Re: [PATCH v1 5/5] mm, hwpoison: enable memory error handling on 1GB hugepage

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/6/8 14:16, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Tue, Jun 07, 2022 at 10:11:24PM +0800, Miaohe Lin wrote:
>> On 2022/6/2 13:06, Naoya Horiguchi wrote:
>>> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
>>>
>>> Now error handling code is prepared, so remove the blocking code and
>>> enable memory error handling on 1GB hugepage.
>>>
>>
>> I'm nervous about this change. It seems there are many code paths not awared of pud swap entry.
>> I browsed some of them:
>> apply_to_pud_range called from apply_to_page_range:
>>
>> apply_to_pud_range:
>> 	next = pud_addr_end(addr, end);
>> 	if (pud_none(*pud) && !create)
>> 		continue;
>> 	if (WARN_ON_ONCE(pud_leaf(*pud)))
>> 		return -EINVAL;
>> 	if (!pud_none(*pud) && WARN_ON_ONCE(pud_bad(*pud))) {
>> 		if (!create)
>> 			continue;
>> 		pud_clear_bad(pud);
>> 	}
>> 	err = apply_to_pmd_range(mm, pud, addr, next,
>> 				 fn, data, create, mask);
>>
>> For !pud_present case, it will mostly reach apply_to_pmd_range and call pmd_offset on it. And invalid
>> pointer will be de-referenced.
> 
> apply_to_pmd_range() has BUG_ON(pud_huge(*pud)) and apply_to_pte_range() has
> BUG_ON(pmd_huge(*pmd)), so this page table walking code seems to not expect
> to handle pmd/pud level mapping.

Yes, you're right. These functions are not intended to handle pmd/pud level mapping.

> 
>>
>> Another example might be copy_pud_range and so on. So I think it might not be prepared to enable the
>> 1GB hugepage or all of these places should be fixed?
> 
> I think that most of page table walker for user address space should first
> check is_vm_hugetlb_page() and call hugetlb specific walking code for vma
> with VM_HUGETLB.
> copy_page_range() is a good example.  It calls copy_hugetlb_page_range()
> for vma with VM_HUGETLB and the function should support hwpoison entry.
> But I feel that I need testing for confirmation.

Sorry, I missed it should be called from hugetlb variants.

> 
> And I'm not sure that all other are prepared for non-present pud-mapping,
> so I'll need somehow code inspection and testing for each.

I browsed the code again, there still might be some problematic code paths:

1.For follow_pud_mask, !pud_present will mostly reach follow_pmd_mask(). This can be
called for hugetlb page. (Note gup_pud_range is fixed at 15494520b776 ("mm: fix gup_pud_range"))

2.Even for huge_pte_alloc, pud_offset will be called in pud_alloc. So pudp will be an invalid pointer.
And it will be de-referenced later.

I hope I'm not miss something again this time. ;)

> 
> Thanks,
> Naoya Horiguchi

Thanks!

>