linux-kernel - Re: [RFC PATCH] mm: net: disable kswapd for high-order network buffer allocation

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Message-ID: <itljl2e4rwbblmnhe2vucmsvxzbu42x5foszf4y5b63evbitpj@qsxj3amwhts3>
Date: Tue, 14 Oct 2025 10:22:03 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Michal Hocko <mhocko@...e.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, Barry Song <21cnbao@...il.com>, 
	netdev@...r.kernel.org, linux-mm@...ck.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>, 
	Jonathan Corbet <corbet@....net>, Eric Dumazet <edumazet@...gle.com>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, Paolo Abeni <pabeni@...hat.com>, 
	Willem de Bruijn <willemb@...gle.com>, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, Brendan Jackman <jackmanb@...gle.com>, 
	Johannes Weiner <hannes@...xchg.org>, Zi Yan <ziy@...dia.com>, Yunsheng Lin <linyunsheng@...wei.com>, 
	Huacai Zhou <zhouhuacai@...o.com>, Alexei Starovoitov <alexei.starovoitov@...il.com>, 
	Harry Yoo <harry.yoo@...cle.com>, David Hildenbrand <david@...hat.com>, 
	Matthew Wilcox <willy@...radead.org>, Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [RFC PATCH] mm: net: disable kswapd for high-order network
 buffer allocation

On Tue, Oct 14, 2025 at 05:14:47PM +0200, Michal Hocko wrote:
> On Tue 14-10-25 07:27:06, Shakeel Butt wrote:
> > On Tue, Oct 14, 2025 at 09:26:49AM +0200, Michal Hocko wrote:
> > > On Mon 13-10-25 20:30:13, Vlastimil Babka wrote:
> > > > On 10/13/25 12:16, Barry Song wrote:
> > > > > From: Barry Song <v-songbaohua@...o.com>
> > > [...]
> > > > I wonder if we should either:
> > > > 
> > > > 1) sacrifice a new __GFP flag specifically for "!allow_spin" case to
> > > > determine it precisely.
> > > 
> > > As said in other reply I do not think this is a good fit for this
> > > specific case as it is all or nothing approach. Soon enough we discover
> > > that "no effort to reclaim/compact" hurts other usecases. So I do not
> > > think we need a dedicated flag for this specific case. We need a way to
> > > tell kswapd/kcompactd how much to try instead.
> > 
> > To me this new floag is to decouple two orthogonal requests i.e. no lock
> > semantic and don't wakeup kswapd. At the moment the lack of kswapd gfp
> > flag convey the semantics of no lock. This can lead to unintended usage
> > of no lock semantics by users which for whatever reason don't want to
> > wakeup kswapd.
> 
> I would argue that callers should have no business into saying whether
> the MM should wake up kswapd or not. The flag name currently suggests
> that but that is mostly for historic reasons. A random page allocator
> user shouldn't really care about this low level detail, really.

I agree but unless we somehow enforce/warn for such cases, there will be
users doing this. A simple grep shows kmsan is doing this. I worry there
might be users who are manually setting up gfp flags for their
allocations and not providing kswapd flag explicitly. Finding such cases
with grep is not easy.