path: root/kernel/bpf/memalloc.c

// SPDX-License-Identifier: GPL-2.0-only
/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
#include <linux/mm.h>
#include <linux/llist.h>
#include <linux/bpf.h>
#include <linux/irq_work.h>
#include <linux/bpf_mem_alloc.h>
#include <linux/memcontrol.h>
#include <asm/local.h>

/* Any context (including NMI) 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 per-cpu per-bucket cache of free elements.
 * Refill this cache asynchronously from irq_work.
 *
 * CPU_0 buckets
 * 16 32 64 96 128 196 256 512 1024 2048 4096
 * ...
 * CPU_N buckets
 * 16 32 64 96 128 196 256 512 1024 2048 4096
 *
 * The buckets are prefilled at the start.
 * 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.
 *
 * Every allocated objected is padded with extra 8 bytes that contains
 * struct llist_node.
 */
#define LLIST_NODE_SZ sizeof(struct llist_node)

/* similar to kmalloc, but sizeof == 8 bucket is gone */
static u8 size_index[24] __ro_after_init = {
	3,	/* 8 */
	3,	/* 16 */
	4,	/* 24 */
	4,	/* 32 */
	5,	/* 40 */
	5,	/* 48 */
	5,	/* 56 */
	5,	/* 64 */
	1,	/* 72 */
	1,	/* 80 */
	1,	/* 88 */
	1,	/* 96 */
	6,	/* 104 */
	6,	/* 112 */
	6,	/* 120 */
	6,	/* 128 */
	2,	/* 136 */
	2,	/* 144 */
	2,	/* 152 */
	2,	/* 160 */
	2,	/* 168 */
	2,	/* 176 */
	2,	/* 184 */
	2	/* 192 */
};

static int bpf_mem_cache_idx(size_t size)
{
	if (!size || size > 4096)
		return -1;

	if (size <= 192)
		return size_index[(size - 1) / 8] - 1;

	return fls(size - 1) - 2;
}

#define NUM_CACHES 11

struct bpf_mem_cache {
	/* per-cpu list of free objects of size 'unit_size'.
	 * All accesses are done with interrupts disabled and 'active' counter
	 * protection with __llist_add() and __llist_del_first().
	 */
	struct llist_head free_llist;
	local_t active;

	/* Operations on the free_list from unit_alloc/unit_free/bpf_mem_refill
	 * are sequenced by per-cpu 'active' counter. But unit_free() cannot
	 * fail. When 'active' is busy the unit_free() will add an object to
	 * free_llist_extra.
	 */
	struct llist_head free_llist_extra;

	struct irq_work refill_work;
	struct obj_cgroup *objcg;
	int unit_size;
	/* count of objects in free_llist */
	int free_cnt;
	int low_watermark, high_watermark, batch;
	int percpu_size;

	struct rcu_head rcu;
	struct llist_head free_by_rcu;
	struct llist_head waiting_for_gp;
	atomic_t call_rcu_in_progress;
};

struct bpf_mem_caches {
	struct bpf_mem_cache cache[NUM_CACHES];
};

static struct llist_node notrace *__llist_del_first(struct llist_head *head)
{
	struct llist_node *entry, *next;

	entry = head->first;
	if (!entry)
		return NULL;
	next = entry->next;
	head->first = next;
	return entry;
}

static void *__alloc(struct bpf_mem_cache *c, int node)
{
	/* Allocate, but don't deplete atomic reserves that typical
	 * GFP_ATOMIC would do. irq_work runs on this cpu and kmalloc
	 * will allocate from the current numa node which is what we
	 * want here.
	 */
	gfp_t flags = GFP_NOWAIT | __GFP_NOWARN | __GFP_ACCOUNT;

	if (c->percpu_size) {
		void **obj = kmalloc_node(c->percpu_size, flags, node);
		void *pptr = __alloc_percpu_gfp(c->unit_size, 8, flags);

		if (!obj || !pptr) {
			free_percpu(pptr);
			kfree(obj);
			return NULL;
		}
		obj[1] = pptr;
		return obj;
	}

	return kmalloc_node(c->unit_size, flags | __GFP_ZERO, node);
}

static struct mem_cgroup *get_memcg(const struct bpf_mem_cache *c)
{
#ifdef CONFIG_MEMCG_KMEM
	if (c->objcg)
		return get_mem_cgroup_from_objcg(c->objcg);
#endif

