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SubjectRe: [PATCH] mm: page_alloc: High-order per-cpu page allocator v4
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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. 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()?

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;

This might result in pcp->count being higher than actual pages. That one would
be introduced by 479f854a207c ("mm, page_alloc: defer debugging checks of pages
allocated from the PCP").

>
> - __free_one_page(page, page_to_pfn(page), zone, 0, mt);
> - trace_mm_page_pcpu_drain(page, 0, mt);
> - } while (--count && --batch_free && !list_empty(list));
> + __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);
> + pcp->count -= nr_freed;
> }
>
> static void free_one_page(struct zone *zone,

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