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

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

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@linux.alibaba.com>
...
>  
>  /*
> - * 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.

> + *
> + * @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.

Thanks,
Naoya Horiguchi

> +
> +		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);