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From:   Roman Gushchin <roman.gushchin@linux.dev>
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Yafang Shao <laoar.shao@gmail.com>, ast@kernel.org,
        daniel@iogearbox.net, andrii@kernel.org, kafai@fb.com,
        songliubraving@fb.com, yhs@fb.com, john.fastabend@gmail.com,
        kpsingh@kernel.org, quentin@isovalent.com, haoluo@google.com,
        bpf@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH bpf-next v2 1/2] bpf: Make non-preallocated allocation
 low priority
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References: <20220706155848.4939-1-laoar.shao@gmail.com>
 <20220706155848.4939-2-laoar.shao@gmail.com>
 <20220707000721.dtl356trspb23ctp@google.com>
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On Thu, Jul 07, 2022 at 12:07:21AM +0000, Shakeel Butt wrote:
> On Wed, Jul 06, 2022 at 03:58:47PM +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.
> 
> Please use GFP_NOWAIT instead of (__GFP_ATOMIC | __GFP_KSWAPD_RECLAIM).
> There is already a plan to completely remove __GFP_ATOMIC and mm-tree
> already have a patch for that.

Oh, I didn't know this, thanks for heads up!
I agree that GFP_NOWAIT is the best choice then.

Btw, we probably shouldn't even add GFP_NOWAIT if the allocation is performed
from the bpf syscall context. Why would we fail to pre-allocate a map if
we can easily go into the reclaim? But probably better to leave it for
a separate change.

Thanks!