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SubjectRe: [PATCH v5 4/4] mm/page_alloc: restrict max order of merging on isolated pageblock
On 10/31/2014 08:25 AM, Joonsoo Kim wrote:
> @@ -571,6 +548,7 @@ static inline void __free_one_page(struct page *page,
> unsigned long combined_idx;
> unsigned long uninitialized_var(buddy_idx);
> struct page *buddy;
> + int max_order = MAX_ORDER;
>
> VM_BUG_ON(!zone_is_initialized(zone));
>
> @@ -579,15 +557,23 @@ static inline void __free_one_page(struct page *page,
> return;
>
> VM_BUG_ON(migratetype == -1);
> - if (!is_migrate_isolate(migratetype))
> + if (is_migrate_isolate(migratetype)) {
> + /*
> + * We restrict max order of merging to prevent merge
> + * between freepages on isolate pageblock and normal
> + * pageblock. Without this, pageblock isolation
> + * could cause incorrect freepage accounting.
> + */
> + max_order = min(MAX_ORDER, pageblock_order + 1);
> + } else
> __mod_zone_freepage_state(zone, 1 << order, migratetype);

Please add { } to the else branch, this looks ugly :)

> - page_idx = pfn & ((1 << MAX_ORDER) - 1);
> + page_idx = pfn & ((1 << max_order) - 1);
>
> VM_BUG_ON_PAGE(page_idx & ((1 << order) - 1), page);
> VM_BUG_ON_PAGE(bad_range(zone, page), page);
>
> - while (order < MAX_ORDER-1) {
> + while (order < max_order - 1) {
> buddy_idx = __find_buddy_index(page_idx, order);
> buddy = page + (buddy_idx - page_idx);
> if (!page_is_buddy(page, buddy, order))
> @@ -1590,7 +1576,7 @@ void split_page(struct page *page, unsigned int order)
> }
> EXPORT_SYMBOL_GPL(split_page);
>
> -static int __isolate_free_page(struct page *page, unsigned int order)
> +int __isolate_free_page(struct page *page, unsigned int order)
> {
> unsigned long watermark;
> struct zone *zone;
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 1fa4a4d..df61c93 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -76,17 +76,48 @@ void unset_migratetype_isolate(struct page *page, unsigned migratetype)
> {
> struct zone *zone;
> unsigned long flags, nr_pages;
> + struct page *isolated_page = NULL;
> + unsigned int order;
> + unsigned long page_idx, buddy_idx;
> + struct page *buddy;
> + int mt;
>
> zone = page_zone(page);
> spin_lock_irqsave(&zone->lock, flags);
> if (get_pageblock_migratetype(page) != MIGRATE_ISOLATE)
> goto out;
> +
> + /*
> + * Because freepage with more than pageblock_order on isolated
> + * pageblock is restricted to merge due to freepage counting problem,
> + * it is possible that there is free buddy page.
> + * move_freepages_block() doesn't care of merge so we need other
> + * approach in order to merge them. Isolation and free will make
> + * these pages to be merged.
> + */
> + if (PageBuddy(page)) {
> + order = page_order(page);
> + if (order >= pageblock_order) {
> + page_idx = page_to_pfn(page) & ((1 << MAX_ORDER) - 1);
> + buddy_idx = __find_buddy_index(page_idx, order);
> + buddy = page + (buddy_idx - page_idx);
> + mt = get_pageblock_migratetype(buddy);
> +
> + if (!is_migrate_isolate(mt)) {

You could use is_migrate_isolate_page(buddy) and save a variable.

> + __isolate_free_page(page, order);
> + set_page_refcounted(page);
> + isolated_page = page;
> + }
> + }
> + }
> nr_pages = move_freepages_block(zone, page, migratetype);

- this is a costly no-op when the whole pageblock is an isolated page,
right?

> __mod_zone_freepage_state(zone, nr_pages, migratetype);

- with isolated_page set, this means you increase freepage_state here,
and then the second time in __free_pages() below? __isolate_free_page()
won't decrease it as the pageblock is still MIGRATE_ISOLATE, so the net
result is overcounting?

> set_pageblock_migratetype(page, migratetype);
> zone->nr_isolate_pageblock--;
> out:
> spin_unlock_irqrestore(&zone->lock, flags);
> + if (isolated_page)
> + __free_pages(isolated_page, order);
> }
>
> static inline struct page *
>



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