Re: [PATCH] mm/migrate.c: also overwrite error when it is bigger than zero

[Yang Shi <shy828301@gmail.com>]

On Fri, Jan 17, 2020 at 3:30 PM Yang Shi <shy828301@gmail.com> wrote:
>
> On Fri, Jan 17, 2020 at 2:27 PM Wei Yang <richardw.yang@linux.intel.com> wrote:
> >
> > On Fri, Jan 17, 2020 at 03:45:34PM +0800, Wei Yang wrote:
> > >If we get here after successfully adding page to list, err would be
> > >the number of pages in the list.
> > >
> > >Current code has two problems:
> > >
> > >  * on success, 0 is not returned
> > >  * on error, the real error code is not returned
> > >
> >
> > Well, this breaks the user interface. User would receive 1 even the migration
> > succeed.
> >
> > The change is introduced by e0153fc2c760 ("mm: move_pages: return valid node
> > id in status if the page is already on the target node").
>
> Yes, it may return a value which is > 0. But, it seems do_pages_move()
> could return > 0 value even before this commit.
>
> For example, if I read the code correctly, it would do:
>
> If we already have some pages on the queue then
> add_page_for_migration() return error, then do_move_pages_to_node() is
> called, but it may return > 0 value (the number of pages that were
> *not* migrated by migrate_pages()), then the code flow would just jump
> to "out" and return the value. And, it may happen to be 1.

Just figured out this is another regression introduced by a49bd4d71637
("mm, numa: rework do_pages_move"). Already posted a patch to fix it.

>
> I'm not sure if it breaks the user interface since the behavior has
> been existed for years, and it looks nobody complains about it. Maybe
> glibc helps hide it or people just care if it is 0 and the status.
>
> >
> > >Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> > >---
> > > mm/migrate.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > >diff --git a/mm/migrate.c b/mm/migrate.c
> > >index 557da996b936..c3ef70de5876 100644
> > >--- a/mm/migrate.c
> > >+++ b/mm/migrate.c
> > >@@ -1677,7 +1677,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
> > >       err1 = do_move_pages_to_node(mm, &pagelist, current_node);
> > >       if (!err1)
> > >               err1 = store_status(status, start, current_node, i - start);
> > >-      if (!err)
> > >+      if (err >= 0)
> > >               err = err1;
> > > out:
> > >       return err;
> > >--
> > >2.17.1
> >
> > --
> > Wei Yang
> > Help you, Help me
> >