ZEN: INTERACTIVE: mm/swap: Disable swap-in readahead

Per an [issue][1] on the chromium project, swap-in readahead causes more
jank than not.  This might be caused by poor optimization on the
swapping code, or the fact under memory pressure, we're pulling in pages
we don't need, causing more swapping.

Either way, this is mainline/upstream to Chromium, and ChromeOS
developers care a lot about system responsiveness. Lets implement the
same change so Zen Kernel users benefit.

[1]: https://bugs.chromium.org/p/chromium/issues/detail?id=263561

Signed-off-by: Kai Krakow <kai@kaishome.de>

Author: damentz

init/Kconfig

@@ -178,6 +178,7 @@ config ZEN_INTERACTIVE
 	    Compact unevictable............:   yes  ->   no
 	    Watermark boost factor.........:   1.5  ->   0
 	    Pageblock order................:  10    ->   3
+	    Swap-in readahead..............:   3    ->   0
 
 	  --- EEVDF CPU Scheduler --------------------------------
 

mm/swap.c

@@ -1080,6 +1080,10 @@ void folio_batch_remove_exceptionals(struct folio_batch *fbatch)
  */
 void __init swap_setup(void)
 {
+#ifdef CONFIG_ZEN_INTERACTIVE
+	/* Only swap-in pages requested, avoid readahead */
+	page_cluster = 0;
+#else
 	unsigned long megs = totalram_pages() >> (20 - PAGE_SHIFT);
 
 	/* Use a smaller cluster for small-memory machines */
@@ -1091,4 +1095,5 @@ void __init swap_setup(void)
 	 * Right now other parts of the system means that we
 	 * _really_ don't want to cluster much more
 	 */
+#endif
 }