Re: [PATCH v4 3/3 update] mm/free_pcppages_bulk: prefetch buddy while not holding lock

[Dave Hansen]

On 03/09/2018 12:24 AM, Aaron Lu wrote:
> +			/*
> +			 * We are going to put the page back to the global
> +			 * pool, prefetch its buddy to speed up later access
> +			 * under zone->lock. It is believed the overhead of
> +			 * an additional test and calculating buddy_pfn here
> +			 * can be offset by reduced memory latency later. To
> +			 * avoid excessive prefetching due to large count, only
> +			 * prefetch buddy for the last pcp->batch nr of pages.
> +			 */
> +			if (count > pcp->batch)
> +				continue;
> +			pfn = page_to_pfn(page);
> +			buddy_pfn = __find_buddy_pfn(pfn, 0);
> +			buddy = page + (buddy_pfn - pfn);
> +			prefetch(buddy);

FWIW, I think this needs to go into a helper function.  Is that possible?

There's too much logic happening here.  Also, 'count' going from
batch_size->0 is totally non-obvious from the patch context.  It makes
this hunk look totally wrong by itself.