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

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Date:   Wed, 22 Nov 2017 14:40:59 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Zi Yan <zi.yan@...t.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Zi Yan <zi.yan@...rutgers.edu>,
        Andrea Reale <ar@...ux.vnet.ibm.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] mm: migrate: fix an incorrect call of
 prep_transhuge_page()

On Wed 22-11-17 09:54:16, Michal Hocko wrote:
> On Mon 20-11-17 21:18:55, Zi Yan wrote:
[...]
> > diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> > index 895ec0c4942e..a2246cf670ba 100644
> > --- a/include/linux/migrate.h
> > +++ b/include/linux/migrate.h
> > @@ -54,7 +54,7 @@ static inline struct page *new_page_nodemask(struct page *page,
> >  	new_page = __alloc_pages_nodemask(gfp_mask, order,
> >  				preferred_nid, nodemask);
> >  
> > -	if (new_page && PageTransHuge(page))
> > +	if (new_page && PageTransHuge(new_page))
> >  		prep_transhuge_page(new_page);
> 
> I would keep the two checks consistent. But that leads to a more
> interesting question. new_page_nodemask does
> 
> 	if (thp_migration_supported() && PageTransHuge(page)) {
> 		order = HPAGE_PMD_ORDER;
> 		gfp_mask |= GFP_TRANSHUGE;
> 	}

And one more question/note. Why do we need thp_migration_supported
in the first place? 9c670ea37947 ("mm: thp: introduce
CONFIG_ARCH_ENABLE_THP_MIGRATION") says
: Introduce CONFIG_ARCH_ENABLE_THP_MIGRATION to limit thp migration
: functionality to x86_64, which should be safer at the first step.

but why is unsafe to enable the feature on other arches which support
THP? Is there any plan to do the next step and remove this config
option?
-- 
Michal Hocko
SUSE Labs