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

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Message-Id: <20190311193112.25527-1-m@lambda.lt>
Date:   Mon, 11 Mar 2019 20:31:12 +0100
From:   Martynas Pumputis <m@...bda.lt>
To:     netdev@...r.kernel.org
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        mhocko@...e.com, m@...bda.lt
Subject: [PATCH] bpf: Try harder when allocating memory for large maps

It has been observed that sometimes a higher order 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 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

Later analysis [1] by Michal Hocko showed that the vmalloc was not trying
to reclaim memory from the page cache and was failing prematurely due to
__GFP_NORETRY.

Considering dcda9b0471 ("mm, tree wide: replace __GFP_REPEAT by
__GFP_RETRY_MAYFAIL with more useful semantic") and [1], 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.

[1]: https://lore.kernel.org/bpf/20190310071318.GW5232@dhcp22.suse.cz/

Signed-off-by: Martynas Pumputis <m@...bda.lt>
---
 kernel/bpf/syscall.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 62f6bced3a3c..1b0a057ed6d5 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -136,20 +136,26 @@ 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_ZERO;
 	void *area;

 	if (size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER)) {
-		area = kmalloc_node(size, GFP_USER | flags, numa_node);
+		/* To avoid bypassing slab alloc for lower order allocs,
+		 * __GFP_NORETRY is used instead of __GFP_RETRY_MAYFAIL.
+		 */
+		area = kmalloc_node(size, GFP_USER | __GFP_NORETRY | flags,
+				    numa_node);
 		if (area != NULL)
 			return area;
 	}

-	return __vmalloc_node_flags_caller(size, numa_node, GFP_KERNEL | flags,
+	return __vmalloc_node_flags_caller(size, numa_node,
+					   GFP_KERNEL | __GFP_RETRY_MAYFAIL |
+					   flags,
 					   __builtin_return_address(0));
 }

-- 
2.21.0