linux-kernel - Re: CXL Boot to Bash - Section 3: Memory (block) Hotplug

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Message-ID: <Z7S9VUqjWOKGxLFi@gourry-fedora-PF4VCD3F>
Date: Tue, 18 Feb 2025 12:03:17 -0500
From: Gregory Price <gourry@...rry.net>
To: David Hildenbrand <david@...hat.com>
Cc: lsf-pc@...ts.linux-foundation.org, linux-mm@...ck.org,
	linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: CXL Boot to Bash - Section 3: Memory (block) Hotplug

On Tue, Feb 18, 2025 at 05:24:30PM +0100, David Hildenbrand wrote:
> > 
> > ZONE_MOVABLE, under normal conditions, disallows most kernel allocations.
> > 
> 
> In essence, only movable allocations (some kernel allcoations are movable).
> 
> > ZONE_MOVABLE does NOT make a *strong* guarantee of hut-unplug-ability.
> > The kernel and privileged users can cause long-term pinning to occur -
> > even in ZONE_MOVABLE.  It should be seen as a best-attempt at providing
> > hot-unplug-ability under normal conditions.
> 
> Yes and no; actual long-term pinning is disallowed (FOLL_LONGTERM), but we
> have a bunch of cases that need fixing. [1]
> 
> Of course, new cases keep popping up. It's a constant fight to make
> hot-unplug as reliable as possible. So yes, we cannot give "strong"
> guarantees, but make it as reliable as possible in sane configurations.
> 
> [1]
> https://lkml.kernel.org/r/882b566c-34d6-4e68-9447-6c74a0693f18@redhat.com
> 

Appreciate the additional context, I missed your topic proposal.  I was
trying to be conservative about the claims ZONE_MOVABLE makes so that I
don't present it as a "This will fix all your hotplug woes" solution.

Looking forward to this LSF :]

~Gregory