Re: [RESEND PATCH v8 2/2] ACPI: APEI: handle synchronous exceptions in task work

[Shuai Xue]

On 2023/10/3 16:28, Naoya Horiguchi wrote:
> On Tue, Sep 19, 2023 at 10:21:27AM +0800, Shuai Xue wrote:
>> Hardware errors could be signaled by synchronous interrupt, e.g.  when an
>> error is detected by a background scrubber, or signaled by synchronous
>> exception, e.g. when an uncorrected error is consumed. Both synchronous and
>> asynchronous error are queued and handled by a dedicated kthread in
>> workqueue.
>>
>> commit 7f17b4a121d0 ("ACPI: APEI: Kick the memory_failure() queue for
>> synchronous errors") keep track of whether memory_failure() work was
>> queued, and make task_work pending to flush out the workqueue so that the
>> work for synchronous error is processed before returning to user-space.
>> The trick ensures that the corrupted page is unmapped and poisoned. And
>> after returning to user-space, the task starts at current instruction which
>> triggering a page fault in which kernel will send SIGBUS to current process
>> due to VM_FAULT_HWPOISON.
>>
>> However, the memory failure recovery for hwpoison-aware mechanisms does not
>> work as expected. For example, hwpoison-aware user-space processes like
>> QEMU register their customized SIGBUS handler and enable early kill mode by
>> seting PF_MCE_EARLY at initialization. Then the kernel will directy notify
>> the process by sending a SIGBUS signal in memory failure with wrong
>> si_code: the actual user-space process accessing the corrupt memory
>> location, but its memory failure work is handled in a kthread context, so
>> it will send SIGBUS with BUS_MCEERR_AO si_code to the actual user-space
>> process instead of BUS_MCEERR_AR in kill_proc().
>>
>> To this end, separate synchronous and asynchronous error handling into
>> different paths like X86 platform does:
>>
>> - valid synchronous errors: queue a task_work to synchronously send SIGBUS
>>   before ret_to_user.
>> - valid asynchronous errors: queue a work into workqueue to asynchronously
>>   handle memory failure.
>> - abnormal branches such as invalid PA, unexpected severity, no memory
>>   failure config support, invalid GUID section, OOM, etc.
>>
>> Then for valid synchronous errors, the current context in memory failure is
>> exactly belongs to the task consuming poison data and it will send SIBBUS
>> with proper si_code.
>>
>> Fixes: 7f17b4a121d0 ("ACPI: APEI: Kick the memory_failure() queue for synchronous errors")
>> Signed-off-by: Shuai Xue <xueshuai@xxxxxxxxxxxxxxxxx>
>> Tested-by: Ma Wupeng <mawupeng1@xxxxxxxxxx>
>> Reviewed-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
>> Reviewed-by: Xiaofei Tan <tanxiaofei@xxxxxxxxxx>
>> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>> ---
>>  arch/x86/kernel/cpu/mce/core.c |  9 +---
>>  drivers/acpi/apei/ghes.c       | 84 +++++++++++++++++++++-------------
>>  include/acpi/ghes.h            |  3 --
>>  mm/memory-failure.c            | 17 ++-----
>>  4 files changed, 56 insertions(+), 57 deletions(-)
>>
> ...
> 
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index 4d6e43c88489..80e1ea1cc56d 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -2163,7 +2163,9 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
>>   *
>>   * Return: 0 for successfully handled the memory error,
>>   *         -EOPNOTSUPP for hwpoison_filter() filtered the error event,
>> - *         < 0(except -EOPNOTSUPP) on failure.
>> + *         -EHWPOISON for already sent SIGBUS to the current process with
>> + *         the proper error info,
> 
> The meaning of this comment is understood, but the sentence seems to be
> a little too long. Could you sort this out with bullet points (like below)?
> 
>  * Return values:
>  *   0             - success
>  *   -EOPNOTSUPP   - hwpoison_filter() filtered the error event.
>  *   -EHWPOISON    - sent SIGBUS to the current process with the proper
>  *                   error info by kill_accessing_process().
>  *   other negative values - failure
> 

Of course, will do it.


>> + *         other negative error code on failure.
>>   */
>>  int memory_failure(unsigned long pfn, int flags)
>>  {
>> @@ -2445,19 +2447,6 @@ static void memory_failure_work_func(struct work_struct *work)
>>  	}
>>  }
>>  
>> -/*
>> - * Process memory_failure work queued on the specified CPU.
>> - * Used to avoid return-to-userspace racing with the memory_failure workqueue.
>> - */
>> -void memory_failure_queue_kick(int cpu)
>> -{
>> -	struct memory_failure_cpu *mf_cpu;
>> -
>> -	mf_cpu = &per_cpu(memory_failure_cpu, cpu);
>> -	cancel_work_sync(&mf_cpu->work);
>> -	memory_failure_work_func(&mf_cpu->work);
>> -}
>> -
> 
> The declaration of memory_failure_queue_kick() still remains in include/linux/mm.h,
> so you can remove it together.

Good catch, will remove it too.

> 
> Thanks,
> Naoya Horiguchi


Thank you for valuable comments.

Best Regards,
Shuai