linux-kernel - Re: [PATCH v1 1/2] mm/vmscan: balance demotion allocation in alloc_demote

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Message-ID: <aWGTNfrLBhCM3_-9@google.com>
Date: Fri, 9 Jan 2026 23:45:57 +0000
From: Bing Jiao <bingjiao@...gle.com>
To: Donet Tom <donettom@...ux.ibm.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	David Hildenbrand <david@...nel.org>,
	Michal Hocko <mhocko@...nel.org>,
	Qi Zheng <zhengqi.arch@...edance.com>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Yuanchu Xie <yuanchu@...gle.com>, Wei Xu <weixugc@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/2] mm/vmscan: balance demotion allocation in
 alloc_demote_folio()

On Thu, Jan 08, 2026 at 06:14:02PM +0530, Donet Tom wrote:
>
> On 1/7/26 12:58 PM, Bing Jiao wrote:
> > +	/* Randomly select a node from fallback nodes for balanced allocation */
> > +	if (allowed_mask) {
> > +		mtc->nid = node_random(allowed_mask);
>
>
> This random selection can cause allocations to fall back to distant memory
> even when the nearer demotion target has sufficient free memory, correct?
> Could this also lead to increased promotion latency?

Hi Donet,

Thanks for your questions.

Yes, the random selection could select a distant node and lead to
incresed promotion latency.

I just realized that the the fallback allocation should not weighted
by a single metric, such as node distance, capacity, free space.
We need a thoroughly study before changing alloc_demote_folio().

Best,
Bing