mm: reimplement si_mem_available()

Reimplement si_mem_available() by reusing si_mem_available_node().

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>

mm/page_alloc.c

@@ -5775,46 +5775,15 @@ long si_mem_available_node(struct sysinfo *val, int nid)
 
 long si_mem_available(void)
 {
-	long available;
-	unsigned long pagecache;
-	unsigned long wmark_low = 0;
-	unsigned long pages[NR_LRU_LISTS];
-	unsigned long reclaimable;
-	struct zone *zone;
-	int lru;
-
-	for (lru = LRU_BASE; lru < NR_LRU_LISTS; lru++)
-		pages[lru] = global_node_page_state(NR_LRU_BASE + lru);
-
-	for_each_zone(zone)
-		wmark_low += low_wmark_pages(zone);
-
-	/*
-	 * Estimate the amount of memory available for userspace allocations,
-	 * without causing swapping.
-	 */
-	available = global_zone_page_state(NR_FREE_PAGES) - totalreserve_pages;
-
-	/*
-	 * Not all the page cache can be freed, otherwise the system will
-	 * start swapping. Assume at least half of the page cache, or the
-	 * low watermark worth of cache, needs to stay.
-	 */
-	pagecache = pages[LRU_ACTIVE_FILE] + pages[LRU_INACTIVE_FILE];
-	pagecache -= min(pagecache / 2, wmark_low);
-	available += pagecache;
+	long available = 0;
+	struct sysinfo i;
+	int nid;
 
-	/*
-	 * Part of the reclaimable slab and other kernel memory consists of
-	 * items that are in use, and cannot be freed. Cap this estimate at the
-	 * low watermark.
-	 */
-	reclaimable = global_node_page_state_pages(NR_SLAB_RECLAIMABLE_B) +
-		global_node_page_state(NR_KERNEL_MISC_RECLAIMABLE);
-	available += reclaimable - min(reclaimable / 2, wmark_low);
+	for_each_online_node(nid) {
+		si_meminfo_node(&i, nid);
+		available += si_mem_available_node(&i, nid);
+	}
 
-	if (available < 0)
-		available = 0;
 	return available;
 }
 EXPORT_SYMBOL_GPL(si_mem_available);