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:   Tue, 27 Feb 2018 11:17:07 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     David Rientjes <rientjes@...gle.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 Tue, Feb 27, 2018 at 10:00:58AM +0800, Aaron Lu wrote:
> On Mon, Feb 26, 2018 at 01:53:10PM -0800, David Rientjes wrote:
> > On Mon, 26 Feb 2018, Aaron Lu wrote:
> > 
> > > @@ -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.
> 
> Thanks for pointing this out.
> 
> I think the comment is useless now since there is a list_add_tail right
> below so it's obvious we need to take the page off its original list.
> I'll remove the comment in an update.

Thinking again, I think I'll change the comment to:

-			/* must delete as __free_one_page list manipulates */
+			/* must delete to avoid corrupting pcp list */
 			list_del(&page->lru);
 			pcp->count--;
 
Meanwhile, I'll add one more comment about why list_for_each_entry_safe
is used:

+	/*
+	 * Use safe version since after __free_one_page(),
+	 * page->lru.next will not point to original list.
+	 */
+	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);
 }