linux-kernel - Re: [PATCH 2/2] liveupdate: kho: allocate metadata directly from the buddy allocator

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Message-ID: <20251024132509.GB760669@ziepe.ca>
Date: Fri, 24 Oct 2025 10:25:09 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: Pratyush Yadav <pratyush@...nel.org>, akpm@...ux-foundation.org,
	brauner@...nel.org, corbet@....net, graf@...zon.com,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	linux-mm@...ck.org, masahiroy@...nel.org, ojeda@...nel.org,
	rdunlap@...radead.org, rppt@...nel.org, tj@...nel.org,
	jasonmiu@...gle.com, dmatlack@...gle.com, skhawaja@...gle.com,
	glider@...gle.com, elver@...gle.com
Subject: Re: [PATCH 2/2] liveupdate: kho: allocate metadata directly from the
 buddy allocator

On Wed, Oct 15, 2025 at 10:19:08AM -0400, Pasha Tatashin wrote:
> On Wed, Oct 15, 2025 at 9:05 AM Pratyush Yadav <pratyush@...nel.org> wrote:
> >
> > +Cc Marco, Alexander
> >
> > On Wed, Oct 15 2025, Pasha Tatashin wrote:
> >
> > > KHO allocates metadata for its preserved memory map using the SLUB
> > > allocator via kzalloc(). This metadata is temporary and is used by the
> > > next kernel during early boot to find preserved memory.
> > >
> > > A problem arises when KFENCE is enabled. kzalloc() calls can be
> > > randomly intercepted by kfence_alloc(), which services the allocation
> > > from a dedicated KFENCE memory pool. This pool is allocated early in
> > > boot via memblock.
> >
> > At some point, we'd probably want to add support for preserving slab
> > objects using KHO. That wouldn't work if the objects can land in scratch
> > memory. Right now, the kfence pools are allocated right before KHO goes
> > out of scratch-only and memblock frees pages to buddy.
> 
> If we do that, most likely we will add a GFP flag that goes with it,
> so the slab can use a special pool of pages that are preservable.
> Otherwise, we are going to be leaking memory from the old kernel in
> the unpreserved parts of the pages. 

That isn't an issue. If we make slab preservable then we'd have to
preserve the page and then somehow record what order is stored in that
page and a bit map of which parts are allocated to restore the slab
state on recovery.

So long as the non-preserved memory comes back as freed on the
sucessor kernel it doesn't matter what was in it in the preceeding
kernel. The new kernel will eventually zero it. So it isn't a 'leak'.

Jason