linux-kernel - Re: [RFC PATCH v4 0/6] Promotion of Unmapped Page Cache Folios.

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Message-ID: <Z_m3VKO2EPd09j4T@gourry-fedora-PF4VCD3F>
Date: Fri, 11 Apr 2025 20:44:04 -0400
From: Gregory Price <gourry@...rry.net>
To: Matthew Wilcox <willy@...radead.org>
Cc: linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	akpm@...ux-foundation.org, mingo@...hat.com, peterz@...radead.org,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	hannes@...xchg.org, mhocko@...nel.org, roman.gushchin@...ux.dev,
	shakeel.butt@...ux.dev, donettom@...ux.ibm.com,
	Huang Ying <ying.huang@...ux.alibaba.com>,
	Keith Busch <kbusch@...a.com>, Feng Tang <feng.tang@...el.com>,
	Neha Gholkar <nehagholkar@...a.com>
Subject: Re: [RFC PATCH v4 0/6] Promotion of Unmapped Page Cache Folios.

On Sat, Apr 12, 2025 at 01:35:56AM +0100, Matthew Wilcox wrote:
> On Fri, Apr 11, 2025 at 08:09:55PM -0400, Gregory Price wrote:
> > On Sat, Apr 12, 2025 at 12:49:18AM +0100, Matthew Wilcox wrote:
> > > On Fri, Apr 11, 2025 at 06:11:05PM -0400, Gregory Price wrote:
> > > > Unmapped page cache pages can be demoted to low-tier memory, but
> > > 
> > > No.  Page cache should never be demoted to low-tier memory.
> > > NACK this patchset.
> > 
> > This wasn't a statement of approval page cache being on lower tiers,
> > it's a statement of fact.  Enabling demotion causes this issue.
> 
> Then that's the bug that needs to be fixed.  Not adding 200+ lines
> of code to recover from a situation that should never happen.

Well, I have a use case that make valuable use of putting the page cache
on a farther node rather than pushing it out to disk.  But this
discussion aside, I think we could simply make this a separate mode of
demotion_enabled

/* Only demote anonymous memory */
echo 2 > /sys/kernel/mm/numa/demotion_enabled

Assuming we can recognize anon from just struct folio

~Gregory