_user_fault(struct task_struct *tsk, unsigned long addr,
>>  			    unsigned int esr, unsigned int sig, int code,
>> -			    struct pt_regs *regs)
>> +			    struct pt_regs *regs, int fault)
>>  {
>>  	struct siginfo si;
>>  	const struct fault_info *inf;
>> +	unsigned int lsb = 0;
>>  
>>  	if (unhandled_signal(tsk, sig) && show_unhandled_signals_ratelimited()) {
>>  		inf = esr_to_fault_info(esr);
>> @@ -259,6 +261,17 @@ static void __do_user_fault(struct task_struct *tsk, unsigned long addr,
>>  	si.si_errno = 0;
>>  	si.si_code = code;
>>  	si.si_addr = (void __user *)addr;
>> +	/*
>> +	 * Either small page or large page may be poisoned.
>> +	 * In other words, VM_FAULT_HWPOISON_LARGE and
>> +	 * VM_FAULT_HWPOISON are mutually exclusive.
>> +	 */
>> +	if (fault & VM_FAULT_HWPOISON_LARGE)
>> +		lsb = hstate_index_to_shift(VM_FAULT_GET_HINDEX(fault));
>> +	else if (fault & VM_FAULT_HWPOISON)
>> +		lsb = PAGE_SHIFT;
>> +	si.si_addr_lsb = lsb;
>> +
>
> If we're going to start handling poison faults, then we should probably
> rejig the perf page fault accounting around here so that we follow x86:
>
>   * Always report PERF_COUNT_SW_PAGE_FAULTS,
>   * Don't report anything else for VM_FAULT_ERROR
>   * Report PERF_COUNT_SW_PAGE_FAULTS_MAJ if VM_FAULT_MAJOR
>   * Otherwise, report PERF_COUNT_SW_PAGE_FAULTS_MIN
>
> at the moment, I think you're accounting VM_FAULT_ERROR as
> PERF_COUNT_SW_PAGE_FAULTS_MIN, which doesn't feel right at all.

Ok. I'd missed the implication of enabling poisoned pages fault handling
on the perf accounting. I'll add a patch to update the reporting.

>
> Will