Re: [PATCH 2/7] mm/thp: try_to_unmap() use TTU_SYNC for safe DEBUG_VM splitting

[Hugh Dickins]

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