Re: [PATCH] mm: migrate: fix an incorrect call of prep_transhuge_page()

[Andrea Reale]

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@xxxxxxxx> 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 THPâs 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