linux-kernel - Re: [PATCH] mm: migrate: fix an incorrect call of prep_transhuge

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Message-Id: <20171122094333.GA24826@samekh>
Date:   Wed, 22 Nov 2017 09:43:34 +0000
From:   Andrea Reale <ar@...ux.vnet.ibm.com>
To:     Zi Yan <zi.yan@...rutgers.edu>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] mm: migrate: fix an incorrect call of
 prep_transhuge_page()

On Tue 21 Nov 2017, 17:35, Zi Yan wrote:
> On 21 Nov 2017, at 17:12, Andrew Morton wrote:
> 
> > On Mon, 20 Nov 2017 21:18:55 -0500 Zi Yan <zi.yan@...t.com> wrote:
> >
> >> This patch fixes it by only calling prep_transhuge_page() when we are
> >> certain that the target page is THP.
> >
> > What are the user-visible effects of the bug?
> 
> By inspecting the code, if called on a non-THP, prep_transhuge_page() will
> 1) change the value of the mapping of (page + 2), since it is used for THP deferred list;
> 2) change the lru value of (page + 1), since it is used for THP’s dtor.
> 
> Both can lead to data corruption of these two pages.

Pragmatically and from the point of view of the memory_hotplug subsys,
the effect is a kernel crash when pages are being migrated during a memory
hot remove offline and migration target pages are found in a bad state.

Best,
Andrea