Re: [PATCH v7 22/25] ACPI / APEI: Kick the memory_failure() queue for synchronous errors

[James Morse <james.morse@arm.com>]

Hi Xie XiuQi,

On 05/12/2018 02:02, Xie XiuQi wrote:
> On 2018/12/4 2:06, James Morse wrote:
>> memory_failure() offlines or repairs pages of memory that have been
>> discovered to be corrupt. These may be detected by an external
>> component, (e.g. the memory controller), and notified via an IRQ.
>> In this case the work is queued as not all of memory_failure()s work
>> can happen in IRQ context.
>>
>> If the error was detected as a result of user-space accessing a
>> corrupt memory location the CPU may take an abort instead. On arm64
>> this is a 'synchronous external abort', and on a firmware first
>> system it is replayed using NOTIFY_SEA.
>>
>> This notification has NMI like properties, (it can interrupt
>> IRQ-masked code), so the memory_failure() work is queued. If we
>> return to user-space before the queued memory_failure() work is
>> processed, we will take the fault again. This loop may cause platform
>> firmware to exceed some threshold and reboot when Linux could have
>> recovered from this error.
>>
>> If a ghes notification type indicates that it may be triggered again
>> when we return to user-space, use the task-work and notify-resume
>> hooks to kick the relevant memory_failure() queue before returning


>> @@ -407,7 +447,22 @@ static void ghes_handle_memory_failure(struct acpi_hest_generic_data *gdata, int
>>  
>>  	if (flags != -1)
>>  		memory_failure_queue(pfn, flags);
> 
> We may need to take MF_ACTION_REQUIRED flags for memory_failure() in SEA condition.

Hmmm, I'd forgotten about the extra flags. They're only used by x86's
do_machine_check(), which knows more about what is going on. I agree we do know
it should be a 'MF_ACTION_REQUIRED' for Synchronous-external-abort, but I'd
really like all the notifications to behave in the same way as we can't change
which notification firmware uses.
(This ghes_is_synchronous() affects when memory_failure() runs, not what it does.)

What happens if we miss MF_ACTION_REQUIRED? Surely the page still gets unmapped
as its PG_Poisoned, an AO signal may be pending, but if user-space touches the
page it will get an AR signal. Is this just about removing an extra AO signal to
user-space?

If we do need this, I'd like to pick it up from the CPER records, as x86's
NOTIFY_NMI looks like it covers both AO/AR cases. (as does NOTIFY_SDEI). The
Master/Target abort or Invalid-address types in the memory-error-section CPER
records look like the best bet.


> And there is no return value check for memory_failure() in memory_failure_work_func(),
> I'm not sure whether we need to check the return value.

What would we do if it fails? The reasons look fairly broad, -EBUSY can mean
"(page) still referenced by [..] users", 'thp split failed' or 'page already
poisoned'. I don't think the behaviour or return-codes are consistent enough to use.


> If the recovery fails here, we need to take other actions, such as force to send a SIGBUS signal.

We don't do this today. If it fixes some mis-behaviour, and we can key it from
something in the CPER records then I'm all ears!


Thanks,

James