On Fri, Dec 4, 2020 at 11:28 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 12/4/20 1:56 PM, Muchun Song wrote:
> > When we free a page whose order is very close to MAX_ORDER and greater
> > than pageblock_order, it wastes some CPU cycles to increase max_order
> > to MAX_ORDER one by one and check the pageblock migratetype of that page
> > repeatedly especially when MAX_ORDER is much larger than pageblock_order.
>
> I would add:
>
> We also should not be checking migratetype of buddy when "order == MAX_ORDER -
> 1" as the buddy pfn may be invalid, so adjust the condition. With the new check,
> we don't need the max_order check anymore, so we replace it.
>
> Also adjust max_order initialization so that it's lower by one than previously,
> which makes the code hopefully more clear.
Got it. Thanks.
>
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>
> Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other
> pageblocks")
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Thanks!
>
> > ---
> > Changes in v2:
> > - Rework the code suggested by Vlastimil. Thanks.
> >
> > mm/page_alloc.c | 8 ++++----
> > 1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index f91df593bf71..56e603eea1dd 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1002,7 +1002,7 @@ static inline void __free_one_page(struct page *page,
> > struct page *buddy;
> > bool to_tail;
> >
> > - max_order = min_t(unsigned int, MAX_ORDER, pageblock_order + 1);
> > + max_order = min_t(unsigned int, MAX_ORDER - 1, pageblock_order);
> >
> > VM_BUG_ON(!zone_is_initialized(zone));
> > VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
> > @@ -1015,7 +1015,7 @@ static inline void __free_one_page(struct page *page,
> > VM_BUG_ON_PAGE(bad_range(zone, page), page);
> >
> > continue_merging:
> > - while (order < max_order - 1) {
> > + while (order < max_order) {
> > if (compaction_capture(capc, page, order, migratetype)) {
> > __mod_zone_freepage_state(zone, -(1 << order),
> > migratetype);
> > @@ -1041,7 +1041,7 @@ static inline void __free_one_page(struct page *page,
> > pfn = combined_pfn;
> > order++;
> > }
> > - if (max_order < MAX_ORDER) {
> > + if (order < MAX_ORDER - 1) {
> > /* If we are here, it means order is >= pageblock_order.
> > * We want to prevent merge between freepages on isolate
> > * pageblock and normal pageblock. Without this, pageblock
> > @@ -1062,7 +1062,7 @@ static inline void __free_one_page(struct page *page,
> > is_migrate_isolate(buddy_mt)))
> > goto done_merging;
> > }
> > - max_order++;
> > + max_order = order + 1;
> > goto continue_merging;
> > }
> >
> >
>
--
Yours,
Muchun
