Re: [External] Re: [PATCH] mm/page_isolation: do not isolate the max order page

[Muchun Song]

On Thu, Dec 3, 2020 at 8:03 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 12/2/20 1:21 PM, Muchun Song wrote:
> > The max order page has no buddy page and never merge to other order.
> > So isolating and then freeing it is pointless.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
>
> > ---
> >  mm/page_isolation.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> > index a254e1f370a3..bddf788f45bf 100644
> > --- a/mm/page_isolation.c
> > +++ b/mm/page_isolation.c
> > @@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
> >        */
> >       if (PageBuddy(page)) {
> >               order = buddy_order(page);
> > -             if (order >= pageblock_order) {
> > +             if (order >= pageblock_order && order < MAX_ORDER - 1) {
> >                       pfn = page_to_pfn(page);
> >                       buddy_pfn = __find_buddy_pfn(pfn, order);
> >                       buddy = page + (buddy_pfn - pfn);
>
> Hm I wonder if order == MAX_ORDER - 1, then the buddy can actually be a
> !pfn_valid() in some corner case? pfn_valid_within(buddy_pfn) that follows would
> only catch it on archs with holes in zone. Then is_migrate_isolate_page(buddy)
> might access an invalid buddy. So this might be actually a bug fix and not just
> optimization, just the bug hasn't been observed in practice.

Agree. Should we add a Fixes tag in the commit log? Thanks.

>
> >
>


-- 
Yours,
Muchun