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

[Yang Shi]

On 11/11/19 3:18 PM, Andrew Morton wrote:
> On Tue, 12 Nov 2019 06:09:25 +0800 Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> 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.
> >
> >
> > --- 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) {
> Is it possible to have (!thp_migration_supported() &&
> PageTransHuge(page) && page_count(page) == 1)?  If so, isn't this new
> behviour?

IMHO it should be possible on some architectures, i.e. aarch64, with 
anonymous THP. I just saw PowerPC and x86_64 have 
CONFIG_ARCH_ENABLE_THP_MIGRATION selected. I'm not quite sure if I miss 
something.

It should be not new behavior since migrate_pages() should just split 
the THP then retry with base pages one by one. Even though it returns 
-EBUSY due to THP split failure in the current code, the behavior sounds 
problematic. We should not return errno for a freed page, right?

> >   		/* 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);