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

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Message-ID: <5ytqerlevrsaldgww6jhiqljim35bxruicrq7hj5rumt3lywto@3ejpuuar45hx>
Date: Mon, 13 Oct 2025 15:25:14 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, Barry Song <21cnbao@...il.com>, 
	Network Development <netdev@...r.kernel.org>, linux-mm <linux-mm@...ck.org>, 
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, LKML <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>, 
	Michal Hocko <mhocko@...e.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>, 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 Mon, Oct 13, 2025 at 02:53:17PM -0700, Alexei Starovoitov wrote:
> On Mon, Oct 13, 2025 at 2:35 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> >
> > On Mon, Oct 13, 2025 at 08:30:13PM +0200, Vlastimil Babka wrote:
[...]
> > >
> > > I'm a bit worried about proliferating "~__GFP_RECLAIM" allocations now that
> > > we introduced alloc_pages_nolock() and kmalloc_nolock() where it's
> > > interpreted as "cannot spin" - see gfpflags_allow_spinning(). Currently it's
> > > fine for the page allocator itself where we have a different entry point
> > > that uses ALLOC_TRYLOCK, but it can affect nested allocations of all kinds
> > > of debugging and accounting metadata (page_owner, memcg, alloc tags for slab
> > > objects etc). kmalloc_nolock() relies on gfpflags_allow_spinning() fully
> > >
> > > I wonder if we should either:
> > >
> > > 1) sacrifice a new __GFP flag specifically for "!allow_spin" case to
> > > determine it precisely.
> > >
> > > 2) keep __GFP_KSWAPD_RECLAIM for allocations that remove it for purposes of
> > > not being disturbing (like proposed here), but that can in fact allow
> > > spinning. Instead, decide to not wake up kswapd by those when other
> > > information indicates it's an opportunistic allocation
> > > (~__GFP_DIRECT_RECLAIM, _GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC,
> > > order > 0...)
> > >
> > > 3) something better?
> > >
> >
> > For the !allow_spin allocations, I think we should just add a new __GFP
> > flag instead of adding more complexity to other allocators which may or
> > may not want kswapd wakeup for many different reasons.
> 
> That's what I proposed long ago, but was convinced that the new flag
> adds more complexity. 

Oh somehow I thought we took that route because we are low on available
bits.

> Looks like we walked this road far enough and
> the new flag will actually make things simpler.
> Back then I proposed __GFP_TRYLOCK which is not a good name.
> How about __GFP_NOLOCK ? or __GFP_NOSPIN ?

Let's go with __GFP_NOLOCK as we already have nolock variants of the
allocation APIs.