linux-kernel - Re: [PATCH v2 07/12] mm: thp: check pmd migration entry in common path

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Message-ID: <014b01d23b34$c7a71600$56f54200$@alibaba-inc.com>
Date:   Thu, 10 Nov 2016 17:28:20 +0800
From:   "Hillf Danton" <hillf.zj@...baba-inc.com>
To:     "'Naoya Horiguchi'" <n-horiguchi@...jp.nec.com>
Cc:     <linux-mm@...ck.org>,
        "'Kirill A. Shutemov'" <kirill.shutemov@...ux.intel.com>,
        "'Hugh Dickins'" <hughd@...gle.com>,
        "'Andrew Morton'" <akpm@...ux-foundation.org>,
        "'Dave Hansen'" <dave.hansen@...el.com>,
        "'Andrea Arcangeli'" <aarcange@...hat.com>,
        "'Mel Gorman'" <mgorman@...hsingularity.net>,
        "'Michal Hocko'" <mhocko@...nel.org>,
        "'Vlastimil Babka'" <vbabka@...e.cz>,
        "'Pavel Emelyanov'" <xemul@...allels.com>,
        "'Zi Yan'" <zi.yan@...rutgers.edu>,
        "'Balbir Singh'" <bsingharora@...il.com>,
        <linux-kernel@...r.kernel.org>,
        "'Naoya Horiguchi'" <nao.horiguchi@...il.com>,
        "'Anshuman Khandual'" <khandual@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 07/12] mm: thp: check pmd migration entry in common path

On Thursday, November 10, 2016 5:22 PM Naoya Horiguchi wrote:
> On Thu, Nov 10, 2016 at 05:08:07PM +0800, Hillf Danton wrote:
> > On Tuesday, November 08, 2016 7:32 AM Naoya Horiguchi wrote:
> > >
> > > @@ -1013,6 +1027,9 @@ int do_huge_pmd_wp_page(struct fault_env *fe, pmd_t orig_pmd)
> > >  	if (unlikely(!pmd_same(*fe->pmd, orig_pmd)))
> > >  		goto out_unlock;
> > >
> > > +	if (unlikely(!pmd_present(orig_pmd)))
> > > +		goto out_unlock;
> > > +
> >
> > Can we encounter a migration entry after acquiring ptl ?
> 
> I think we can. thp migration code releases ptl after converting pmd into
> migration entry, so other code can see it even within ptl.
> 
But we have a pmd_same check there, you see.