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

[Baolin Wang]

On 5/2/2022 10:02 PM, Gerald Schaefer wrote:
> On Sat, 30 Apr 2022 11:22:33 +0800
> Baolin Wang <baolin.wang at linux.alibaba.com> wrote:
> 
>>
>>
>> On 4/30/2022 4:02 AM, Gerald Schaefer wrote:
>>> On Fri, 29 Apr 2022 16:14:43 +0800
>>> Baolin Wang <baolin.wang at 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 at 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);
>> 	}
>>
>> }
> 
> OK, but wouldn't the pteval be overwritten here with
> pteval = swp_entry_to_pte(make_hwpoison_entry(subpage))?
> IOW, what sense does it make to save the returned pteval from
> huge_ptep_clear_flush(), when it is never being used anywhere?

Please see previous code, we'll use the original pte value to check if 
it is uffd-wp armed, and if need to mark it dirty though the hugetlbfs 
is set noop_dirty_folio().

pte_install_uffd_wp_if_needed(vma, address, pvmw.pte, pteval);

/* Set the dirty flag on the folio now the pte is gone. */
if (pte_dirty(pteval))
	folio_mark_dirty(folio);