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

[Shuai Xue <xueshuai@linux.alibaba.com>]

On 2023/9/25 23:00, Jarkko Sakkinen wrote:
> On Tue Sep 19, 2023 at 5:21 AM EEST, 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@linux.alibaba.com>
>> Tested-by: Ma Wupeng <mawupeng1@huawei.com>
>> Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> Reviewed-by: Xiaofei Tan <tanxiaofei@huawei.com>
>> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> 
> Did 7f17b4a121d0 actually break something that was not broken before?
> 
> If not, this is (afaik) not a bug fix.

Hi, Jarkko,

It did not. It keeps track of whether memory_failure() work was queued,
and makes task_work pending to flush out the queue. But if no work queued for
synchronous error due to abnormal branches, it does not do a force kill to
current process resulting a hard lockup due to exception loop.

It is fine to me to remove the bug fix tag if you insist on removing it.

Best Regards,
Shuai