[PATCH v4 bpf-next 15/15] bpf: Introduce sysctl kernel.bpf_force_dyn_alloc.

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

From: Alexei Starovoitov <ast@kernel.org>

Introduce sysctl kernel.bpf_force_dyn_alloc to force dynamic allocation in bpf
hash map. All selftests/bpf should pass with bpf_force_dyn_alloc 0 or 1 and all
bpf programs (both sleepable and not) should not see any functional difference.
The sysctl's observable behavior should only be improved memory usage.

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 include/linux/filter.h | 2 ++
 kernel/bpf/core.c      | 2 ++
 kernel/bpf/hashtab.c   | 5 +++++
 kernel/bpf/syscall.c   | 9 +++++++++
 4 files changed, 18 insertions(+)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index a5f21dc3c432..eb4d4a0c0bde 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1009,6 +1009,8 @@ bpf_run_sk_reuseport(struct sock_reuseport *reuse, struct sock *sk,
 }
 #endif
 
+extern int bpf_force_dyn_alloc;
+
 #ifdef CONFIG_BPF_JIT
 extern int bpf_jit_enable;
 extern int bpf_jit_harden;
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 639437f36928..a13e78ea4b90 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -533,6 +533,8 @@ void bpf_prog_kallsyms_del_all(struct bpf_prog *fp)
 	bpf_prog_kallsyms_del(fp);
 }
 
+int bpf_force_dyn_alloc __read_mostly;
+
 #ifdef CONFIG_BPF_JIT
 /* All BPF JIT sysctl knobs here. */
 int bpf_jit_enable   __read_mostly = IS_BUILTIN(CONFIG_BPF_JIT_DEFAULT_ON);
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 89f26cbddef5..f68a3400939e 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -505,6 +505,11 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
 
 	bpf_map_init_from_attr(&htab->map, attr);
 
+	if (!lru && bpf_force_dyn_alloc) {
+		prealloc = false;
+		htab->map.map_flags |= BPF_F_NO_PREALLOC;
+	}
+
 	if (percpu_lru) {
 		/* ensure each CPU's lru list has >=1 elements.
 		 * since we are at it, make each lru list has the same
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 074c901fbb4e..5c631244b63b 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -5299,6 +5299,15 @@ static struct ctl_table bpf_syscall_table[] = {
 		.mode		= 0644,
 		.proc_handler	= bpf_stats_handler,
 	},
+	{
+		.procname	= "bpf_force_dyn_alloc",
+		.data		= &bpf_force_dyn_alloc,
+		.maxlen		= sizeof(int),
+		.mode		= 0600,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
+	},
 	{ }
 };
 
-- 
2.30.2