Re: [PATCH RFC v2 02/12] mm/hugetlb: Move swap entry handling into vma lock for fault

[Peter Xu]

On Thu, Nov 17, 2022 at 08:10:15PM -0500, Peter Xu wrote:
> In hugetlb_fault(), there used to have a special path to handle swap entry
> at the entrance using huge_pte_offset().  That's unsafe because
> huge_pte_offset() for a pmd sharable range can access freed pgtables if
> without either the walker lock or vma lock.
> 
> Here the simplest solution for making it safe is just to move the swap
> handling to be after the vma lock being held.  We may need to take the
> fault mutex on either migration or hwpoison entries now (also the vma lock,
> but that's really needed), however neither of them is hot path so it should
> be fine.
> 
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  mm/hugetlb.c | 24 +++++++-----------------
>  1 file changed, 7 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index c3aab6d5b7aa..62ff3fc51d4e 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5824,22 +5824,6 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  	int need_wait_lock = 0;
>  	unsigned long haddr = address & huge_page_mask(h);
>  
> -	ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
> -	if (ptep) {
> -		/*
> -		 * Since we hold no locks, ptep could be stale.  That is
> -		 * OK as we are only making decisions based on content and
> -		 * not actually modifying content here.
> -		 */
> -		entry = huge_ptep_get(ptep);
> -		if (unlikely(is_hugetlb_entry_migration(entry))) {
> -			migration_entry_wait_huge(vma, ptep);
> -			return 0;
> -		} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
> -			return VM_FAULT_HWPOISON_LARGE |
> -				VM_FAULT_SET_HINDEX(hstate_index(h));
> -	}
> -
>  	/*
>  	 * Serialize hugepage allocation and instantiation, so that we don't
>  	 * get spurious allocation failures if two CPUs race to instantiate
> @@ -5886,8 +5870,14 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  	 * fault, and is_hugetlb_entry_(migration|hwpoisoned) check will
>  	 * properly handle it.
>  	 */
> -	if (!pte_present(entry))
> +	if (!pte_present(entry)) {
> +		if (unlikely(is_hugetlb_entry_migration(entry)))
> +			migration_entry_wait_huge(vma, ptep);

Hmm no, need to release the vma lock and fault mutex.. So I remembered why
I had a note that I need to rework migration wait code..

I'll try that on next version, it would be a callback just to release the
proper locks in migration_entry_wait_huge() right after releasing the
pgtable lock, in e.g. migration_entry_wait_on_locked().

> +		else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
> +			ret = VM_FAULT_HWPOISON_LARGE |
> +			    VM_FAULT_SET_HINDEX(hstate_index(h));
>  		goto out_mutex;
> +	}
>  
>  	/*
>  	 * If we are going to COW/unshare the mapping later, we examine the
> -- 
> 2.37.3
> 

-- 
Peter Xu