Re: [PATCH] mm: fix is_pinnable_page against on cma page

[Minchan Kim <minchan@kernel.org>]

On Mon, May 02, 2022 at 08:02:31PM +0200, David Hildenbrand wrote:
> On 02.05.22 19:35, Minchan Kim wrote:
> > Pages on CMA area could have MIGRATE_ISOLATE as well as MIGRATE_CMA
> > so current is_pinnable_page could miss CMA pages which has MIGRATE_
> > ISOLATE. It ends up putting CMA pages longterm pinning possible on
> > pin_user_pages APIs so CMA allocation fails.
> > 
> > The CMA allocation path protects the migration type change race
> > using zone->lock but what GUP path need to know is just whether the
> > page is on CMA area or not rather than exact type. Thus, we don't
> > need zone->lock but just checks the migratype in either of
> > (MIGRATE_ISOLATE and MIGRATE_CMA).
> > 
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > ---
> >  include/linux/mm.h | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 6acca5cecbc5..f59bbe3296e3 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1625,8 +1625,10 @@ static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma,
> >  #ifdef CONFIG_MIGRATION
> >  static inline bool is_pinnable_page(struct page *page)
> >  {
> > -	return !(is_zone_movable_page(page) || is_migrate_cma_page(page)) ||
> > -		is_zero_pfn(page_to_pfn(page));
> > +	int mt = get_pageblock_migratetype(page);
> > +
> > +	return !(is_zone_movable_page(page) || mt == MIGRATE_CMA ||
> > +		mt == MIGRATE_ISOLATE || is_zero_pfn(page_to_pfn(page)));
> >  }
> >  #else
> >  static inline bool is_pinnable_page(struct page *page)
> 
> That implies that other memory ranges that are currently isolated
> (memory offlining, alloc_contig_range()) cannot be pinned. That might
> not be a bad thing, however, I think we could end up failing to pin
> something that's temporarily unmovable (due to temporary references).

Sure.

> 
> However, I assume we have the same issue right now already with
> ZONE_MOVABLE and MIGRATE_CMA when trying to pin a page residing on these

ZONE_MOVALBE is also changed dynamically?

> there are temporarily unmovable and we fail to migrate. But it would now
> apply even without ZONE_MOVABLE or MIGRATE_CMA. Hm...

Didn't parse your last mention.