Re: [PATCH bpf-next] selftsets/bpf: Retry map update on helper_fill_hashmap()

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Thu Jun 11, 2026 at 11:07 AM EDT, Gabriele Monaco wrote:
> helper_fill_hashmap() is used also on parallel and stress map tests.
> Those are consistently failing with ENOMEM on kernels built with
> PREEMPT_RT if preallocation is disabled. The failure is transient and
> only called by the memory cache refill running in a preemptible
> irq_work, which can easily stall in case of contention.
>
> Use a retriable update in those cases to handle transient ENOMEM and
> make the test more stable also on PREEMPT_RT.
> Also fix the sign of the value printed in case of error (strerror()
> expects a positive errno while updates return it negative).
>
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

The bot has a point about changing the code to & BPF_F_PREALLOC, but
we touch test_maps.c twice a year and that only for bugfixes. There will
probably be no other consumer of this function in the foreseeable future.
Similarly, the whole file uses CHECK() so imo it's fine to keep this as-is.

> ---
>  tools/testing/selftests/bpf/test_maps.c | 23 +++++++++++++----------
>  1 file changed, 13 insertions(+), 10 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/test_maps.c b/tools/testing/selftests/bpf/test_maps.c
> index ccc5acd55f..c32da7bd8b 100644
> --- a/tools/testing/selftests/bpf/test_maps.c
> +++ b/tools/testing/selftests/bpf/test_maps.c
> @@ -260,6 +260,16 @@ static void test_hashmap_percpu(unsigned int task, void *data)
>  	close(fd);
>  }
>  
> +#define MAP_RETRIES 20
> +
> +static bool can_retry(int err)
> +{
> +	return (err == EAGAIN || err == EBUSY ||
> +		((err == ENOMEM || err == E2BIG) &&
> +		 map_opts.map_flags == BPF_F_NO_PREALLOC));
> +}
> +
> +
>  #define VALUE_SIZE 3
>  static int helper_fill_hashmap(int max_entries)
>  {
> @@ -274,10 +284,11 @@ static int helper_fill_hashmap(int max_entries)
>  
>  	for (i = 0; i < max_entries; i++) {
>  		key = i; value[0] = key;
> -		ret = bpf_map_update_elem(fd, &key, value, BPF_NOEXIST);
> +		ret = map_update_retriable(fd, &key, value, BPF_NOEXIST,
> +					   MAP_RETRIES, can_retry);
>  		CHECK(ret != 0,
>  		      "can't update hashmap",
> -		      "err: %s\n", strerror(ret));
> +		      "err: %s\n", strerror(-ret));
>  	}
>  
>  	return fd;
> @@ -1392,17 +1403,9 @@ static void test_map_stress(void)
>  #define DO_UPDATE 1
>  #define DO_DELETE 0
>  
> -#define MAP_RETRIES 20
>  #define MAX_DELAY_US 50000
>  #define MIN_DELAY_RANGE_US 5000
>  
> -static bool can_retry(int err)
> -{
> -	return (err == EAGAIN || err == EBUSY ||
> -		((err == ENOMEM || err == E2BIG) &&
> -		 map_opts.map_flags == BPF_F_NO_PREALLOC));
> -}
> -
>  int map_update_retriable(int map_fd, const void *key, const void *value, int flags, int attempts,
>  			 retry_for_error_fn need_retry)
>  {
>
> base-commit: 2d3090a8aeb596a26935db0955d46c9a5db5c6ce