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

From: Gerald Schaefer <[email-masked]>
Date: Mon, 2 May 2022 16:02:32 +0200

On Sat, 30 Apr 2022 11:22:33 +0800
Baolin Wang <[email-masked]> wrote:

> 
> 
> On 4/30/2022 4:02 AM, Gerald Schaefer wrote:
> > On Fri, 29 Apr 2022 16:14:43 +0800
> > Baolin Wang <[email-masked]> 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 <[email-masked]>
> >> ---
> >>   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?