Re: [PATCH] mm: fix apply_to_existing_page_range()

Re: [PATCH] mm: fix apply_to_existing_page_range()

[David Hildenbrand]

On 09.04.25 11:40, Kirill A. Shutemov wrote:
> In the case of apply_to_existing_page_range(), apply_to_pte_range() is
> reached with 'create' set to false. When !create, the loop over the PTE
> page table is broken.
> 
> apply_to_pte_range() will only move to the next PTE entry if 'create' is
> true or if the current entry is not pte_none().
> 
> This means that the user of apply_to_existing_page_range() will not have
> 'fn' called for any entries after the first pte_none() in the PTE page
> table.
> 
> Fix the loop logic in apply_to_pte_range().
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Fixes: be1db4753ee6 ("mm/memory.c: add apply_to_existing_page_range() helper")
> Cc: Daniel Axtens <dja@axtens.net>
> ---
>   mm/memory.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index fb7b8dc75167..2094564f4dfb 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2907,11 +2907,11 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
>   	if (fn) {
>   		do {
>   			if (create || !pte_none(ptep_get(pte))) {
> -				err = fn(pte++, addr, data);
> +				err = fn(pte, addr, data);
>   				if (err)
>   					break;
>   			}
> -		} while (addr += PAGE_SIZE, addr != end);
> +		} while (pte++, addr += PAGE_SIZE, addr != end);
>   	}
>   	*mask |= PGTBL_PTE_MODIFIED;
>   

LGTM. just curious, did you run into any actual issues that are worth 
describing?

It should affect apply_to_existing_page_range() users where 
create==false. There are not many, and likely most PTEs in the range 
they are passing are all non-none.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH] mm: fix apply_to_existing_page_range()

[Kirill A. Shutemov]

On Wed, Apr 09, 2025 at 11:52:43AM +0200, David Hildenbrand wrote:
> On 09.04.25 11:40, Kirill A. Shutemov wrote:
> > In the case of apply_to_existing_page_range(), apply_to_pte_range() is
> > reached with 'create' set to false. When !create, the loop over the PTE
> > page table is broken.
> > 
> > apply_to_pte_range() will only move to the next PTE entry if 'create' is
> > true or if the current entry is not pte_none().
> > 
> > This means that the user of apply_to_existing_page_range() will not have
> > 'fn' called for any entries after the first pte_none() in the PTE page
> > table.
> > 
> > Fix the loop logic in apply_to_pte_range().
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Fixes: be1db4753ee6 ("mm/memory.c: add apply_to_existing_page_range() helper")
> > Cc: Daniel Axtens <dja@axtens.net>
> > ---
> >   mm/memory.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index fb7b8dc75167..2094564f4dfb 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2907,11 +2907,11 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
> >   	if (fn) {
> >   		do {
> >   			if (create || !pte_none(ptep_get(pte))) {
> > -				err = fn(pte++, addr, data);
> > +				err = fn(pte, addr, data);
> >   				if (err)
> >   					break;
> >   			}
> > -		} while (addr += PAGE_SIZE, addr != end);
> > +		} while (pte++, addr += PAGE_SIZE, addr != end);
> >   	}
> >   	*mask |= PGTBL_PTE_MODIFIED;
> 
> LGTM. just curious, did you run into any actual issues that are worth
> describing?

I stepped on it in my non-upstream code debugging. I am not sure how it
affects existing users.

> It should affect apply_to_existing_page_range() users where create==false.
> There are not many, and likely most PTEs in the range they are passing are
> all non-none.

Or we just silently leak memory :P

> Acked-by: David Hildenbrand <david@redhat.com>

Thanks!

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

Re: [PATCH] mm: fix apply_to_existing_page_range()

[David Hildenbrand]

On 09.04.25 12:23, Kirill A. Shutemov wrote:
> On Wed, Apr 09, 2025 at 11:52:43AM +0200, David Hildenbrand wrote:
>> On 09.04.25 11:40, Kirill A. Shutemov wrote:
>>> In the case of apply_to_existing_page_range(), apply_to_pte_range() is
>>> reached with 'create' set to false. When !create, the loop over the PTE
>>> page table is broken.
>>>
>>> apply_to_pte_range() will only move to the next PTE entry if 'create' is
>>> true or if the current entry is not pte_none().
>>>
>>> This means that the user of apply_to_existing_page_range() will not have
>>> 'fn' called for any entries after the first pte_none() in the PTE page
>>> table.
>>>
>>> Fix the loop logic in apply_to_pte_range().
>>>
>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>> Fixes: be1db4753ee6 ("mm/memory.c: add apply_to_existing_page_range() helper")
>>> Cc: Daniel Axtens <dja@axtens.net>
>>> ---
>>>    mm/memory.c | 4 ++--
>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index fb7b8dc75167..2094564f4dfb 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -2907,11 +2907,11 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
>>>    	if (fn) {
>>>    		do {
>>>    			if (create || !pte_none(ptep_get(pte))) {
>>> -				err = fn(pte++, addr, data);
>>> +				err = fn(pte, addr, data);
>>>    				if (err)
>>>    					break;
>>>    			}
>>> -		} while (addr += PAGE_SIZE, addr != end);
>>> +		} while (pte++, addr += PAGE_SIZE, addr != end);
>>>    	}
>>>    	*mask |= PGTBL_PTE_MODIFIED;
>>
>> LGTM. just curious, did you run into any actual issues that are worth
>> describing?
> 
> I stepped on it in my non-upstream code debugging. I am not sure how it
> affects existing users.
> 
>> It should affect apply_to_existing_page_range() users where create==false.
>> There are not many, and likely most PTEs in the range they are passing are
>> all non-none.
> 
> Or we just silently leak memory :P

That's exactly what I am trying to figure out: is there something 
upstream that could actually run into this such that we should CC stable?

-- 
Cheers,

David / dhildenb

Re: [PATCH] mm: fix apply_to_existing_page_range()

[Kirill A. Shutemov]

On Wed, Apr 09, 2025 at 12:26:09PM +0200, David Hildenbrand wrote:
> > > It should affect apply_to_existing_page_range() users where create==false.
> > > There are not many, and likely most PTEs in the range they are passing are
> > > all non-none.
> > 
> > Or we just silently leak memory :P
> 
> That's exactly what I am trying to figure out: is there something upstream
> that could actually run into this such that we should CC stable?

From a quick glance, I don't see any of them to have a problem, but the
fix is trivial enough for stable@ even without a known buggy user.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov