On Tue, Apr 14, 2020 at 01:32:34PM -0300, Jason Gunthorpe wrote:
> On Tue, Apr 14, 2020 at 05:10:01PM +0200, Alexander Gordeev wrote:
> > Commit 0005d20 ("mm/gup: Move page table entry dereference
> > into helper function") wrapped access to page table entries
> > larger than sizeof(long) into a race-aware accessor. One of
> > the two dereferences in gup_fast path was however overlooked.
> >
> > CC: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > CC: linux-mm@xxxxxxxxx
> > Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
> > mm/gup.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index d53f7dd..eceb98b 100644
> > +++ b/mm/gup.c
> > @@ -2208,7 +2208,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> > if (!head)
> > goto pte_unmap;
> >
> > - if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> > + if (unlikely(pte_val(pte) != pte_val(gup_get_pte(ptep)))) {
>
> It doesn't seem like this needs the special helper as it is just
> checking that the pte hasn't changed, it doesn't need to be read
> exactly.
>
> But it probably should technically still be a READ_ONCE. Although I
> think the atomic inside try_grab_compound_head prevents any real
> problems.
I think we should go for consistency here and use the helper function.
Ira
>
> Jason
>
