linux-kernel - Re: [RFC PATCH 1/4] mm: migrate: Allow misplaced migration without VMA too

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Message-ID: <Z8nsgr46eVH7W7SJ@casper.infradead.org>
Date: Thu, 6 Mar 2025 18:42:10 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Gregory Price <gourry@...rry.net>
Cc: Bharata B Rao <bharata@....com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, AneeshKumar.KizhakeVeetil@....com,
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Subject: Re: [RFC PATCH 1/4] mm: migrate: Allow misplaced migration without
 VMA too

On Thu, Mar 06, 2025 at 01:19:41PM -0500, Gregory Price wrote:
> On Thu, Mar 06, 2025 at 05:45:34PM +0000, Matthew Wilcox wrote:
> > On Thu, Mar 06, 2025 at 12:24:16PM -0500, Gregory Price wrote:
> > > we may also need this:
> > > https://lore.kernel.org/linux-mm/20250107000346.1338481-4-gourry@gourry.net/
> > > 
> > > May be worth just pulling these ahead to avoid conflict.
> > 
> > Or not putting them in at all because this whole thing is a magnificent
> > waste of time?
> 
> Divorced from the tiering mechanisms, is making misplaced migration able
> to migrate unmapped pages not generally useful?

The only thing I can think of is if you have a process or set of
processes on node A calling read() and the file is cached on node B.
But in order to decide if the page is on the wrong node, you'd need
to track a lot of information about which nodes the page is being
accessed from.  Which is probably why we've never bothered to do it.

This is not a large patch for you to carry as part of your patchset.
There's nothing intrinsically wrong with it; it just has no users in
mainline and no real prospect of any being added soon.