linux-kernel - Re: [PATCH] Revert "mm, hugetlb: remove hugepages_treat_as

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Message-ID: <aObCks3bwrT27CIu@gourry-fedora-PF4VCD3F>
Date: Wed, 8 Oct 2025 15:59:14 -0400
From: Gregory Price <gourry@...rry.net>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, corbet@....net, muchun.song@...ux.dev,
	osalvador@...e.de, akpm@...ux-foundation.org, hannes@...xchg.org,
	laoar.shao@...il.com, brauner@...nel.org, mclapinski@...gle.com,
	joel.granados@...nel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
	Michal Hocko <mhocko@...e.com>,
	Alexandru Moise <00moses.alexander00@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	Joshua Hahn <joshua.hahnjy@...il.com>
Subject: Re: [PATCH] Revert "mm, hugetlb: remove hugepages_treat_as_movable
 sysctl"

On Wed, Oct 08, 2025 at 09:52:09PM +0200, David Hildenbrand wrote:
> On 08.10.25 21:44, Gregory Price wrote:
> > On Wed, Oct 08, 2025 at 09:01:09PM +0200, David Hildenbrand wrote:
> > > > 
> > > > fwiw this works cleanly.  Just dropping this here, but should continue
> > > > the zone conversation.  I need to check, but does this actually allow
> > > > pinnable allocations?  I thought pinning kicked off migration.
> > > 
> > > Yes, it should because longterm pinning -> unmovable.
> > > 
> > 
> > You know i just realized, my test here only works before I allocated 1GB
> > pages on both node0 and node1.  If I only allocate 1gb hugetlb on node1,
> > then the migrate pages call fails - because there are no 1gb pages
> > available there.
> > 
> > I imagine this would cause hot-unplug/offline to fail since it uses the
> > same migration mechanisms.
> > 
> > Worse I would imagine this would fail for 2MB.
> > 
> > Seems like the 1GB limitation is arbitrary if 2MB causes the same issue.
> 
> Yeah, with hugetlb allocations there are no guarantees either. It's just
> that page compaction / defragmentation makes it much less likely to fail in
> many scenarios.
> 

Gotcha, well I am open to suggestions.  This chicken bit here feels like
a sufficient guardrail, but I'm happy to explore the ZONE discussion
further if we think that's fruitful.

Joshua Hahn (cc) did privately question whether zonelist ordering breaks
for such a configuable zone.  If memory can't live in ZONE_NORMAL or
ZONE_MOVABLE, but you want it to have some combination of attributes
between the two, it can't also live above ZONE_MOVABLE I don't think.

~Gregory