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

[Kirill A. Shutemov]

On Thu, Jun 03, 2021 at 07:54:11PM -0700, Hugh Dickins wrote:
> 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)).

Oh.. It made me scratch head too. Could you rewrite it in a more readable
way?

-- 
 Kirill A. Shutemov