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

[Changwoo Min]

Hello Song,

Thank you for the review!

On 25. 2. 13. 03:33, Song Liu wrote:
> 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?

It would be a bit tricky to create an IRQ-disabled case in a BPF
program. However, I think it will be possible to reproduce the
allocation failure issue when allocating sufficiently enough
small allocations.

> > 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"?

"retried" reads betters. Will fix it.

> >          /* 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);

Sure, will change it as suggested.

> 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?

Yup, I will change the alloc_bulk() as it returns 0 or -ENOMEM.

Regards,
Changwoo Min