#ifdef CONFIG_MEMCG
	return root_mem_cgroup;
#else
	return NULL;
#endif
}

/* Mostly runs from irq_work except __init phase. */
static void alloc_bulk(struct bpf_mem_cache *c, int cnt, int node)
{
	struct mem_cgroup *memcg = NULL, *old_memcg;
	unsigned long flags;
	void *obj;
	int i;

	memcg = get_memcg(c);
	old_memcg = set_active_memcg(memcg);
	for (i = 0; i < cnt; i++) {
		/*
		 * free_by_rcu is only manipulated by irq work refill_work().
		 * IRQ works on the same CPU are called sequentially, so it is
		 * safe to use __llist_del_first() here. If alloc_bulk() is
		 * invoked by the initial prefill, there will be no running
		 * refill_work(), so __llist_del_first() is fine as well.
		 *
		 * In most cases, objects on free_by_rcu are from the same CPU.
		 * If some objects come from other CPUs, it doesn't incur any
		 * harm because NUMA_NO_NODE means the preference for current
		 * numa node and it is not a guarantee.
		 */
		obj = __llist_del_first(&c->free_by_rcu);
		if (!obj) {
			obj = __alloc(c, node);
			if (!obj)
				break;
		}
		if (IS_ENABLED(CONFIG_PREEMPT_RT))
			/* In RT irq_work runs in per-cpu kthread, so disable
			 * interrupts to avoid preemption and interrupts and
			 * reduce the chance of bpf prog executing on this cpu
			 * when active counter is busy.
			 */
			local_irq_save(flags);
		/* alloc_bulk runs from irq_work which will not preempt a bpf
		 * program that does unit_alloc/unit_free since IRQs are
		 * disabled there. There is no race to increment 'active'
		 * counter. It protects free_llist from corruption in case NMI
		 * bpf prog preempted this loop.
		 */
		WARN_ON_ONCE(local_inc_return(&c->active) != 1);
		__llist_add(obj, &c->free_llist);
		c->free_cnt++;
		local_dec(&c->active);
		if (IS_ENABLED(CONFIG_PREEMPT_RT))
			local_irq_restore(flags);
	}
	set_active_memcg(old_memcg);
	mem_cgroup_put(memcg);
}

static void free_one(struct bpf_mem_cache *c, void *obj)
{
	if (c->percpu_size) {
		free_percpu(((void **)obj)[1]);
		kfree(obj);
		return;
	}

	kfree(obj);
}

static void __free_rcu(struct rcu_head *head)
{
	struct bpf_mem_cache *c = container_of(head, struct bpf_mem_cache, rcu);
	struct llist_node *llnode = llist_del_all(&c->waiting_for_gp);
	struct llist_node *pos, *t;

	llist_for_each_safe(pos, t, llnode)
		free_one(c, pos);
	atomic_set(&c->call_rcu_in_progress, 0);
}

static void __free_rcu_tasks_trace(struct rcu_head *head)
{
	/* If RCU Tasks Trace grace period implies RCU grace period,
	 * there is no need to invoke call_rcu().
	 */
	if (rcu_trace_implies_rcu_gp())
		__free_rcu(head);
	else
		call_rcu(head, __free_rcu);
}

static void enque_to_free(struct bpf_mem_cache *c, void *obj)
{
	struct llist_node *llnode = obj;

	/* bpf_mem_cache is a per-cpu object. Freeing happens in irq_work.
	 * Nothing races to add to free_by_rcu list.
	 */
	__llist_add(llnode, &c->free_by_rcu);
}

static void do_call_rcu(struct bpf_mem_cache *c)
{
	struct llist_node *llnode, *t;

	if (atomic_xchg(&c->call_rcu_in_progress, 1))
		return;

	WARN_ON_ONCE(!llist_empty(&c->waiting_for_gp));
	llist_for_each_safe(llnode, t, __llist_del_all(&c->free_by_rcu))
		/* There is no concurrent __llist_add(waiting_for_gp) access.
		 * It doesn't race with llist_del_all either.
		 * But there could be two concurrent llist_del_all(waiting_for_gp):
		 * from __free_rcu() and from drain_mem_cache().
		 */
		__llist_add(llnode, &c->waiting_for_gp);
	/* Use call_rcu_tasks_trace() to wait for sleepable progs to finish.
	 * If RCU Tasks Trace grace period implies RCU grace period, free
	 * these elements directly, else use call_rcu() to wait for normal
	 * progs to finish and finally do free_one() on each element.
	 */
	call_rcu_tasks_trace(&c->rcu, __free_rcu_tasks_trace);
}

static void free_bulk(struct bpf_mem_cache *c)
{
	struct llist_node *llnode, *t;
	unsigned long flags;
	int cnt;

	do {
		if (IS_ENABLED(CONFIG_PREEMPT_RT))
			local_irq_save(flags);
		WARN_ON_ONCE(local_inc_return(&c->active) != 1);
		llnode = __llist_del_first(&c->free_llist);
		if (llnode)
			cnt = --c->free_cnt;
		else
			cnt = 0;
		local_dec(&c->active);
		if (IS_ENABLED(CONFIG_PREEMPT_RT))
			local_irq_restore(flags);
		if (llnode)
			enque_to_free(c, llnode);
	} while (cnt > (c->high_watermark + c->low_watermark) / 2);

	/* and drain free_llist_extra */
	llist_for_each_safe(llnode, t, llist_del_all(&c->free_llist_extra))
		enque_to_free(c, llnode);
	do_call_rcu(c);
}

static void bpf_mem_refill(struct irq_work *work)
{
	struct bpf_mem_cache *c = container_of(work, struct bpf_mem_cache, refill_work);
	int cnt;

	/* Racy access to free_cnt. It doesn't need to be 100% accurate */
	cnt =