Re: [PATCH bpf-next] bpf: Add a retry after refilling the free list when unit_alloc() fails

[Song Liu]

On Wed, Feb 12, 2025 at 12:49 AM Changwoo Min <changwoo@xxxxxxxxxx> wrote:
>
> When there is no entry in the free list (c->free_llist), unit_alloc()
> fails even when there is available memory in the system, causing allocation
> failure in various BPF calls -- such as bpf_mem_alloc() and
> bpf_cpumask_create().
>
> Such allocation failure can happen, especially when a BPF program tries many
> allocations -- more than a delta between high and low watermarks -- in an
> IRQ-disabled context.

Can we add a selftests for this scenario?

>
> To address the problem, when there is no free entry, refill one entry on the
> free list (alloc_bulk) and then retry the allocation procedure on the free
> list. Note that since some callers of unit_alloc() do not allow to block
> (e.g., bpf_cpumask_create), allocate the additional free entry in an atomic
> manner (atomic = true in alloc_bulk).
>
> Signed-off-by: Changwoo Min <changwoo@xxxxxxxxxx>
> ---
>  kernel/bpf/memalloc.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
> index 889374722d0a..22fe9cfb2b56 100644
> --- a/kernel/bpf/memalloc.c
> +++ b/kernel/bpf/memalloc.c
> @@ -784,6 +784,7 @@ static void notrace *unit_alloc(struct bpf_mem_cache *c)
>         struct llist_node *llnode = NULL;
>         unsigned long flags;
>         int cnt = 0;
> +       bool retry = false;

"retry = false;" reads weird to me. Maybe rename it as "retried"?

>
>         /* Disable irqs to prevent the following race for majority of prog types:
>          * prog_A
> @@ -795,6 +796,7 @@ static void notrace *unit_alloc(struct bpf_mem_cache *c)
>          * Use per-cpu 'active' counter to order free_list access between
>          * unit_alloc/unit_free/bpf_mem_refill.
>          */
> +retry_alloc:
>         local_irq_save(flags);
>         if (local_inc_return(&c->active) == 1) {
>                 llnode = __llist_del_first(&c->free_llist);
> @@ -815,6 +817,13 @@ static void notrace *unit_alloc(struct bpf_mem_cache *c)
>          */
>         local_irq_restore(flags);
>
> +       if (unlikely(!llnode && !retry)) {
> +               int cpu = smp_processor_id();
> +               alloc_bulk(c, 1, cpu_to_node(cpu), true);
cpu_to_node() is not necessary, we can just do

alloc_bulk(c, 1, NUMA_NO_NODE, true);

Also, maybe we can let alloc_bulk return int (0 or -ENOMEM).
For -ENOMEM, there is no need to goto retry_alloc.

Does this make sense?

Thanks,
Song

> +               retry = true;
> +               goto retry_alloc;
> +       }
> +
>         return llnode;
>  }
>
> --
> 2.48.1
>