Re: [PATCH v8 1/4] mm,hwpoison,hugetlb,memory_hotplug: hotremove memory section with hwpoisoned hugepage

[Miaohe Lin]

On 2022/10/25 13:35, Naoya Horiguchi wrote:
> On Tue, Oct 25, 2022 at 10:38:11AM +0800, Miaohe Lin wrote:
>> On 2022/10/24 14:20, Naoya Horiguchi wrote:
>>> From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
>>>
>>> HWPoisoned page is not supposed to be accessed once marked, but currently
>>> such accesses can happen during memory hotremove because do_migrate_range()
>>> can be called before dissolve_free_huge_pages() is called.
>>>
>>> Clear HPageMigratable for hwpoisoned hugepages to prevent them from being
>>> migrated.  This should be done in hugetlb_lock to avoid race against
>>> isolate_hugetlb().
>>>
>>> get_hwpoison_huge_page() needs to have a flag to show it's called from
>>> unpoison to take refcount of hwpoisoned hugepages, so add it.
>>>
>>> Reported-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
>>> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
>>> Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>> ---
>>> ChangeLog v3 -> v7:
>>> - introduce TESTCLEARHPAGEFLAG() to determine the value of migratable_cleared
>>
>> Many thanks for update, Naoya. I'm sorry but TestClearHPageMigratable() might be somewhat
>> overkill. As we discussed in previous thread:
>>
>> """
>> I think I might be nitpicking... But it seems ClearHPageMigratable is not enough here.
>>   1. In MF_COUNT_INCREASED case, we don't know whether HPageMigratable is set.
>>   2. Even if HPageMigratable is set, there might be a race window before we clear HPageMigratable?
>> So "*migratable_cleared = TestClearHPageMigratable" might be better? But I might be wrong.
>> """
>>
>> The case 2 should be a dumb problem(sorry about it). HPageMigratable() is always cleared while holding
>> the hugetlb_lock which is already held by get_huge_page_for_hwpoison(). So the only case we should care
>> about is case 1 and that can be handled by below more efficient pattern:
>> 	if (HPageMigratable)
>> 		ClearHPageMigratable()
>>
>> So the overhead of test and clear atomic ops can be avoided. But this is trival.
>>
>> Anyway, this patch still looks good to me. And my Reviewed-by tag still applies. Many thanks.
> 
> OK, so I replace this 1/4 with the following one, thank you.

Many thanks for your update, Naoya. ;) This version looks good to me too.

Thanks,
Miaohe Lin