Re: [PATCH] mm/memory-failure.c: bail out early if huge zero page

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/4/12 17:30, Oscar Salvador wrote:
> On Tue, Apr 12, 2022 at 05:25:52PM +0800, Miaohe Lin wrote:
>> On 2022/4/12 16:31, Oscar Salvador wrote:
>>> On Sun, Apr 10, 2022 at 11:22:34PM +0800, Xu Yu wrote:
>>>> Kernel panic when injecting memory_failure for the global huge_zero_page,
>>>> when CONFIG_DEBUG_VM is enabled, as follows.
>>> ... 
>>>> In fact, huge_zero_page is unhandlable currently in either soft offline
>>>> or memory failure injection.  With CONFIG_DEBUG_VM disabled,
>>>> huge_zero_page is bailed out when checking HWPoisonHandlable() in
>>>> get_any_page(), or checking page mapping in split_huge_page_to_list().
>>>>
>>>> This makes huge_zero_page bail out early in madvise_inject_error(), and
>>>> panic above won't happen again.
>>>
>>> I would not special case this in madvise_inject_error() but rather
>>> handle it in memory-failure code.
>>> We do already have HWPoisonHandlable(), which tells us whether the page
>>> is of a type we can really do something about, so why not add another
>>> check in HWPoisonHandlable() for huge_zero_page(), and have that checked
>>> in memory_failure().
>>
>> IIUC, this does not work. Because HWPoisonHandlable is only called in !MF_COUNT_INCREASED case.
>> But MF_COUNT_INCREASED is always set when called from madvise_inject_error, so HWPoisonHandlable
>> is not even called in this scene. Or am I miss something?
> 
> But nothing stops you from calling it in memory_failure(), right?
> 
>  if (MF_COUNT_INCREASED not set) {
>   ....
>   ...
>  } else if(!HWPoisonHandable(p)) {
>          action_result(pfn, MF_MSG_UNKNOWN, MF_IGNORED);
>          res = -EBUSY;
>          goto unlock_mutex;
>  }

Yes, I somewhat misread the proposed code. Thanks for clarifying. :)

> 
>> BTW: IIRC, LRU isn't set on huge_zero_page. So the origin HWPoisonHandlable can already filter out this page.
> 
> I would rather have it as a explicit check than buried in that kind of
> assumption.
> 
> But after all, Naoya's suggestion might just be better and more focused.
> 
>