Re: [PATCH v4 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when unmapping

[David Hildenbrand <david@redhat.com>]

On 11.05.22 14:04, Baolin Wang 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 try_to_unmap() is only passed a hugetlb page in the case
> where the hugetlb page is poisoned. 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.
> 
> We've already used set_huge_swap_pte_at() to set a poisoned
> swap entry for a poisoned hugetlb page. Meanwhile adding a VM_BUG_ON()
> to make sure the passed hugetlb page is poisoned in try_to_unmap().
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>  mm/rmap.c | 39 ++++++++++++++++++++++-----------------
>  1 file changed, 22 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 4e96daf..219e287 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1528,6 +1528,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  
>  		if (folio_test_hugetlb(folio)) {
>  			/*
> +			 * The try_to_unmap() is only passed a hugetlb page
> +			 * in the case where the hugetlb page is poisoned.
> +			 */
> +			VM_BUG_ON_PAGE(!PageHWPoison(subpage), subpage);
> +			/*
>  			 * huge_pmd_unshare may unmap an entire PMD page.
>  			 * There is no way of knowing exactly which PMDs may
>  			 * be cached for this mm, so we must flush them all.
> @@ -1562,28 +1567,28 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  					break;
>  				}
>  			}
> +			pteval = huge_ptep_clear_flush(vma, address, pvmw.pte);
>  		} else {
>  			flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
> -		}
> -
> -		/*
> -		 * Nuke the page table entry. When having to clear
> -		 * PageAnonExclusive(), we always have to flush.
> -		 */
> -		if (should_defer_flush(mm, flags) && !anon_exclusive) {
>  			/*
> -			 * We clear the PTE but do not flush so potentially
> -			 * a remote CPU could still be writing to the folio.
> -			 * If the entry was previously clean then the
> -			 * architecture must guarantee that a clear->dirty
> -			 * transition on a cached TLB entry is written through
> -			 * and traps if the PTE is unmapped.
> +			 * Nuke the page table entry. When having to clear
> +			 * PageAnonExclusive(), we always have to flush.
>  			 */
> -			pteval = ptep_get_and_clear(mm, address, pvmw.pte);
> +			if (should_defer_flush(mm, flags) && !anon_exclusive) {
> +				/*
> +				 * We clear the PTE but do not flush so potentially
> +				 * a remote CPU could still be writing to the folio.
> +				 * If the entry was previously clean then the
> +				 * architecture must guarantee that a clear->dirty
> +				 * transition on a cached TLB entry is written through
> +				 * and traps if the PTE is unmapped.
> +				 */
> +				pteval = ptep_get_and_clear(mm, address, pvmw.pte);
>  
> -			set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
> -		} else {
> -			pteval = ptep_clear_flush(vma, address, pvmw.pte);
> +				set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
> +			} else {
> +				pteval = ptep_clear_flush(vma, address, pvmw.pte);
> +			}
>  		}
>  
>  		/*

LGTM

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb