linux-kernel - Re: [PATCH v6 09/10] liveupdate: kho: Increase metadata bitmap size to PAGE

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Message-ID: <aPXszzdehAbkPOAh@kernel.org>
Date: Mon, 20 Oct 2025 11:03:27 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: akpm@...ux-foundation.org, brauner@...nel.org, corbet@....net,
	graf@...zon.com, jgg@...pe.ca, linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
	masahiroy@...nel.org, ojeda@...nel.org, pratyush@...nel.org,
	rdunlap@...radead.org, tj@...nel.org, jasonmiu@...gle.com,
	dmatlack@...gle.com, skhawaja@...gle.com
Subject: Re: [PATCH v6 09/10] liveupdate: kho: Increase metadata bitmap size
 to PAGE_SIZE

On Sat, Oct 18, 2025 at 01:17:55PM -0400, Pasha Tatashin wrote:
> Metadata is preserved via 512-bytes, which requires using slabs. Slabs

KHO memory preservation metadata is preserved in 512 byte chunks which
requires their allocation from slab allocator.

> are not safe to be used with KHO because of kfence, and because partial

Please add more details why kfence is not safe here.

> slabs may lead leaks to the next kernel. Change the size to be
> PAGE_SIZE.
> 
> While this change could potentially increase metadata overhead on
> systems with sparsely preserved memory, this is being mitigated by
> ongoing work to reduce sparseness during preservation via 1G guest
> pages. Furthermore, this change aligns with future work on a stateless
> KHO, which will also use page-sized bitmaps for its radix tree metadata.

With the stateless memory preservation there would be no problem with
kfence, right?
Maybe we should accelerate the stateless preservation work?
 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>

-- 
Sincerely yours,
Mike.