bpf: Introduce any context BPF specific memory allocator.

Tracing BPF programs can attach to kprobe and fentry. Hence they
run in unknown context where calling plain kmalloc() might not be safe.

Front-end kmalloc() with minimal per-cpu cache of free elements.
Refill this cache asynchronously from irq_work.

BPF programs always run with migration disabled.
It's safe to allocate from cache of the current cpu with irqs disabled.
Free-ing is always done into bucket of the current cpu as well.
irq_work trims extra free elements from buckets with kfree
and refills them with kmalloc, so global kmalloc logic takes care
of freeing objects allocated by one cpu and freed on another.

struct bpf_mem_alloc supports two modes:
- When size != 0 create kmem_cache and bpf_mem_cache for each cpu.
  This is typical bpf hash map use case when all elements have equal size.
- When size == 0 allocate 11 bpf_mem_cache-s for each cpu, then rely on
  kmalloc/kfree. Max allocation size is 4096 in this case.
  This is bpf_dynptr and bpf_kptr use case.

bpf_mem_alloc/bpf_mem_free are bpf specific 'wrappers' of kmalloc/kfree.
bpf_mem_cache_alloc/bpf_mem_cache_free are 'wrappers' of kmem_cache_alloc/kmem_cache_free.

The allocators are NMI-safe from bpf programs only. They are not NMI-safe in general.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf_mem_alloc.h

@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
+#ifndef _BPF_MEM_ALLOC_H
+#define _BPF_MEM_ALLOC_H
+#include <linux/compiler_types.h>
+
+struct bpf_mem_cache;
+struct bpf_mem_caches;
+
+struct bpf_mem_alloc {
+	struct bpf_mem_caches __percpu *caches;
+	struct bpf_mem_cache __percpu *cache;
+};
+
+int bpf_mem_alloc_init(struct bpf_mem_alloc *ma, int size);
+void bpf_mem_alloc_destroy(struct bpf_mem_alloc *ma);
+
+/* kmalloc/kfree equivalent: */
+void *bpf_mem_alloc(struct bpf_mem_alloc *ma, size_t size);
+void bpf_mem_free(struct bpf_mem_alloc *ma, void *ptr);
+
+/* kmem_cache_alloc/free equivalent: */
+void *bpf_mem_cache_alloc(struct bpf_mem_alloc *ma);
+void bpf_mem_cache_free(struct bpf_mem_alloc *ma, void *ptr);
+
+#endif /* _BPF_MEM_ALLOC_H */

kernel/bpf/Makefile

@@ -13,7 +13,7 @@ obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o bpf_task_storage.o
 obj-${CONFIG_BPF_LSM}	  += bpf_inode_storage.o
 obj-$(CONFIG_BPF_SYSCALL) += disasm.o
 obj-$(CONFIG_BPF_JIT) += trampoline.o
-obj-$(CONFIG_BPF_SYSCALL) += btf.o
+obj-$(CONFIG_BPF_SYSCALL) += btf.o memalloc.o
 obj-$(CONFIG_BPF_JIT) += dispatcher.o
 ifeq ($(CONFIG_NET),y)
 obj-$(CONFIG_BPF_SYSCALL) += devmap.o