Re: [PATCH 06/10] mm: vmscan: demote anon DRAM pages to PMEM node

[Keith Busch]

On Sat, Mar 23, 2019 at 12:44:31PM +0800, Yang Shi wrote:
>  		/*
> +		 * Demote DRAM pages regardless the mempolicy.
> +		 * Demot anonymous pages only for now and skip MADV_FREE
> +		 * pages.
> +		 */
> +		if (PageAnon(page) && !PageSwapCache(page) &&
> +		    (node_isset(page_to_nid(page), def_alloc_nodemask)) &&
> +		    PageSwapBacked(page)) {
> +
> +			if (has_nonram_online()) {
> +				list_add(&page->lru, &demote_pages);
> +				unlock_page(page);
> +				continue;
> +			}
> +		}
> +
> +		/*
>  		 * Anonymous process memory has backing store?
>  		 * Try to allocate it some swap space here.
>  		 * Lazyfree page could be freed directly
> @@ -1477,6 +1507,25 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
>  	}
>  
> +	/* Demote pages to PMEM */
> +	if (!list_empty(&demote_pages)) {
> +		int err, target_nid;
> +		nodemask_t used_mask;
> +
> +		nodes_clear(used_mask);
> +		target_nid = find_next_best_node(pgdat->node_id, &used_mask,
> +						 true);
> +
> +		err = migrate_pages(&demote_pages, alloc_new_node_page, NULL,
> +				    target_nid, MIGRATE_ASYNC, MR_DEMOTE);
> +
> +		if (err) {
> +			putback_movable_pages(&demote_pages);
> +
> +			list_splice(&ret_pages, &demote_pages);
> +		}
> +	}
> +
>  	mem_cgroup_uncharge_list(&free_pages);
>  	try_to_unmap_flush();
>  	free_unref_page_list(&free_pages);

How do these pages eventually get to swap when migration fails? Looks
like that's skipped.

And page cache demotion is useful too, we shouldn't consider only
anonymous for this feature.