Re: [PATCH bpf-next 1/4] bpf: Make non-preallocated allocation low priority

[Daniel Borkmann <daniel@iogearbox.net>]

Hi Yafang,

On 6/29/22 5:48 PM, Yafang Shao wrote:
> GFP_ATOMIC doesn't cooperate well with memcg pressure so far, especially
> if we allocate too much GFP_ATOMIC memory. For example, when we set the
> memcg limit to limit a non-preallocated bpf memory, the GFP_ATOMIC can
> easily break the memcg limit by force charge. So it is very dangerous to
> use GFP_ATOMIC in non-preallocated case. One way to make it safe is to
> remove __GFP_HIGH from GFP_ATOMIC, IOW, use (__GFP_ATOMIC |
> __GFP_KSWAPD_RECLAIM) instead, then it will be limited if we allocate
> too much memory.
> 
> We introduced BPF_F_NO_PREALLOC is because full map pre-allocation is
> too memory expensive for some cases. That means removing __GFP_HIGH
> doesn't break the rule of BPF_F_NO_PREALLOC, but has the same goal with
> it-avoiding issues caused by too much memory. So let's remove it.
> 
> __GFP_KSWAPD_RECLAIM doesn't cooperate well with memcg pressure neither
> currently. But the memcg code can be improved to make
> __GFP_KSWAPD_RECLAIM work well under memcg pressure.

Ok, but could you also explain in commit desc why it's a specific problem
to BPF hashtab?

Afaik, there is plenty of other code using GFP_ATOMIC | __GFP_NOWARN outside
of BPF e.g. under net/, so it's a generic memcg problem?

Why are lpm trie and local storage map for BPF not affected (at least I don't
see them covered in the patch)?

Thanks,
Daniel

> It also fixes a typo in the comment.
> 
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> Cc: Roman Gushchin <roman.gushchin@linux.dev>
> ---
>   kernel/bpf/hashtab.c | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 17fb69c0e0dc..9d4559a1c032 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -61,7 +61,7 @@
>    *
>    * As regular device interrupt handlers and soft interrupts are forced into
>    * thread context, the existing code which does
> - *   spin_lock*(); alloc(GPF_ATOMIC); spin_unlock*();
> + *   spin_lock*(); alloc(GFP_ATOMIC); spin_unlock*();
>    * just works.
>    *
>    * In theory the BPF locks could be converted to regular spinlocks as well,
> @@ -978,7 +978,8 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
>   				goto dec_count;
>   			}
>   		l_new = bpf_map_kmalloc_node(&htab->map, htab->elem_size,
> -					     GFP_ATOMIC | __GFP_NOWARN,
> +					     __GFP_ATOMIC | __GFP_NOWARN |
> +					     __GFP_KSWAPD_RECLAIM,
>   					     htab->map.numa_node);
>   		if (!l_new) {
>   			l_new = ERR_PTR(-ENOMEM);
> @@ -996,7 +997,8 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
>   		} else {
>   			/* alloc_percpu zero-fills */
>   			pptr = bpf_map_alloc_percpu(&htab->map, size, 8,
> -						    GFP_ATOMIC | __GFP_NOWARN);
> +						    __GFP_ATOMIC | __GFP_NOWARN |
> +						    __GFP_KSWAPD_RECLAIM);
>   			if (!pptr) {
>   				kfree(l_new);
>   				l_new = ERR_PTR(-ENOMEM);
>