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

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

On 5/3/2022 6:03 PM, Gerald Schaefer wrote:
> On Tue, 3 May 2022 10:19:46 +0800
> Baolin Wang <baolin.wang@linux.alibaba.com> wrote:
> 
>>
>>
>> On 5/2/2022 10:02 PM, Gerald Schaefer wrote:
>>> On Sat, 30 Apr 2022 11:22:33 +0800
>>> Baolin Wang <baolin.wang@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@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);
>>>> 	}
>>>>
>>>> }
>>>
>>> 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);
> 
> Uh, ok, that wouldn't work on s390, but we also don't have
> CONFIG_PTE_MARKER_UFFD_WP / HAVE_ARCH_USERFAULTFD_WP set, so
> I guess we will be fine (for now).

OK.

> 
> Still, I find it a bit unsettling that pte_install_uffd_wp_if_needed()
> would work on a potential hugetlb *pte, directly de-referencing it
> instead of using huge_ptep_get().
> 
> The !pte_none(*pte) check at the beginning would be broken in the
> hugetlb case for s390 (not sure about other archs, but I think s390
> might be the only exception strictly requiring huge_ptep_get()
> for de-referencing hugetlb *pte pointers).

Right, I think so too. I'll look at the uffd code in detail, seems need 
another patch to fix the hugetlb for uffd. Thanks for your comments.