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

["Huang, Ying" <ying.huang@intel.com>]

Miaohe Lin <linmiaohe@huawei.com> writes:

> On 2022/3/7 13:37, Huang, Ying wrote:
>> Miaohe Lin <linmiaohe@huawei.com> 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.
>> 
>> This seems a possible issue.
>> 
>> Have you tested it?  It appears that it's possible to trigger the
>> issue.  If so, you can paste the error log here.
>
> This might happen iff on x86 machine with HIGHMEM enabled which is uncommon
> now (at least in my work environment).

Yes.  32-bit isn't popular now.  But you can always test it via virtual
machine.

> So I can't paste the err log. And The
> issues from this series mainly come from the code investigating with some tests.

If not too complex, I still think it's better to test your code and
verify the problem.

Best Regards,
Huang, Ying

> Thanks. :)
>
>> 
>> BTW: have you tested the other functionality issues in your patchset?
>> 
>> Best Regards,
>> Huang, Ying
>> 
>>> Fixes: 30dad30922cc ("mm: migration: add migrate_entry_wait_huge()")
>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>> ---
>>>  include/linux/migrate.h |  2 +-
>>>  include/linux/swapops.h |  4 ++--
>>>  mm/filemap.c            | 10 +++++-----
>>>  mm/hugetlb.c            |  2 +-
>>>  mm/migrate.c            | 14 ++++++++------
>>>  5 files changed, 17 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
>>> index 66a34eae8cb6..3ef4ff699bef 100644
>>> --- a/include/linux/migrate.h
>>> +++ b/include/linux/migrate.h
>>> @@ -41,7 +41,7 @@ extern int migrate_huge_page_move_mapping(struct address_space *mapping,
>>>  extern int migrate_page_move_mapping(struct address_space *mapping,
>>>  		struct page *newpage, struct page *page, int extra_count);
>>>  void migration_entry_wait_on_locked(swp_entry_t entry, pte_t *ptep,
>>> -				spinlock_t *ptl);
>>> +				spinlock_t *ptl, bool unmap);
>>>  void folio_migrate_flags(struct folio *newfolio, struct folio *folio);
>>>  void folio_migrate_copy(struct folio *newfolio, struct folio *folio);
>>>  int folio_migrate_mapping(struct address_space *mapping,
>>> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
>>> index d356ab4047f7..d66556875d7d 100644
>>> --- a/include/linux/swapops.h
>>> +++ b/include/linux/swapops.h
>>> @@ -213,7 +213,7 @@ static inline swp_entry_t make_writable_migration_entry(pgoff_t offset)
>>>  }
>>>  
>>>  extern void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
>>> -					spinlock_t *ptl);
>>> +					spinlock_t *ptl, bool unmap);
>>>  extern void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
>>>  					unsigned long address);
>>>  extern void migration_entry_wait_huge(struct vm_area_struct *vma,
>>> @@ -235,7 +235,7 @@ static inline int is_migration_entry(swp_entry_t swp)
>>>  }
>>>  
>>>  static inline void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
>>> -					spinlock_t *ptl) { }
>>> +					spinlock_t *ptl, bool unmap) { }
>>>  static inline void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
>>>  					 unsigned long address) { }
>>>  static inline void migration_entry_wait_huge(struct vm_area_struct *vma,
>>> 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)
>>>  {
>>>  	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);
>>>  
>>>  	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);
>> .
>>