bpf: Avoid setting bpf insns pages read-only when prog is jited

For the case where the interpreter is compiled out or when the prog is jited
it is completely unnecessary to set the BPF insn pages as read-only. In fact,
on frequent churn of BPF programs, it could lead to performance degradation of
the system over time since it would break the direct map down to 4k pages when
calling set_memory_ro() for the insn buffer on x86-64 / arm64 and there is no
reverse operation. Thus, avoid breaking up large pages for data maps, and only
limit this to the module range used by the JIT where it is necessary to set
the image read-only and executable.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191129222911.3710-1-daniel@iogearbox.net

include/linux/filter.h

@@ -776,8 +776,12 @@ bpf_ctx_narrow_access_offset(u32 off, u32 size, u32 size_default)
 
 static inline void bpf_prog_lock_ro(struct bpf_prog *fp)
 {
-	set_vm_flush_reset_perms(fp);
-	set_memory_ro((unsigned long)fp, fp->pages);
+#ifndef CONFIG_BPF_JIT_ALWAYS_ON
+	if (!fp->jited) {
+		set_vm_flush_reset_perms(fp);
+		set_memory_ro((unsigned long)fp, fp->pages);
+	}
+#endif
 }
 
 static inline void bpf_jit_binary_lock_ro(struct bpf_binary_header *hdr)