On Thu, 3 Jun 2021, Peter Xu wrote: > On Tue, Jun 01, 2021 at 02:07:53PM -0700, Hugh Dickins wrote: > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > > index 2cf01d933f13..b45d22738b45 100644 > > --- a/mm/page_vma_mapped.c > > +++ b/mm/page_vma_mapped.c > > @@ -212,6 +212,14 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > > pvmw->ptl = NULL; > > } > > } else if (!pmd_present(pmde)) { > > + /* > > + * If PVMW_SYNC, take and drop THP pmd lock so that we > > + * cannot return prematurely, while zap_huge_pmd() has > > + * cleared *pmd but not decremented compound_mapcount(). > > + */ > > + if ((pvmw->flags & PVMW_SYNC) && > > + PageTransCompound(pvmw->page)) > > + spin_unlock(pmd_lock(mm, pvmw->pmd)); > > return false; > > } > > if (!map_pte(pvmw)) > > Sorry if I missed something important, but I'm totally confused on how this > unlock is pairing with another lock().. I imagine you're reading that as spin_unlock(pmd_lockptr(blah)); no, the lock is right there, inside spin_unlock(pmd_lock(blah)). > > And.. isn't PVMW_SYNC only meaningful for pte-level only (as I didn't see a > reference of it outside map_pte)? But you are pointing directly to its reference outside map_pte()! Hugh