Re: [PATCH bpf-next v2 1/2] bpf: Make non-preallocated allocation low priority

From: Roman Gushchin <roman.gushchin@linux.dev>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
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>
Subject: Re: [PATCH bpf-next v2 1/2] bpf: Make non-preallocated allocation low priority
Date: Wed, 6 Jul 2022 20:36:33 -0700	[thread overview]
Message-ID: <YsZUwf8539ztHHeP@castle> (raw)
In-Reply-To: <CAADnVQKxKMcXcVra-+A8UVEUmp2h8GWotbLRi65-gBfAzJ37Ew@mail.gmail.com>

On Wed, Jul 06, 2022 at 07:09:22PM -0700, Alexei Starovoitov wrote:
> 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.

Right. Sorry, my bad, for some reason I was under an impression it's a common
path for all allocations.

Thanks!