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

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Message-ID: <aO89YYhEJ-GDPWFg@tiehlicka>
Date: Wed, 15 Oct 2025 08:21:21 +0200
From: Michal Hocko <mhocko@...e.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
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 14-10-25 10:22:03, Shakeel Butt wrote:
> 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.

You are right but this is inherent problem of our gfp interface. It is
too late to have a defensive interface I am afraid.

-- 
Michal Hocko
SUSE Labs