linux-kernel - Re: [PATCH v4 3/3] mm/vmscan: don't demote if there is not enough free memory in the lower memory tier

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Message-ID: <aXrKctiYCIAGyOWV@gourry-fedora-PF4VCD3F>
Date: Wed, 28 Jan 2026 21:48:18 -0500
From: Gregory Price <gourry@...rry.net>
To: Akinobu Mita <akinobu.mita@...il.com>
Cc: Michal Hocko <mhocko@...e.com>, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, axelrasmussen@...gle.com,
	yuanchu@...gle.com, weixugc@...gle.com, hannes@...xchg.org,
	david@...nel.org, zhengqi.arch@...edance.com,
	shakeel.butt@...ux.dev, lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org,
	surenb@...gle.com, ziy@...dia.com, matthew.brost@...el.com,
	joshua.hahnjy@...il.com, rakie.kim@...com, byungchul@...com,
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	bingjiao@...gle.com, jonathan.cameron@...wei.com,
	pratyush.brahma@....qualcomm.com
Subject: Re: [PATCH v4 3/3] mm/vmscan: don't demote if there is not enough
 free memory in the lower memory tier

On Thu, Jan 29, 2026 at 09:51:44AM +0900, Akinobu Mita wrote:
> > I'm not sure there's a best-option here, we may need additional input to
> > determine what the least-worst option is.  Causing LRU inversion when
> > all the nodes are pressured but swap is available is not preferable.
> 
> Would it be better if can_demote() returned false after checking that
> there is no free swap space at all and that there is not enough free space
> on the demote target node or its lower nodes?
>

I need some time to think on this.

If we take your patch, I think we essentially default to the same
behavior as-if demotion was wholesale disabled in the first place -
toptier nodes would reclaim space directly into swap.  zswap would
probably get skipped if we're already in direct reclaim (if we can't
allocate a page, neither can zswap).

The alternative is reclaim makes absolutely no progress, even if there
is (z)swap space available - which is essentially what you're
experiencing.

Maybe there is an argument for simply falling back to swap if there's no
room on any node further away.

Will ponder this for a bit and get back to you.

~Gregory