linux-kernel - Re: [PATCH] mm: page_alloc: High-order per-cpu page allocator v4

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Date:   Thu, 1 Dec 2016 14:24:29 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: page_alloc: High-order per-cpu page allocator v4

On Thu, Dec 01, 2016 at 02:41:29PM +0100, Vlastimil Babka wrote:
> On 12/01/2016 01:24 AM, Mel Gorman wrote:
> 
> ...
> 
> > @@ -1096,28 +1097,29 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> >  	if (nr_scanned)
> >  		__mod_node_page_state(zone->zone_pgdat, NR_PAGES_SCANNED, -nr_scanned);
> > 
> > -	while (count) {
> > +	while (count > 0) {
> >  		struct page *page;
> >  		struct list_head *list;
> > +		unsigned int order;
> > 
> >  		/*
> >  		 * Remove pages from lists in a round-robin fashion. A
> >  		 * batch_free count is maintained that is incremented when an
> > -		 * empty list is encountered.  This is so more pages are freed
> > -		 * off fuller lists instead of spinning excessively around empty
> > -		 * lists
> > +		 * empty list is encountered. This is not exact due to
> > +		 * high-order but percision is not required.
> >  		 */
> >  		do {
> >  			batch_free++;
> > -			if (++migratetype == MIGRATE_PCPTYPES)
> > -				migratetype = 0;
> > -			list = &pcp->lists[migratetype];
> > +			if (++pindex == NR_PCP_LISTS)
> > +				pindex = 0;
> > +			list = &pcp->lists[pindex];
> >  		} while (list_empty(list));
> > 
> >  		/* This is the only non-empty list. Free them all. */
> > -		if (batch_free == MIGRATE_PCPTYPES)
> > +		if (batch_free == NR_PCP_LISTS)
> >  			batch_free = count;
> > 
> > +		order = pindex_to_order(pindex);
> >  		do {
> >  			int mt;	/* migratetype of the to-be-freed page */
> > 
> > @@ -1135,11 +1137,14 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> >  			if (bulkfree_pcp_prepare(page))
> >  				continue;
> 
> Hmm I think that if this hits, we don't decrease count/increase nr_freed and
> pcp->count will become wrong.

Ok, I think you're right but I also think it's relatively trivial to fix
with

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 94808f565f74..8777aefc1b8e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1134,13 +1134,13 @@ static void free_pcppages_bulk(struct zone *zone, int count,
 			if (unlikely(isolated_pageblocks))
 				mt = get_pageblock_migratetype(page);
 
+			nr_freed += (1 << order);
+			count -= (1 << order);
 			if (bulkfree_pcp_prepare(page))
 				continue;
 
 			__free_one_page(page, page_to_pfn(page), zone, order, mt);
 			trace_mm_page_pcpu_drain(page, order, mt);
-			nr_freed += (1 << order);
-			count -= (1 << order);
 		} while (count > 0 && --batch_free && !list_empty(list));
 	}
 	spin_unlock(&zone->lock);

> And if we are unlucky/doing full drain, all
> lists will get empty, but as count stays e.g. 1, we loop forever on the
> outer while()?
> 

Potentially yes. Granted the system is already in a bad state as pages
are being freed in a bad or unknown state but we haven't halted the
system for that in the past.

> BTW, I think there's a similar problem (but not introduced by this patch) in
> rmqueue_bulk() and its
> 
>     if (unlikely(check_pcp_refill(page)))
>             continue;
> 

Potentially yes. It's outside the scope of this patch but it needs
fixing.

If you agree with the above fix, I'll roll it into a v5 and append
another patch for this issue.

-- 
Mel Gorman
SUSE Labs