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

[Roman Gushchin <roman.gushchin@linux.dev>]

On Thu, Jun 30, 2022 at 11:47:00PM +0200, Daniel Borkmann wrote:
> 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?

I'd be careful here and not change it all together.

__GFP_NOWARN might be used to suppress warnings which otherwise would be too
verbose and disruptive (especially if we talk about /net allocations in
conjunction with netconsole) and might not mean a low/lower priority.

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

Yes, it would be nice to fix this consistently over the bpf code.
Yafang, would you mind to fix it too?

Thanks!

Roman