Subject: Re: [PATCH v2 07/14] mm: khugepaged: collapse_pte_mapped_thp() use pte_offset_map_rw_nolock()

From: Muchun Song <[email-masked]>
Date: Thu, 29 Aug 2024 16:10:22 +0800

On 2024/8/22 15:13, Qi Zheng wrote:
> In collapse_pte_mapped_thp(), we may modify the pte and pmd entry after
> acquring the ptl, so convert it to using pte_offset_map_rw_nolock(). At
> this time, the write lock of mmap_lock is not held, and the pte_same()
> check is not performed after the PTL held. So we should get pgt_pmd and do
> pmd_same() check after the ptl held.
>
> For the case where the ptl is released first and then the pml is acquired,
> the PTE page may have been freed, so we must do pmd_same() check before
> reacquiring the ptl.
>
> Signed-off-by: Qi Zheng <[email-masked]>
> ---
>   mm/khugepaged.c | 16 +++++++++++++++-
>   1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 53bfa7f4b7f82..15d3f7f3c65f2 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1604,7 +1604,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>   	if (userfaultfd_armed(vma) && !(vma->vm_flags & VM_SHARED))
>   		pml = pmd_lock(mm, pmd);
>   
> -	start_pte = pte_offset_map_nolock(mm, pmd, haddr, &ptl);
> +	start_pte = pte_offset_map_rw_nolock(mm, pmd, haddr, &pgt_pmd, &ptl);
>   	if (!start_pte)		/* mmap_lock + page lock should prevent this */
>   		goto abort;
>   	if (!pml)
> @@ -1612,6 +1612,9 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>   	else if (ptl != pml)
>   		spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
>   
> +	if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd))))
> +		goto abort;
> +
>   	/* step 2: clear page table and adjust rmap */
>   	for (i = 0, addr = haddr, pte = start_pte;
>   	     i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {
> @@ -1657,6 +1660,16 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>   	/* step 4: remove empty page table */
>   	if (!pml) {
>   		pml = pmd_lock(mm, pmd);
> +		/*
> +		 * We called pte_unmap() and release the ptl before acquiring
> +		 * the pml, which means we left the RCU critical section, so the
> +		 * PTE page may have been freed, so we must do pmd_same() check
> +		 * before reacquiring the ptl.
> +		 */
> +		if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) {
> +			spin_unlock(pml);
> +			goto pmd_change;

Seems we forget to flush TLB since we've cleared some pte entry?

> +		}
>   		if (ptl != pml)
>   			spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
>   	}
> @@ -1688,6 +1701,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>   		pte_unmap_unlock(start_pte, ptl);
>   	if (pml && pml != ptl)
>   		spin_unlock(pml);
> +pmd_change:
>   	if (notified)
>   		mmu_notifier_invalidate_range_end(&range);
>   drop_folio: