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@xxxxxxxxxxxxxxx>
> > Fixes: be1db4753ee6 ("mm/memory.c: add apply_to_existing_page_range() helper")
> > Cc: Daniel Axtens <dja@xxxxxxxxxx>
> > ---
> >   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@xxxxxxxxxx>

Thanks!

-- 
  Kiryl Shutsemau / Kirill A. Shutemov