Re: [PATCH 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when unmapping

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 4/30/2022 4:02 AM, Gerald Schaefer wrote:
> On Fri, 29 Apr 2022 16:14:43 +0800
> Baolin Wang <baolin.wang@linux.alibaba.com> wrote:
> 
>> On some architectures (like ARM64), it can support CONT-PTE/PMD size
>> hugetlb, which means it can support not only PMD/PUD size hugetlb:
>> 2M and 1G, but also CONT-PTE/PMD size: 64K and 32M if a 4K page
>> size specified.
>>
>> When unmapping a hugetlb page, we will get the relevant page table
>> entry by huge_pte_offset() only once to nuke it. This is correct
>> for PMD or PUD size hugetlb, since they always contain only one
>> pmd entry or pud entry in the page table.
>>
>> However this is incorrect for CONT-PTE and CONT-PMD size hugetlb,
>> since they can contain several continuous pte or pmd entry with
>> same page table attributes, so we will nuke only one pte or pmd
>> entry for this CONT-PTE/PMD size hugetlb page.
>>
>> And now we only use try_to_unmap() to unmap a poisoned hugetlb page,
>> which means now we will unmap only one pte entry for a CONT-PTE or
>> CONT-PMD size poisoned hugetlb page, and we can still access other
>> subpages of a CONT-PTE or CONT-PMD size poisoned hugetlb page,
>> which will cause serious issues possibly.
>>
>> So we should change to use huge_ptep_clear_flush() to nuke the
>> hugetlb page table to fix this issue, which already considered
>> CONT-PTE and CONT-PMD size hugetlb.
>>
>> Note we've already used set_huge_swap_pte_at() to set a poisoned
>> swap entry for a poisoned hugetlb page.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   mm/rmap.c | 34 +++++++++++++++++-----------------
>>   1 file changed, 17 insertions(+), 17 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 7cf2408..1e168d7 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1564,28 +1564,28 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>   					break;
>>   				}
>>   			}
>> +			pteval = huge_ptep_clear_flush(vma, address, pvmw.pte);
> 
> Unlike in your patch 2/3, I do not see that this (huge) pteval would later
> be used again with set_huge_pte_at() instead of set_pte_at(). Not sure if
> this (huge) pteval could end up at a set_pte_at() later, but if yes, then
> this would be broken on s390, and you'd need to use set_huge_pte_at()
> instead of set_pte_at() like in your patch 2/3.

IIUC, As I said in the commit message, we will only unmap a poisoned 
hugetlb page by try_to_unmap(), and the poisoned hugetlb page will be 
remapped with a poisoned entry by set_huge_swap_pte_at() in 
try_to_unmap_one(). So I think no need change to use set_huge_pte_at() 
instead of set_pte_at() for other cases, since the hugetlb page will not 
hit other cases.

if (PageHWPoison(subpage) && !(flags & TTU_IGNORE_HWPOISON)) {
	pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
	if (folio_test_hugetlb(folio)) {
		hugetlb_count_sub(folio_nr_pages(folio), mm);
		set_huge_swap_pte_at(mm, address, pvmw.pte, pteval,
				     vma_mmu_pagesize(vma));
	} else {
		dec_mm_counter(mm, mm_counter(&folio->page));
		set_pte_at(mm, address, pvmw.pte, pteval);
	}

}

> 
> Please note that huge_ptep_get functions do not return valid PTEs on s390,
> and such PTEs must never be set directly with set_pte_at(), but only with
> set_huge_pte_at().
> 
> Background is that, for hugetlb pages, we are of course not really dealing
> with PTEs at this level, but rather PMDs or PUDs, depending on hugetlb size.
> On s390, the layout is quite different for PTEs and PMDs / PUDs, and
> unfortunately the hugetlb code is not properly reflecting this by using
> PMD or PUD types, like the THP code does.
> 
> So, as work-around, on s390, the huge_ptep_xxx functions will return
> only fake PTEs, which must be converted again to a proper PMD or PUD,
> before writing them to the page table, which is what happens in
> set_huge_pte_at(), but not in set_pte_at().

Thanks for your explanation. As I said as above, I think we've already 
handled the hugetlb with set_huge_swap_pte_at() in try_to_unmap_one().