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

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Date:   Tue, 28 Aug 2018 18:10:43 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Zi Yan <zi.yan@...rutgers.edu>
Cc:     Jerome Glisse <jglisse@...hat.com>, 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 11:54:33, Zi Yan wrote:
> Hi Michal,
> 
> On 28 Aug 2018, at 11:45, Michal Hocko wrote:
> 
> > 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.
> 
> is_migration_entry() is a general check for swp_entry_t, so it can return
> true even if THP migration is not enabled. is_pmd_migration_entry() always
> returns false when THP migration is not enabled.
> 
> So the code can be changed in two ways, either replacing is_migration_entry()
> with is_pmd_migration_entry() or adding thp_migration_supported() check
> like Jerome did.
> 
> Does this clarify your question?

Not really. IIUC the code checks for the pmd. So even though
is_migration_entry is a more generic check it should never return true
for thp_migration_supported() == F because we simply never have those
unless I am missing something.

is_pmd_migration_entry is much more readable of course and I suspect it
can save few cycles as well.
-- 
Michal Hocko
SUSE Labs