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

[Yang Shi]

On 11/12/19 12:04 AM, Michal Hocko wrote:
> On Tue 12-11-19 06:09:25, 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 did two things before handling freed page:
> >
> > 1. Return -ENOMEM if the page is THP and THP migration is not supported.
> > 2. Allocate target page unconditionally.
> >
> > Both makes not too much sense.  If we handle freed page at the first place
> > we don't have to worry about allocating/freeing target page and split
> > THP at all.
> >
> > For example (worst case) if we are trying to migrate a freed THP without
> > THP migration supported, the migrate_pages() would just split the THP then
> > retry to migrate base pages one by one by pointless allocating and freeing
> > pages, this is just waste of time.
> >
> > I didn't run into any actual problem with the current code (or I may
> > just not notice it yet), it was found by visual inspection.
> It would be preferable to accompany a change like this with some actual
> numbers. A race with page freeing should be a very rare situation. Maybe
> it is not under some workloads but that would better be checked and
> documented. I also do not like to do page state changes for THP
> migration without a support. I cannot really say this is 100% correct

However that THP will not be migrated actually, just will be freed by 
put_page().

> from top of my head and I do not see a sufficient justification to go
> and chase all those tiny details because that is time consuming.
>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/migrate.c | 18 +++++++-----------
> >   1 file changed, 7 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 4fe45d1..ef96997 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1170,13 +1170,6 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
> >   	int rc = MIGRATEPAGE_SUCCESS;
> >   	struct page *newpage;
> >   
> > -	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 +1180,16 @@ 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;
> >   	}
> >   
> > +	if (!thp_migration_supported() && PageTransHuge(page))
> > +		return -ENOMEM;
> > +
> > +	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