Re: [PATCH v5 4/4] mm/page_alloc: restrict max order of merging on isolated pageblock

[Joonsoo Kim]

On Fri, Oct 31, 2014 at 03:39:13PM +0100, Vlastimil Babka wrote:
> 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 :)

Sure.

> 
> >-	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.

Okay.

> >+				__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?

Okay. I will fix it.

> 
> >  	__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?

After __isolate_free_page(), freepage has no buddy flag and
move_freepages_block() doesn't move and count it. So, freepage_state
only increase in __free_pages(). So net result will be correct.

Below is the update for your comment.

Thanks.

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