[PATCH 2/2] mm/vmalloc: Add attempt_larger_order_alloc parameter

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

Introduce a module parameter to enable or disable the large-order
allocation path in vmalloc. High-order allocations are disabled by
default so far, but users may explicitly enable them at runtime if
desired.

High-order pages allocated for vmalloc are immediately split into
order-0 pages and later freed as order-0, which means they do not
feed the per-CPU page caches. As a result, high-order attempts tend
to bypass the PCP fastpath and fall back to the buddy allocator that
can affect performance.

However, when the PCP caches are empty, high-order allocations may
show better performance characteristics especially for larger
allocation requests.

Since the best strategy is workload-dependent, this patch adds a
parameter letting users to choose whether vmalloc should try
high-order allocations or stay strictly on the order-0 fastpath.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 mm/vmalloc.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d3a4725e15ca..f66543896b16 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -43,6 +43,7 @@
 #include <asm/tlbflush.h>
 #include <asm/shmparam.h>
 #include <linux/page_owner.h>
+#include <linux/moduleparam.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/vmalloc.h>
@@ -3671,6 +3672,9 @@ vm_area_alloc_pages_large_order(gfp_t gfp, int nid, unsigned int order,
 	return nr_allocated;
 }
 
+static int attempt_larger_order_alloc;
+module_param(attempt_larger_order_alloc, int, 0644);
+
 static inline unsigned int
 vm_area_alloc_pages(gfp_t gfp, int nid,
 		unsigned int order, unsigned int nr_pages, struct page **pages)
@@ -3679,8 +3683,9 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
 	struct page *page;
 	int i;
 
-	nr_allocated = vm_area_alloc_pages_large_order(gfp, nid,
-		order, nr_pages, pages);
+	if (attempt_larger_order_alloc)
+		nr_allocated = vm_area_alloc_pages_large_order(gfp, nid,
+			order, nr_pages, pages);
 
 	/*
 	 * For order-0 pages we make use of bulk allocator, if
-- 
2.47.3