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

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Date:   Fri, 1 Jul 2022 20:54:40 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org,
        quentin@...valent.com, netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/4] bpf: Make non-preallocated allocation low
 priority

On Wed, Jun 29, 2022 at 03:48:29PM +0000, 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.
> 
> It also fixes a typo in the comment.
> 
> Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> Cc: Roman Gushchin <roman.gushchin@...ux.dev>

I agree, it makes total sense to me. Bpf allocations are not high priority
and should not be enforced both on memcg and page allocator levels.

Reviewed-by: Roman Gushchin <roman.gushchin@...ux.dev>

Thanks, Yafang!