Re: [PATCH] mm: Avoid calling folio_page() with an out-of-bounds index

[Li Zhe]

On Wed, 25 Feb 2026 14:40:41 +0100 david@xxxxxxxxxx wrote:

> On 2/25/26 10:26, Li Zhe wrote:
> > In folio_zero_user(), the page pointer is calculated via folio_page()
> > before checking if the number of pages to be cleared is greater than zero.
> > Furthermore, folio_page() does not verify that the page number lies
> > within folio.
> > 
> > When 'addr_hint' is near the end of a large folio, the range 'r[0]'
> > represents an empty interval. In this scenario, 'nr_pages' will be
> > calculated as 0 and 'r[0].start' can be an index that is out-of-bounds
> > for folio_page(). The code unconditionally calls folio_page() on a wrong
> > index, even though the subsequent clearing logic is correctly skipped.
> > 
> > While this does not cause a functional bug today, calculating a page
> > pointer for an out-of-bounds index is logically unsound and fragile. It
> > could pose a risk for future refactoring or trigger warnings from static
> > analysis tools.
> > 
> > To fix this, move the call to folio_page() inside the 'if (nr_pages > 0)'
> > block. This ensures that the page pointer is only calculated when it is
> > actually needed for a valid, non-empty range of pages, thus making the code
> > more robust and logically correct.
> > 
> > Signed-off-by: Li Zhe <lizhe.67@xxxxxxxxxxxxx>
> > ---
> >  mm/memory.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 07778814b4a8..6f8c55d604b5 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -7343,12 +7343,14 @@ void folio_zero_user(struct folio *folio, unsigned long addr_hint)
> >  	r[0] = DEFINE_RANGE(r[2].end + 1, pg.end);
> >  
> >  	for (i = 0; i < ARRAY_SIZE(r); i++) {
> > -		const unsigned long addr = base_addr + r[i].start * PAGE_SIZE;
> >  		const long nr_pages = (long)range_len(&r[i]);
> > -		struct page *page = folio_page(folio, r[i].start);
> >  
> > -		if (nr_pages > 0)
> > +		if (nr_pages > 0) {
> > +			const unsigned long addr = base_addr + r[i].start * PAGE_SIZE;
> > +			struct page *page = folio_page(folio, r[i].start);
> > +
> >  			clear_contig_highpages(page, addr, nr_pages);
> > +		}
> >  	}
> >  }
> >  
> 
> It's all just arithmetic operations. Just like Willy says, I'd assume
> that the compiler can just move them around as it pleases. No dependencies.
> 
> So I don't see any real benefit with this patch.

Yes, this change does not offer any performance benefits under the
current conditions. I proposed it primarily as a code cleanup effort.

> > While this does not cause a functional bug today, calculating a page
> > pointer for an out-of-bounds index is logically unsound and fragile. It
> > could pose a risk for future refactoring or trigger warnings from static
> > analysis tools.

This is what I see as the primary benefit of this patch. Should an
out-of-bounds issue surface later, it would still need to be addressed.

Thanks,
Zhe