Re: [PATCH 16/16] mm/migration: fix potential pte_unmap on an not mapped pte

[Alistair Popple]

Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:

> __migration_entry_wait and migration_entry_wait_on_locked assume pte is
> always mapped from caller. But this is not the case when it's called from
> migration_entry_wait_huge and follow_huge_pmd. And a parameter unmap to
> indicate whether pte needs to be unmapped to fix this issue.

[...]

> diff --git a/mm/filemap.c b/mm/filemap.c
> index 8f7e6088ee2a..18c353d52aae 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1389,6 +1389,7 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,
>   * @ptep: mapped pte pointer. Will return with the ptep unmapped. Only required
>   *        for pte entries, pass NULL for pmd entries.
>   * @ptl: already locked ptl. This function will drop the lock.
> + * @unmap: indicating whether ptep need to be unmapped.
>   *
>   * Wait for a migration entry referencing the given page to be removed. This is
>   * equivalent to put_and_wait_on_page_locked(page, TASK_UNINTERRUPTIBLE) except
> @@ -1402,7 +1403,7 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,
>   * there.
>   */
>  void migration_entry_wait_on_locked(swp_entry_t entry, pte_t *ptep,
> -				spinlock_t *ptl)
> +				spinlock_t *ptl, bool unmap)

It's a pity we have to pass unmap all the way down to
migration_entry_wait_on_locked().

>  {
>  	struct wait_page_queue wait_page;
>  	wait_queue_entry_t *wait = &wait_page.wait;
> @@ -1439,10 +1440,9 @@ void migration_entry_wait_on_locked(swp_entry_t entry, pte_t *ptep,
>  	 * a valid reference to the page, and it must take the ptl to remove the
>  	 * migration entry. So the page is valid until the ptl is dropped.
>  	 */
> -	if (ptep)
> -		pte_unmap_unlock(ptep, ptl);
> -	else
> -		spin_unlock(ptl);
> +	spin_unlock(ptl);
> +	if (unmap && ptep)
> +		pte_unmap(ptep);

However we might not have to - afaict this is the only usage of ptep so callers
could do the pte_unmap() prior to calling migration_entry_wait_on_locked(). We
could then remove both the `ptep` and `unmap` parameters. Ie:

migration_entry_wait_on_locked(swp_entry_t entry, spinlock_t *ptl)

This does assume it's ok to change the order of pte unmap/ptl unlock operations.
I'm not terribly familiar with CONFIG_HIGHPTE systems, but it seems like that
should be ok.

- Alistair

>
>  	for (;;) {
>  		unsigned int flags;
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 07668781c246..8088128c25db 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6713,7 +6713,7 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
>  	} else {
>  		if (is_hugetlb_entry_migration(pte)) {
>  			spin_unlock(ptl);
> -			__migration_entry_wait(mm, (pte_t *)pmd, ptl);
> +			__migration_entry_wait(mm, (pte_t *)pmd, ptl, false);
>  			goto retry;
>  		}
>  		/*
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 98a968e6f465..5519261f54fe 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -281,7 +281,7 @@ void remove_migration_ptes(struct folio *src, struct folio *dst, bool locked)
>   * When we return from this function the fault will be retried.
>   */
>  void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
> -				spinlock_t *ptl)
> +				spinlock_t *ptl, bool unmap)
>  {
>  	pte_t pte;
>  	swp_entry_t entry;
> @@ -295,10 +295,12 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
>  	if (!is_migration_entry(entry))
>  		goto out;
>
> -	migration_entry_wait_on_locked(entry, ptep, ptl);
> +	migration_entry_wait_on_locked(entry, ptep, ptl, unmap);
>  	return;
>  out:
> -	pte_unmap_unlock(ptep, ptl);
> +	spin_unlock(ptl);
> +	if (unmap)
> +		pte_unmap(ptep);
>  }
>
>  void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
> @@ -306,14 +308,14 @@ void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
>  {
>  	spinlock_t *ptl = pte_lockptr(mm, pmd);
>  	pte_t *ptep = pte_offset_map(pmd, address);
> -	__migration_entry_wait(mm, ptep, ptl);
> +	__migration_entry_wait(mm, ptep, ptl, true);
>  }
>
>  void migration_entry_wait_huge(struct vm_area_struct *vma,
>  		struct mm_struct *mm, pte_t *pte)
>  {
>  	spinlock_t *ptl = huge_pte_lockptr(hstate_vma(vma), mm, pte);
> -	__migration_entry_wait(mm, pte, ptl);
> +	__migration_entry_wait(mm, pte, ptl, false);
>  }
>
>  #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> @@ -324,7 +326,7 @@ void pmd_migration_entry_wait(struct mm_struct *mm, pmd_t *pmd)
>  	ptl = pmd_lock(mm, pmd);
>  	if (!is_pmd_migration_entry(*pmd))
>  		goto unlock;
> -	migration_entry_wait_on_locked(pmd_to_swp_entry(*pmd), NULL, ptl);
> +	migration_entry_wait_on_locked(pmd_to_swp_entry(*pmd), NULL, ptl, false);
>  	return;
>  unlock:
>  	spin_unlock(ptl);