[PATCH bpf v2] bpf/helpers: Use __GFP_HIGH instead of GFP_ATOMIC in __bpf_async_init()

[Peilin Ye <yepeilin@google.com>]

Currently, calling bpf_map_kmalloc_node() from __bpf_async_init() can
cause various locking issues; see the following stack trace (edited for
style) as one example:

...
 [10.011566]  do_raw_spin_lock.cold
 [10.011570]  try_to_wake_up             (5) double-acquiring the same
 [10.011575]  kick_pool                      rq_lock, causing a hardlockup
 [10.011579]  __queue_work
 [10.011582]  queue_work_on
 [10.011585]  kernfs_notify
 [10.011589]  cgroup_file_notify
 [10.011593]  try_charge_memcg           (4) memcg accounting raises an
 [10.011597]  obj_cgroup_charge_pages        MEMCG_MAX event
 [10.011599]  obj_cgroup_charge_account
 [10.011600]  __memcg_slab_post_alloc_hook
 [10.011603]  __kmalloc_node_noprof
...
 [10.011611]  bpf_map_kmalloc_node
 [10.011612]  __bpf_async_init
 [10.011615]  bpf_timer_init             (3) BPF calls bpf_timer_init()
 [10.011617]  bpf_prog_xxxxxxxxxxxxxxxx_fcg_runnable
 [10.011619]  bpf__sched_ext_ops_runnable
 [10.011620]  enqueue_task_scx           (2) BPF runs with rq_lock held
 [10.011622]  enqueue_task
 [10.011626]  ttwu_do_activate
 [10.011629]  sched_ttwu_pending         (1) grabs rq_lock
...

The above was reproduced on bpf-next (b338cf849ec8) by modifying
./tools/sched_ext/scx_flatcg.bpf.c to call bpf_timer_init() during
ops.runnable(), and hacking [1] the memcg accounting code a bit to make
a bpf_timer_init() call much more likely to raise an MEMCG_MAX event.

We have also run into other similar variants (both internally and on
bpf-next), including double-acquiring cgroup_file_kn_lock, the same
worker_pool::lock, etc.

As suggested by Shakeel, fix this by using __GFP_HIGH instead of
GFP_ATOMIC in __bpf_async_init(), so that e.g. if try_charge_memcg()
raises an MEMCG_MAX event, we call __memcg_memory_event() with
@allow_spinning=false and avoid calling cgroup_file_notify() there.

Depends on mm patch "memcg: skip cgroup_file_notify if spinning is not
allowed".  Tested with vmtest.sh (llvm-18, x86-64):

 $ ./test_progs -a '*timer*' -a '*wq*'
...
 Summary: 7/12 PASSED, 0 SKIPPED, 0 FAILED

[1] Making bpf_timer_init() much more likely to raise an MEMCG_MAX event
(gist-only, for brevity):

kernel/bpf/helpers.c:__bpf_async_init():
 -        cb = bpf_map_kmalloc_node(map, size, GFP_ATOMIC, map->numa_node);
 +        cb = bpf_map_kmalloc_node(map, size, GFP_ATOMIC | __GFP_HACK,
 +                                  map->numa_node);

mm/memcontrol.c:try_charge_memcg():
          if (!do_memsw_account() ||
 -            page_counter_try_charge(&memcg->memsw, batch, &counter)) {
 -                if (page_counter_try_charge(&memcg->memory, batch, &counter))
 +            page_counter_try_charge_hack(&memcg->memsw, batch, &counter,
 +                                         gfp_mask & __GFP_HACK)) {
 +                if (page_counter_try_charge_hack(&memcg->memory, batch,
 +                                                 &counter,
 +                                                 gfp_mask & __GFP_HACK))
                          goto done_restock;

mm/page_counter.c:page_counter_try_charge():
 -bool page_counter_try_charge(struct page_counter *counter,
 -                             unsigned long nr_pages,
 -                             struct page_counter **fail)
 +bool page_counter_try_charge_hack(struct page_counter *counter,
 +                                  unsigned long nr_pages,
 +                                  struct page_counter **fail, bool hack)
 {
...
 -                if (new > c->max) {
 +                if (hack || new > c->max) {     // goto failed;
                          atomic_long_sub(nr_pages, &c->usage);

Fixes: b00628b1c7d5 ("bpf: Introduce bpf timers.")
Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Peilin Ye <yepeilin@google.com>
---
v1: https://lore.kernel.org/bpf/20250905234547.862249-1-yepeilin@google.com/
change since v1:
 - simplify comment, and mention kmalloc_nolock() (Shakeel)

 kernel/bpf/helpers.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index b9b0c5fe33f6..8af62cb243d9 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1274,8 +1274,11 @@ static int __bpf_async_init(struct bpf_async_kern *async, struct bpf_map *map, u
 		goto out;
 	}
 
-	/* allocate hrtimer via map_kmalloc to use memcg accounting */
-	cb = bpf_map_kmalloc_node(map, size, GFP_ATOMIC, map->numa_node);
+	/* Allocate via bpf_map_kmalloc_node() for memcg accounting. Until
+	 * kmalloc_nolock() is available, avoid locking issues by using
+	 * __GFP_HIGH (GFP_ATOMIC & ~__GFP_RECLAIM).
+	 */
+	cb = bpf_map_kmalloc_node(map, size, __GFP_HIGH, map->numa_node);
 	if (!cb) {
 		ret = -ENOMEM;
 		goto out;
-- 
2.51.0.384.g4c02a37b29-goog