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

[Michal Hocko]

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@xxxxxxxx>
> > > > [...]
> > > > > 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