linux-kernel - Re: [PATCH 5/6] mm/page_alloc: Protect PCP lists with a spinlock

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Message-ID: <20220429090511.GA3441@techsingularity.net>
Date:   Fri, 29 Apr 2022 10:05:11 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 5/6] mm/page_alloc: Protect PCP lists with a spinlock

On Tue, Apr 26, 2022 at 12:24:56PM -0700, Minchan Kim wrote:
> > @@ -3450,10 +3496,19 @@ void free_unref_page(struct page *page, unsigned int order)
> >  void free_unref_page_list(struct list_head *list)
> >  {
> >  	struct page *page, *next;
> > +	struct per_cpu_pages *pcp;
> > +	struct zone *locked_zone;
> >  	unsigned long flags;
> >  	int batch_count = 0;
> >  	int migratetype;
> >  
> > +	/*
> > +	 * An empty list is possible. Check early so that the later
> > +	 * lru_to_page() does not potentially read garbage.
> > +	 */
> > +	if (list_empty(list))
> > +		return;
> > +
> >  	/* Prepare pages for freeing */
> >  	list_for_each_entry_safe(page, next, list, lru) {
> >  		unsigned long pfn = page_to_pfn(page);
> > @@ -3474,8 +3529,26 @@ void free_unref_page_list(struct list_head *list)
> >  		}
> >  	}
> >  
> > +	VM_BUG_ON(in_hardirq());
> 
> You need to check the list_empty here again and bail out if it is.
> 

You're right, every page could have failed to prepare or was isolated.

-- 
Mel Gorman
SUSE Labs