Re: [PATCH 04/10] mm/migrate: make migrate_pages() return nr_succeeded

[Yang Shi]

On Thu, Apr 8, 2021 at 3:14 AM Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> On Thu, Apr 01, 2021 at 11:32:23AM -0700, Dave Hansen wrote:
> >
> > From: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> >
> > The migrate_pages() returns the number of pages that were not migrated,
> > or an error code.  When returning an error code, there is no way to know
> > how many pages were migrated or not migrated.
> >
> > In the following patch, migrate_pages() is used to demote pages to PMEM
> > node, we need account how many pages are reclaimed (demoted) since page
> > reclaim behavior depends on this.  Add *nr_succeeded parameter to make
> > migrate_pages() return how many pages are demoted successfully for all
> > cases.
> >
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
> > Cc: Wei Xu <weixugc@xxxxxxxxxx>
> > Cc: Huang Ying <ying.huang@xxxxxxxxx>
> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > Cc: osalvador <osalvador@xxxxxxx>
> >
>
> ...
> >  int migrate_pages(struct list_head *from, new_page_t get_new_page,
> >               free_page_t put_new_page, unsigned long private,
> > -             enum migrate_mode mode, int reason)
> > +             enum migrate_mode mode, int reason, unsigned int *nr_succeeded)
> >  {
> >       int retry = 1;
> >       int thp_retry = 1;
> >       int nr_failed = 0;
> > -     int nr_succeeded = 0;
> >       int nr_thp_succeeded = 0;
> >       int nr_thp_failed = 0;
> >       int nr_thp_split = 0;
> > @@ -1611,10 +1611,10 @@ retry:
> >                       case MIGRATEPAGE_SUCCESS:
> >                               if (is_thp) {
> >                                       nr_thp_succeeded++;
> > -                                     nr_succeeded += nr_subpages;
> > +                                     *nr_succeeded += nr_subpages;
> >                                       break;
> >                               }
> > -                             nr_succeeded++;
> > +                             (*nr_succeeded)++;
> >                               break;
> >                       default:
> >                               /*
> > @@ -1643,12 +1643,12 @@ out:
> >        */
> >       list_splice(&ret_pages, from);
> >
> > -     count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
> > +     count_vm_events(PGMIGRATE_SUCCESS, *nr_succeeded);
> >       count_vm_events(PGMIGRATE_FAIL, nr_failed);
> >       count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);
> >       count_vm_events(THP_MIGRATION_FAIL, nr_thp_failed);
> >       count_vm_events(THP_MIGRATION_SPLIT, nr_thp_split);
> > -     trace_mm_migrate_pages(nr_succeeded, nr_failed, nr_thp_succeeded,
> > +     trace_mm_migrate_pages(*nr_succeeded, nr_failed, nr_thp_succeeded,
> >                              nr_thp_failed, nr_thp_split, mode, reason);
>
> It seems that reclaiming is the only user who cared about how many pages
> could we migrated, could not do the following instead:
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 695a594e5860..d4170b7ea2fe 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1503,7 +1503,7 @@ static inline int try_split_thp(struct page *page, struct page **page2,
>   */
>  int migrate_pages(struct list_head *from, new_page_t get_new_page,
>                 free_page_t put_new_page, unsigned long private,
> -               enum migrate_mode mode, int reason)
> +               enum migrate_mode mode, int reason, unsigned int *ret_succeeded)
>  {
>         int retry = 1;
>         int thp_retry = 1;
> @@ -1654,6 +1654,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>         if (!swapwrite)
>                 current->flags &= ~PF_SWAPWRITE;
>
> +       if (ret_succedded)
> +               *ret_succedded = nr_succedded;
> +
>         return rc;
>  }
>
>  And pass only a valid pointer from demote_page_list() and NULL from all
>  the others?
>  I was just wondered after all those "unsigned int nr_succedded" in all
>  other functions.
>  This would also solve the "be careful to initialize nr_succedded"
>  problem?

Thanks, Oscar. Yes, kind of. But we have to remember to initialize
"nr_succedded" pointer properly for every migrate_pages() callsite,
right? And it doesn't prevent from returning wrong value if
migrate_pages() is called multiple times by one caller although there
might be not such case (calls migrate_pages() multiple times and care
about nr_succeded) for now.

So IMHO I do prefer Wei's suggestion to have migrate_pages()
initialize nr_succeeded. This seems simpler.


>
>
> --
> Oscar Salvador
> SUSE L3