Re: [PATCH v2 2/2] ACPI: APEI: handle synchronous exceptions in task work

[Shuai Xue]

On 2023/3/16 PM3:21, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Mon, Feb 27, 2023 at 01:03:15PM +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:
>>
>> - task work for synchronous errors.
>> - and workqueue for asynchronous errors.
>>
>> Then for 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>
> ...
>>  
>>  /*
>> - * Called as task_work before returning to user-space.
>> - * Ensure any queued work has been done before we return to the context that
>> - * triggered the notification.
>> + * struct mce_task_work - for synchronous RAS event
> 
> This seems to handle synchronous memory errors, not limited to MCE?
> So naming this struct as such (more generally) might be better.

Yes. How about `sync_task_work`?

> 
>> + *
>> + * @twork:                callback_head for task work
>> + * @pfn:                  page frame number of corrupted page
>> + * @flags:                fine tune action taken
>> + *
>> + * Structure to pass task work to be handled before
>> + * ret_to_user via task_work_add().
>>   */
> ...
> 
>>  }
>>  
>> -static bool ghes_do_memory_failure(u64 physical_addr, int flags)
>> +static void ghes_do_memory_failure(u64 physical_addr, int flags)
>>  {
>>  	unsigned long pfn;
>> +	struct mce_task_work *twcb;
>>  
>>  	if (!IS_ENABLED(CONFIG_ACPI_APEI_MEMORY_FAILURE))
>> -		return false;
>> +		return;
>>  
>>  	pfn = PHYS_PFN(physical_addr);
>>  	if (!pfn_valid(pfn) && !arch_is_platform_page(physical_addr)) {
>>  		pr_warn_ratelimited(FW_WARN GHES_PFX
>>  		"Invalid address in generic error data: %#llx\n",
>>  		physical_addr);
>> -		return false;
>> +		return;
>> +	}
>> +
>> +	if (flags == MF_ACTION_REQUIRED && current->mm) {
>> +		twcb = kmalloc(sizeof(*twcb), GFP_ATOMIC);
>> +		if (!twcb)
>> +			return;
> 
> When this kmalloc() fails, the error event might be silently dropped?
> If so, some warning messages could be helpful.

Yes, I was going to add a warning messages like:

    pr_err("Failed to handle memory failure due to out of memory\n");

But got a warning about patch when checked by checkpatch.pl.

   WARNING: Possible unnecessary 'out of memory' message

I will add it back in next version :)

> 
> Thanks,
> Naoya Horiguchi

Thank you for your comments.

Cheer,
Shuai

> 
>> +
>> +		twcb->pfn = pfn;
>> +		twcb->flags = flags;
>> +		init_task_work(&twcb->twork, memory_failure_cb);
>> +		task_work_add(current, &twcb->twork, TWA_RESUME);
>> +		return;
>>  	}
>>  
>>  	memory_failure_queue(pfn, flags);