linux-kernel - Re: [PATCH 4/7] mm/hmm: properly handle migration pmd

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Date:   Tue, 28 Aug 2018 17:45:55 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Zi Yan <zi.yan@...rutgers.edu>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH 4/7] mm/hmm: properly handle migration pmd

On Tue 28-08-18 17:42:06, Michal Hocko wrote:
> On Tue 28-08-18 11:36:59, Jerome Glisse wrote:
> > On Tue, Aug 28, 2018 at 05:24:14PM +0200, Michal Hocko wrote:
> > > On Fri 24-08-18 20:05:46, Zi Yan wrote:
> > > [...]
> > > > > +	if (!pmd_present(pmd)) {
> > > > > +		swp_entry_t entry = pmd_to_swp_entry(pmd);
> > > > > +
> > > > > +		if (is_migration_entry(entry)) {
> > > > 
> > > > I think you should check thp_migration_supported() here, since PMD migration is only enabled in x86_64 systems.
> > > > Other architectures should treat PMD migration entries as bad.
> > > 
> > > How can we have a migration pmd entry when the migration is not
> > > supported?
> > 
> > Not sure i follow here, migration can happen anywhere (assuming
> > that something like compaction is active or numa or ...). So this
> > code can face pmd migration entry on architecture that support
> > it. What is missing here is thp_migration_supported() call to
> > protect the is_migration_entry() to avoid false positive on arch
> > which do not support thp migration.
> 
> I mean that architectures which do not support THP migration shouldn't
> ever see any migration entry. So is_migration_entry should be always
> false. Or do I miss something?

And just to be clear. thp_migration_supported should be checked only
when we actually _do_ the migration or evaluate migratability of the
page. We definitely do want to sprinkle this check to all places where
is_migration_entry is checked.
-- 
Michal Hocko
SUSE Labs