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Subject: Re: [PATCH v5 1/3] arm64/ras: support sea error recovery
To:     James Morse <james.morse@arm.com>
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From:   Xie XiuQi <xiexiuqi@huawei.com>
Message-ID: <7dacf375-4645-ba34-62d1-96d9f67dbcc2@huawei.com>
Date:   Thu, 8 Feb 2018 16:35:44 +0800
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Hi James,

Sorry for reply late.

On 2018/2/8 3:03, James Morse wrote:
> Hi Xie XiuQi,
> 
> On 30/01/18 19:19, James Morse wrote:
>> On 26/01/18 12:31, Xie XiuQi wrote:
>>> With ARM v8.2 RAS Extension, SEA are usually triggered when memory errors
>>> are consumed. According to the existing process, errors occurred in the
>>> kernel, leading to direct panic, if it occurred the user-space, we should
>>> just kill process.
>>>
>>> But there is a class of error, in fact, is not necessary to kill
>>> process, you can recover and continue to run the process. Such as
>>> the instruction data corrupted, where the memory page might be
>>> read-only, which is has not been modified, the disk might have the
>>> correct data, so you can directly drop the page, ant reload it when
>>> necessary.
>>
>> With firmware-first support, we do all this...
>>
>>
>>> So this patchset is just try to solve such problem: if the error is
>>> consumed in user-space and the error occurs on a clean page, you can
>>> directly drop the memory page without killing process.
>>>
>>> If the corrupted page is clean, just dropped it and return to user-space
>>> without side effects. And if corrupted page is dirty, memory_failure()
>>> will send SIGBUS with code=BUS_MCEERR_AR. While without this patchset,
>>> do_sea() will just send SIGBUS, so the process was killed in the same place.
>>
>> ... but this happens too. I agree its something we should fix, but I don't think
>> this is the best way to do it.
>>
>> This series is pulling the memory-failure-queue details back into the arch-code
>> to build a second list, that gets processed as extra work when we return to
>> user-space.
>>
>>
>> The root of the issue is ghes_notify_sea() claims the notification as something
>> APEI has dealt with, ... but it hasn't done it yet. The signals will be
>> generated by something currently stuck in a queue. (Evidently x86 doesn't handle
>> synchronous errors like this using firmware-first).
>>
>> I think a smaller fix is to give the queues that may be holding the
>> memory_failure() work a kick as part of the code that calls ghes_notify_sea().
>> This means that by the time we return to do_sea() ghes_notify_sea()'s claim that
>> APEI has dealt with it is true as any generated signals are pending. We can then
>> skip the existing SIGBUS generation code.
>>
>>
>>> Because memory_failure() may sleep, we can not call it directly in SEA
>>
>> (this one is more serious, I've attempted to fix it by moving all NMI-like
>> GHES-notifications to use the estatus queue).
>>
>>
>>> exception context. So we saved faulting physical address associated with
>>> a process in the ghes handler and set __TIF_SEA_NOTIFY. When we return
>>> from SEA exception context and get into do_notify_resume() before the
>>> process running, we could check it and call memory_failure() to do
>>> recovery.
>>
>>> It's safe, because we are in process context.
>>
>> I think this is the trick. When we take a Synchronous-external-abort out of
>> userspace, we're in process context too. We can add helpers to drain the
>> memory_failure_queue which can be called when do_sea() when we know we're
>> preemptible and interrupts-et-al are unmasked.
> 
> Something like... base on [0], in arch/arm64/kernel/acpi.c:

I am very glad that you are trying to solve the problem, which is very helpful.
I agree with your proposal, and I'll test it on by box latter.

Indeed, we're in precess context when we are in sea handler. I was thought we
can't call schedule() in the exception handler before.

Thank you very much!

> -----------------%<-----------------
> int apei_claim_sea(struct pt_regs *regs)
> {
>         int cpu;
>         int err = -ENOENT;
>         unsigned long current_flags = arch_local_save_flags();
>         unsigned long interrupted_flags = current_flags;
> 
>         if (!IS_ENABLED(CONFIG_ACPI_APEI_SEA))
>                 return err;
> 
>         if (regs)
>                 interrupted_flags = regs->pstate;
> 
>         /*
>          * APEI expects an NMI-like notification to always be called
>          * in NMI context.
>          */
>         local_daif_restore(DAIF_ERRCTX);
>         nmi_enter();
>         err = ghes_notify_sea();
>         cpu = smp_processor_id();
>         nmi_exit();
> 
>         /*
>          * APEI NMI-like notifications are deferred to irq_work. Unless
>          * we interrupted irqs-masked code, we can do that now.
>          */
>         if (!err) {
>                 if (!arch_irqs_disabled_flags(interrupted_flags)) {
>                         local_daif_restore(DAIF_PROCCTX_NOIRQ);
>                         irq_work_run();
>                 } else {
>                         err = -EINPROGRESS;
>                 }
>         }
> 
>         local_daif_restore(current_flags);
> 
>         if (IS_ENABLED(CONFIG_ACPI_APEI_MEMORY_FAILURE) && !err) {
>                 /*
>                  * Memory failure work is scheduled on the local CPU.
>                  * If we interrupted userspace, or are in process context
>                  * we can do that now.
>                  */
>                 if ((regs && !user_mode(regs)) || !preemptible())
>                         err = -EINPROGRESS;
>                 else
>                         memory_failure_queue_kick(cpu);
>         }
> 
>         return err;
> }
> -----------------%<-----------------
> 
> 
> and to mm/memory-failure.c:
> -----------------%<-----------------
> @@ -1355,7 +1355,7 @@ static void memory_failure_work_func(struct work_struct *w
> ork)
>         unsigned long proc_flags;
>         int gotten;
> 
> -       mf_cpu = this_cpu_ptr(&memory_failure_cpu);
> +       mf_cpu = container_of(work, struct memory_failure_cpu, work);
>         for (;;) {
>                 spin_lock_irqsave(&mf_cpu->lock, proc_flags);
>                 gotten = kfifo_get(&mf_cpu->fifo, &entry);
> 
> @@ -1369,6 +1369,22 @@ 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)
> +{
> +       unsigned long flags;
> +       struct memory_failure_cpu *mf_cpu;
> +
> +       might_sleep();
> +
> +       mf_cpu = &per_cpu(memory_failure_cpu, cpu);
> +       cancel_work_sync(&mf_cpu->work);
> +       memory_failure_work_func(&mf_cpu->work);
> +}
> +
>  static int __init memory_failure_init(void)
>  {
>         struct memory_failure_cpu *mf_cpu;
> -----------------%<-----------------
> 
> I've cooked up some NOTFIY_SEA-ing APEI firmware using kvmtool to test this. I
> haven't yet managed to hit irq-masked code with NOTIFY_SEA. I'll try and tidy
> this up and post a branch to make it easier to test...
> 
> I prefer this as it doesn't duplicate the state then come back on a TIF flag.
> I'd like to move the kicking logic into ghes.c, as that is where the queueing
> happened, but the 'do-this, restore these flags, do-that' is somewhat tasteless,
> and it looks like on arm64 has synchronous nmi-like notifications that must be
> handled before returning to user-space...
> 
> Thanks,
> 
> James
> 
> [0] https://www.spinics.net/lists/linux-acpi/msg80149.html
> 


-- 
Thanks,
Xie XiuQi