linux-kernel - Re: [PATCH v3] mm, hugetlb: implement movable_gigantic

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Message-ID: <aTAEguKBAXN1p7o7@gourry-fedora-PF4VCD3F>
Date: Wed, 3 Dec 2025 04:36:02 -0500
From: Gregory Price <gourry@...rry.net>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: linux-mm@...ck.org, kernel-team@...a.com, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, osalvador@...e.de,
	akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org,
	surenb@...gle.com, mhocko@...e.com, corbet@....net,
	muchun.song@...ux.dev, Mel Gorman <mgorman@...e.de>,
	Alexandru Moise <00moses.alexander00@...il.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v3] mm, hugetlb: implement movable_gigantic_pages sysctl

On Wed, Dec 03, 2025 at 10:26:20AM +0100, David Hildenbrand (Red Hat) wrote:
> On 12/3/25 07:38, Gregory Price wrote:
> > This reintroduces a concept removed by:
> > commit d6cb41cc44c6 ("mm, hugetlb: remove hugepages_treat_as_movable sysctl")
> > 
> > This sysctl provides flexibility between ZONE_MOVABLE use cases:
> > 1) onlining memory in ZONE_MOVABLE to maintain hotplug compatibility
> > 2) onlining memory in ZONE_MOVABLE to make hugepage allocate reliable
> > 
> > When ZONE_MOVABLE is used to make huge page allocation more reliable,
> > disallowing gigantic pages memory in this region is pointless.  If
> > hotplug is not a requirement, we can loosen the restrictions to allow
> > 1GB gigantic pages in ZONE_MOVABLE.
> > 
> > Since 1GB can be difficult to migrate / has impacts on compaction /
> > defragmentation, we don't enable this by default. Notably, 1GB pages
> > can only be migrated if another 1GB page is available - so hot-unplug
> > will fail if such a page cannot be found.
> 
> In light of the other discussion: will it fail or will it simplt retry
> forever, until there is a free 1g page?
>
...
> > +- When an admin sets the ``movable_gigantic_pages`` sysctl to true, gigantic
> > +  pages are allowed in ZONE_MOVABLE.  This only allows migratable gigantic
> > +  pages to be allocated; however, if there are no eligible destination gigantic
> > +  pages at offline, the offlining operation will fail.
> 
> Same question here.
> 

Hah, great question. I will make a note to try this in the morning.

> Nothing else jumped at me, in general as discussed, as long as it is opt-in
> behavior
> 
> Acked-by: David Hildenbrand (Red Hat) <david@...nel.org>
> 
> -- 
> Cheers
> 
> David