mm: refactor si_mem_available()

si_mem_available() needlessly places LRU statistics into an array before
retrieving only two of them, simply access those directly.

In addition, refactor the code so that the blocks of code which calculate
the page cache and reclaimable components each resemble one another to
clearly indicate we cap both against wmark_low in the same fashion.

Link: https://lkml.kernel.org/r/20230827110848.43510-1-lstoakes@gmail.comSigned-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/show_mem.c

@@ -34,13 +34,8 @@ 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);
@@ -56,7 +51,8 @@ long si_mem_available(void)
 	 * start swapping or thrashing. 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 = global_node_page_state(NR_ACTIVE_FILE) +
+		global_node_page_state(NR_INACTIVE_FILE);
 	pagecache -= min(pagecache / 2, wmark_low);
 	available += pagecache;
 
@@ -67,7 +63,8 @@ long si_mem_available(void)
 	 */
 	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);
+	reclaimable -= min(reclaimable / 2, wmark_low);
+	available += reclaimable;
 
 	if (available < 0)
 		available = 0;