Re: [v2 PATCH] mm: migrate: handle freed page at the first place

[Yang Shi]

On 11/14/19 10:56 AM, Michal Hocko wrote:
> On Fri 15-11-19 02:24:29, Yang Shi wrote:
> > When doing migration if the freed page is met, we just return without
> > migrating it since it is pointless to migrate a freed page.  But, the
> > current code allocates target page unconditionally before handling freed
> > page, if the page is freed, the newly allocated will be just freed.  It
> > doesn't make too much sense and is just a waste of time although
> > migrating freed page is rare.
> >
> > So, handle freed page at the before that to avoid unnecessary page
> > allocation and free.
> >
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> I would be really surprised if this led to any runtime visible effect
> but I do agree that one less put_page path looks slightly better. For
> that reason
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> > ---
> > v2: * Keep thp migration support check before handling freed page per Michal Hocko
> >      * Fixed the build warning reported by 0-day
> >
> >   mm/migrate.c | 14 +++++---------
> >   1 file changed, 5 insertions(+), 9 deletions(-)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 4fe45d1..a8f87cb 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1168,15 +1168,11 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
> >   				   enum migrate_reason reason)
> >   {
> >   	int rc = MIGRATEPAGE_SUCCESS;
> > -	struct page *newpage;
> > +	struct page *newpage = NULL;
> >   
> >   	if (!thp_migration_supported() && PageTransHuge(page))
> >   		return -ENOMEM;
> >   
> > -	newpage = get_new_page(page, private);
> > -	if (!newpage)
> > -		return -ENOMEM;
> > -
> >   	if (page_count(page) == 1) {
> >   		/* page was freed from under us. So we are done. */
> >   		ClearPageActive(page);
> > @@ -1187,13 +1183,13 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
> >   				__ClearPageIsolated(page);
> >   			unlock_page(page);
> >   		}
> > -		if (put_new_page)
> > -			put_new_page(newpage, private);
> > -		else
> > -			put_page(newpage);
> >   		goto out;
> >   	}
> >   
> > +	newpage = get_new_page(page, private);
> > +	if (!newpage)
> > +		return -ENOMEM;
> > +
> >   	rc = __unmap_and_move(page, newpage, force, mode);
> >   	if (rc == MIGRATEPAGE_SUCCESS)
> >   		set_page_owner_migrate_reason(newpage, reason);
> > --
> > 1.8.3.1