linux-kernel - Re: [PATCH v3 2/3] mm/free_pcppages_bulk: do not hold lock when picking pages to free

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Date:   Mon, 26 Feb 2018 13:53:10 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Aaron Lu <aaron.lu@...el.com>
cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Kemi Wang <kemi.wang@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 2/3] mm/free_pcppages_bulk: do not hold lock when
 picking pages to free

On Mon, 26 Feb 2018, Aaron Lu wrote:

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3154859cccd6..35576da0a6c9 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1116,13 +1116,11 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  	int migratetype = 0;
>  	int batch_free = 0;
>  	bool isolated_pageblocks;
> +	struct page *page, *tmp;
> +	LIST_HEAD(head);
>  
>  	pcp->count -= count;
> -	spin_lock(&zone->lock);
> -	isolated_pageblocks = has_isolate_pageblock(zone);
> -
>  	while (count) {
> -		struct page *page;
>  		struct list_head *list;
>  
>  		/*
> @@ -1144,26 +1142,31 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  			batch_free = count;
>  
>  		do {
> -			int mt;	/* migratetype of the to-be-freed page */
> -
>  			page = list_last_entry(list, struct page, lru);
>  			/* must delete as __free_one_page list manipulates */

Looks good in general, but I'm not sure how I reconcile this comment with 
the new implementation that later links page->lru again.

>  			list_del(&page->lru);
>  
> -			mt = get_pcppage_migratetype(page);
> -			/* MIGRATE_ISOLATE page should not go to pcplists */
> -			VM_BUG_ON_PAGE(is_migrate_isolate(mt), page);
> -			/* Pageblock could have been isolated meanwhile */
> -			if (unlikely(isolated_pageblocks))
> -				mt = get_pageblock_migratetype(page);
> -
>  			if (bulkfree_pcp_prepare(page))
>  				continue;
>  
> -			__free_one_page(page, page_to_pfn(page), zone, 0, mt);
> -			trace_mm_page_pcpu_drain(page, 0, mt);
> +			list_add_tail(&page->lru, &head);
>  		} while (--count && --batch_free && !list_empty(list));
>  	}
> +
> +	spin_lock(&zone->lock);
> +	isolated_pageblocks = has_isolate_pageblock(zone);
> +
> +	list_for_each_entry_safe(page, tmp, &head, lru) {
> +		int mt = get_pcppage_migratetype(page);
> +		/* MIGRATE_ISOLATE page should not go to pcplists */
> +		VM_BUG_ON_PAGE(is_migrate_isolate(mt), page);
> +		/* Pageblock could have been isolated meanwhile */
> +		if (unlikely(isolated_pageblocks))
> +			mt = get_pageblock_migratetype(page);
> +
> +		__free_one_page(page, page_to_pfn(page), zone, 0, mt);
> +		trace_mm_page_pcpu_drain(page, 0, mt);
> +	}
>  	spin_unlock(&zone->lock);
>  }
>