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

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On 2022/3/7 13:37, Huang, Ying wrote:
> 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.
> 
> 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). So I can't paste the err log. And The
issues from this series mainly come from the code investigating with some tests.

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@xxxxxxxxxx>
>> ---
>>  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);
> .
> 





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