Re: Re: Re: [PATCH 3/4] mm/memofy-failure.c: optimize hwpoison_filter

[zhenwei pi <pizhenwei@bytedance.com>]

On 5/7/22 16:20, Naoya Horiguchi wrote:
> On Sat, May 07, 2022 at 08:28:05AM +0800, zhenwei pi wrote:
>>
>> On 5/7/22 00:28, David Hildenbrand wrote:
>>> On 06.05.22 15:38, zhenwei pi wrote:
>>>>
>>>>
>>>> On 5/6/22 16:59, Naoya Horiguchi wrote:
>>>>> On Fri, Apr 29, 2022 at 10:22:05PM +0800, zhenwei pi wrote:
>>>>>> In the memory failure procedure, hwpoison_filter has higher priority,
>>>>>> if memory_filter() filters the error event, there is no need to do
>>>>>> the further work.
>>>>>
>>>>> Could you clarify what problem you are trying to solve (what does
>>>>> "optimize" mean in this context or what is the benefit)?
>>>>>
>>>>
>>>> OK. The background of this work:
>>>> As well known, the memory failure mechanism handles memory corrupted
>>>> event, and try to send SIGBUS to the user process which uses this
>>>> corrupted page.
>>>>
>>>> For the virtualization case, QEMU catches SIGBUS and tries to inject MCE
>>>> into the guest, and the guest handles memory failure again. Thus the
>>>> guest gets the minimal effect from hardware memory corruption.
>>>>
>>>> The further step I'm working on:
>>>> 1, try to modify code to decrease poisoned pages in a single place
>>>> (mm/memofy-failure.c: simplify num_poisoned_pages_dec in this series).
> 
> This is fine to me.
> 
>>>>
>>>> 2, try to use page_handle_poison() to handle SetPageHWPoison() and
>>>> num_poisoned_pages_inc() together. It would be best to call
>>>> num_poisoned_pages_inc() in a single place too. I'm not sure if this is
>>>> possible or not, please correct me if I misunderstand.
> 
> SetPageHWPoison() can be cancelled in memory_failure(), so simply bundling
> it with num_poisoned_pages_inc() might not be optimal.  I think that
> action_result() is supposed to be called when memory error handling is
> effective (not filtered, not cancelled). So moving num_poisoned_pages_inc()
> (and notification code in your plan) into this function might be good.
> 
OK, I'll remove this patch(mm/memofy-failure.c: optimize 
hwpoison_filter) from this series, and fix the other 3 patches in the v2 
version. Then try to implement/test as your suggestion in another series.

>>>>
>>>> 3, introduce memory failure notifier list in memory-failure.c: notify
>>>> the corrupted PFN to someone who registers this list.
>>>> If I can complete [1] and [2] part, [3] will be quite easy(just call
>>>> notifier list after increasing poisoned page).
>>>>
>>>> 4, introduce memory recover VQ for memory balloon device, and registers
>>>> memory failure notifier list. During the guest kernel handles memory
>>>> failure, balloon device gets notified by memory failure notifier list,
>>>> and tells the host to recover the corrupted PFN(GPA) by the new VQ.
>>>
>>> Most probably you might want to do that asynchronously, and once the
>>> callback succeeds, un-poison the page.
>>>
>> Yes!
>>
>>>>
>>>> 5, host side remaps the corrupted page(HVA), and tells the guest side to
>>>> unpoison the PFN(GPA). Then the guest fixes the corrupted page(GPA)
>>>> dynamically.
>>>
>>> I think QEMU already does that during reboots. Now it would be triggered
>>> by the guest for individual pages.
>>>
>> Yes, currently QEMU supports to un-poison corrupted pages during
>> reset/reboot. We can reuse some code to do the work in this case, this
>> allows a VM to fix corrupted pages as soon as possible(also no need to
>> reset/reboot).
> 
> So this finally allows to replace broken page mapped to guest with
> a healthy page without rebooting the guest. That sounds helpful.
> 
> Thanks,
> Naoya Horiguchi
> 
Yes, it's my plan. Thanks for your suggestions!

>>
>>>>
>>>> Because [4] and [5] are related to balloon device, also CC Michael,
>>>> David and Jason.
>>>
>>> Doesn't sound too crazy for me, although it's a shame that we always
>>> have to use virtio-balloon for such fairly balloon-unrelated things.
>>>
>> Thanks!
>>
>> -- 
>> zhenwei pi

-- 
zhenwei pi