Re: [RFC PATCH] mm: correct status code which move_pages() returns for zero page

[Li Wang <liwang@redhat.com>]

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On Tue, Apr 17, 2018 at 10:14 PM, Michal Hocko <mhocko@suse.com> wrote:

> On Tue 17-04-18 15:03:00, Michal Hocko wrote:
> > On Tue 17-04-18 19:06:15, Li Wang wrote:
> > [...]
> > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > index f65dd69..2b315fc 100644
> > > --- a/mm/migrate.c
> > > +++ b/mm/migrate.c
> > > @@ -1608,7 +1608,7 @@ static int do_pages_move(struct mm_struct *mm,
> nodemask_t task_nodes,
> > >                     continue;
> > >
> > >             err = store_status(status, i, err, 1);
> > > -           if (err)
> > > +           if (!err)
> > >                     goto out_flush;
> >
> > This change just doesn't make any sense to me. Why should we bail out if
> > the store_status is successul? I am trying to wrap my head around the
> > test case. 6b9d757ecafc ("mm, numa: rework do_pages_move") tried to
> > explain that move_pages has some semantic issues and the new
> > implementation might be not 100% replacement. Anyway I am studying the
> > test case to come up with a proper fix.
>
> OK, I get what the test cases does. I've failed to see the subtle
> difference between alloc_pages_on_node and numa_alloc_onnode. The later
> doesn't faul in anything.
>
> Why are we getting EPERM is quite not yet clear to me.
> add_page_for_migration uses FOLL_DUMP which should return EFAULT on
> zero pages (no_page_table()).
>
>         err = PTR_ERR(page);
>         if (IS_ERR(page))
>                 goto out;
>
> therefore bails out from add_page_for_migration and store_status should
> store that value. There shouldn't be any EPERM on the way.
>

Yes, I print the the return value and confirmed the
add_page_for_migration()​
do right things for zero page. and after store_status(...) the status saves
-EFAULT.
So I did the change above.



>
> Let me try to reproduce and see what is going on. Btw. which kernel do
> you try this on?
>

​The latest mainline kernel-4.17-rc1.



-- 
Li Wang
liwang@redhat.com

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