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Date:   Thu, 3 Jun 2021 19:54:11 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Peter Xu <peterx@redhat.com>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Yang Shi <shy828301@gmail.com>,
        Wang Yugui <wangyugui@e16-tech.com>,
        Matthew Wilcox <willy@infradead.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Alistair Popple <apopple@nvidia.com>,
        Ralph Campbell <rcampbell@nvidia.com>, Zi Yan <ziy@nvidia.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Minchan Kim <minchan@kernel.org>, Jue Wang <juew@google.com>,
        Jan Kara <jack@suse.cz>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/7] mm/thp: try_to_unmap() use TTU_SYNC for safe DEBUG_VM
 splitting
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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