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References: <20220706155848.4939-1-laoar.shao@gmail.com> <20220706155848.4939-2-laoar.shao@gmail.com>
 <20220707000721.dtl356trspb23ctp@google.com> <YsYn3HoqQ4JtTaO6@castle>
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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Wed, 6 Jul 2022 19:09:22 -0700
Message-ID: <CAADnVQKxKMcXcVra-+A8UVEUmp2h8GWotbLRi65-gBfAzJ37Ew@mail.gmail.com>
Subject: Re: [PATCH bpf-next v2 1/2] bpf: Make non-preallocated allocation low priority
To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     Shakeel Butt <shakeelb@google.com>,
        Yafang Shao <laoar.shao@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Quentin Monnet <quentin@isovalent.com>,
        Hao Luo <haoluo@google.com>, bpf <bpf@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>
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On Wed, Jul 6, 2022 at 5:25 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
>
> 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.

The places affected by this patch are in atomic context.
Prealloc path from syscall is using GFP_USER.