[PATCH] bpf: Try harder when allocating memory for maps

[Martynas Pumputis <m@lambda.lt>]

It has been observed that sometimes memory allocation for BPF maps
fails when there is no obvious memory pressure in a system.

E.g. the map (BPF_MAP_TYPE_LRU_HASH, key=38, value=56, max_elems=524288)
could not be created due to due to vmalloc unable to allocate 75497472B,
when the system's memory consumption (in MB) was the following:

    Total: 3942 Used: 837 (21.24%) Free: 138 Buffers: 239 Cached: 2727

Considering dcda9b0471 ("mm, tree wide: replace __GFP_REPEAT by
__GFP_RETRY_MAYFAIL with more useful semantic") we can replace
__GFP_NORETRY with __GFP_RETRY_MAYFAIL, as it won't invoke OOM killer
and will try harder to fulfil allocation requests.

The change has been tested with the workloads mentioned above and by
observing oom_kill value from /proc/vmstat.

Signed-off-by: Martynas Pumputis <m@lambda.lt>
---
 kernel/bpf/syscall.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 62f6bced3a3c..eb5cefe44af3 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -136,11 +136,11 @@ static struct bpf_map *find_and_alloc_map(union bpf_attr *attr)
 
 void *bpf_map_area_alloc(size_t size, int numa_node)
 {
-	/* We definitely need __GFP_NORETRY, so OOM killer doesn't
-	 * trigger under memory pressure as we really just want to
-	 * fail instead.
+	/* We definitely need __GFP_NORETRY or __GFP_RETRY_MAYFAIL, so
+	 * OOM killer doesn't trigger under memory pressure as we really
+	 * just want to fail instead.
 	 */
-	const gfp_t flags = __GFP_NOWARN | __GFP_NORETRY | __GFP_ZERO;
+	const gfp_t flags = __GFP_NOWARN | __GFP_RETRY_MAYFAIL | __GFP_ZERO;
 	void *area;
 
 	if (size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER)) {
-- 
2.21.0