Re: [PATCH v3 2/6] mm: gup: clarify a reason to check pte/pmd in gup_fast_pte_range()

[Yeoreum Yun <yeoreum.yun@arm.com>]

On Mon, Nov 09, 2026 at 10:50:32PM -0500, Rik van Riel wrote:
> On Mon, 2026-11-09 at 12:32 +0000, Yeoreum Yun wrote:
> > Clarify a reason to check pte/pmd in gup_fast_pte_range().
> > 
> > Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> > ---
> >  mm/gup.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 7d43ce717b246..7f9348c633d58 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2881,6 +2881,14 @@ static unsigned long gup_fast_pte_range(pmd_t
> > pmd, pmd_t *pmdp,
> >  		if (!folio)
> >  			goto pte_unmap;
> >  
> > +		/*
> > +		 * Recheck the PTE after grabbing the folio to
> > detect concurrent
> > +		 * unmapping or protection changes such as by KSM.
> > +		 *
> > +		 * Also, recheck the PMD: THP collpase can detach
> > the PTE table
> 
> collapse

Okay.

> 
> > +		 * while its entries remain unchaged. Checking the
> > PTE alone
> 
> unchanged

Acked.

> 
> > +		 * would not dtect that we are walking a detached
> > page table.
> 
> detect

Sorry for typos..


> 
> Other than that, Reviewed-by: Rik van Riel <riel@surriel.com>

Thanks ;)


> 
> > +		 */
> >  		if (unlikely(pmd_val(pmd) !=
> > pmd_val(pmdp_get_lockless(pmdp))) ||
> >  		    unlikely(pte_val(pte) !=
> > pte_val(ptep_get_lockless(ptep)))) {
> >  			gup_put_folio(folio, 1, flags);
> 
> -- 
> All Rights Reversed.

-- 
Sincerely,
Yeoreum Yun