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

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Date:   Thu, 3 Dec 2020 16:49:24 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [External] Re: [PATCH] mm/page_isolation: do not isolate the max
 order page

On 12/3/20 3:43 AM, Muchun Song wrote:
> On Thu, Dec 3, 2020 at 8:03 AM Vlastimil Babka <vbabka@...e.cz> 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@...edance.com>
>>
>> Acked-by: Vlastimil Babka <vbabka@...e.cz>
>>
>> > ---
>> >  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.

Right.

Fixes: 3c605096d315 ("mm/page_alloc: restrict max order of merging on isolated
pageblock")

The criteria for CC stable is not met though as it's theoretical.

>>
>> >
>>
> 
>