Re: [PATCH] bpf: don't call mmap_read_trylock() from IRQ context

From: Tetsuo Handa
Date: Sat Jun 15 2024 - 10:27:16 EST

On 2024/06/15 19:59, Tetsuo Handa wrote:
> Is the reason because &buf[idx] in get_memcg_path_buf() might become out of
> bounds due to preemption in normal context if PREEMPT_RT=y ? If so, can't we
> add "idx >=0 && idx < CONTEXT_COUNT" check into get_memcg_path_buf() and
> return NULL if preemption (or interrupt or recursion if any) exhausted the per
> cpu buffer?

More simply, why not use on-stack buffer, for MEMCG_PATH_BUF_SIZE is only 256?

mm/mmap_lock.c | 175 ++++++-------------------------------------------
1 file changed, 20 insertions(+), 155 deletions(-)

diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index 1854850b4b89..59165c01c960 100644
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c
@@ -19,14 +19,7 @@ EXPORT_TRACEPOINT_SYMBOL(mmap_lock_released);

#ifdef CONFIG_MEMCG

-/*
- * Our various events all share the same buffer (because we don't want or need
- * to allocate a set of buffers *per event type*), so we need to protect against
- * concurrent _reg() and _unreg() calls, and count how many _reg() calls have
- * been made.
- */
-static DEFINE_MUTEX(reg_lock);
-static int reg_refcount; /* Protected by reg_lock. */
+static atomic_t reg_refcount;

/*
* Size of the buffer for memcg path names. Ignoring stack trace support,
@@ -34,136 +27,22 @@ static int reg_refcount; /* Protected by reg_lock. */
*/
#define MEMCG_PATH_BUF_SIZE MAX_FILTER_STR_VAL

-/*
- * How many contexts our trace events might be called in: normal, softirq, irq,
- * and NMI.
- */
-#define CONTEXT_COUNT 4
-
-struct memcg_path {
- local_lock_t lock;
- char __rcu *buf;
- local_t buf_idx;
-};
-static DEFINE_PER_CPU(struct memcg_path, memcg_paths) = {
- .lock = INIT_LOCAL_LOCK(lock),
- .buf_idx = LOCAL_INIT(0),
-};
-
-static char **tmp_bufs;
-
-/* Called with reg_lock held. */
-static void free_memcg_path_bufs(void)
-{
- struct memcg_path *memcg_path;
- int cpu;
- char **old = tmp_bufs;
-
- for_each_possible_cpu(cpu) {
- memcg_path = per_cpu_ptr(&memcg_paths, cpu);
- *(old++) = rcu_dereference_protected(memcg_path->buf,
- lockdep_is_held(&reg_lock));
- rcu_assign_pointer(memcg_path->buf, NULL);
- }
-
- /* Wait for inflight memcg_path_buf users to finish. */
- synchronize_rcu();
-
- old = tmp_bufs;
- for_each_possible_cpu(cpu) {
- kfree(*(old++));
- }
-
- kfree(tmp_bufs);
- tmp_bufs = NULL;
-}
-
int trace_mmap_lock_reg(void)
{
- int cpu;
- char *new;
-
- mutex_lock(&reg_lock);
-
- /* If the refcount is going 0->1, proceed with allocating buffers. */
- if (reg_refcount++)
- goto out;
-
- tmp_bufs = kmalloc_array(num_possible_cpus(), sizeof(*tmp_bufs),
- GFP_KERNEL);
- if (tmp_bufs == NULL)
- goto out_fail;
-
- for_each_possible_cpu(cpu) {
- new = kmalloc(MEMCG_PATH_BUF_SIZE * CONTEXT_COUNT, GFP_KERNEL);
- if (new == NULL)
- goto out_fail_free;
- rcu_assign_pointer(per_cpu_ptr(&memcg_paths, cpu)->buf, new);
- /* Don't need to wait for inflights, they'd have gotten NULL. */
- }
-
-out:
- mutex_unlock(&reg_lock);
+ atomic_inc(&reg_refcount);
return 0;
-
-out_fail_free:
- free_memcg_path_bufs();
-out_fail:
- /* Since we failed, undo the earlier ref increment. */
- --reg_refcount;
-
- mutex_unlock(&reg_lock);
- return -ENOMEM;
}

void trace_mmap_lock_unreg(void)
{
- mutex_lock(&reg_lock);
-
- /* If the refcount is going 1->0, proceed with freeing buffers. */
- if (--reg_refcount)
- goto out;
-
- free_memcg_path_bufs();
-
-out:
- mutex_unlock(&reg_lock);
-}
-
-static inline char *get_memcg_path_buf(void)
-{
- struct memcg_path *memcg_path = this_cpu_ptr(&memcg_paths);
- char *buf;
- int idx;
-
- rcu_read_lock();
- buf = rcu_dereference(memcg_path->buf);
- if (buf == NULL) {
- rcu_read_unlock();
- return NULL;
- }
- idx = local_add_return(MEMCG_PATH_BUF_SIZE, &memcg_path->buf_idx) -
- MEMCG_PATH_BUF_SIZE;
- return &buf[idx];
+ atomic_dec(&reg_refcount);
}

-static inline void put_memcg_path_buf(void)
-{
- local_sub(MEMCG_PATH_BUF_SIZE, &this_cpu_ptr(&memcg_paths)->buf_idx);
- rcu_read_unlock();
-}
-
-#define TRACE_MMAP_LOCK_EVENT(type, mm, ...) \
- do { \
- const char *memcg_path; \
- local_lock(&memcg_paths.lock); \
- memcg_path = get_mm_memcg_path(mm); \
- trace_mmap_lock_##type(mm, \
- memcg_path != NULL ? memcg_path : "", \
- ##__VA_ARGS__); \
- if (likely(memcg_path != NULL)) \
- put_memcg_path_buf(); \
- local_unlock(&memcg_paths.lock); \
+#define TRACE_MMAP_LOCK_EVENT(type, mm, ...) \
+ do { \
+ char buf[MEMCG_PATH_BUF_SIZE]; \
+ get_mm_memcg_path(mm, buf, sizeof(buf)); \
+ trace_mmap_lock_##type(mm, buf, ##__VA_ARGS__); \
} while (0)

#else /* !CONFIG_MEMCG */
@@ -185,37 +64,23 @@ void trace_mmap_lock_unreg(void)
#ifdef CONFIG_TRACING
#ifdef CONFIG_MEMCG
/*
- * Write the given mm_struct's memcg path to a percpu buffer, and return a
- * pointer to it. If the path cannot be determined, or no buffer was available
- * (because the trace event is being unregistered), NULL is returned.
- *
- * Note: buffers are allocated per-cpu to avoid locking, so preemption must be
- * disabled by the caller before calling us, and re-enabled only after the
- * caller is done with the pointer.
- *
- * The caller must call put_memcg_path_buf() once the buffer is no longer
- * needed. This must be done while preemption is still disabled.
+ * Write the given mm_struct's memcg path to on-stack buffer. If the path cannot be
+ * determined or the trace event is being unregistered, empty string is written.
*/
-static const char *get_mm_memcg_path(struct mm_struct *mm)
+static void get_mm_memcg_path(struct mm_struct *mm, char *buf, size_t buflen)
{
- char *buf = NULL;
- struct mem_cgroup *memcg = get_mem_cgroup_from_mm(mm);
+ struct mem_cgroup *memcg;

+ buf[0] = '\0';
+ /* No need to get path if no trace event is registered. */
+ if (!atomic_read(&reg_refcount))
+ return;
+ memcg = get_mem_cgroup_from_mm(mm);
if (memcg == NULL)
- goto out;
- if (unlikely(memcg->css.cgroup == NULL))
- goto out_put;
-
- buf = get_memcg_path_buf();
- if (buf == NULL)
- goto out_put;
-
- cgroup_path(memcg->css.cgroup, buf, MEMCG_PATH_BUF_SIZE);
-
-out_put:
+ return;
+ if (memcg->css.cgroup)
+ cgroup_path(memcg->css.cgroup, buf, buflen);
css_put(&memcg->css);
-out:
- return buf;
}

#endif /* CONFIG_MEMCG */
--
2.18.4