linux-kernel - Re: [PATCH v1 04/13] kho: Verify deserialization status and fix FDT alignment access

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Message-ID: <aR7v2VyYfs2HAUbW@kernel.org>
Date: Thu, 20 Nov 2025 12:39:21 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>, akpm@...ux-foundation.org,
	bhe@...hat.com, jasonmiu@...gle.com, arnd@...db.de, coxu@...hat.com,
	dave@...ilevsky.ca, ebiggers@...gle.com, graf@...zon.com,
	kees@...nel.org, linux-kernel@...r.kernel.org,
	kexec@...ts.infradead.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 04/13] kho: Verify deserialization status and fix FDT
 alignment access

On Tue, Nov 18, 2025 at 06:11:24PM +0100, Pratyush Yadav wrote:
> On Tue, Nov 18 2025, Pasha Tatashin wrote:
> 
> >> > This page is never freed, so adding it to zone managed pages or keeping it
> >> > reserved does not change anything.
> >>
> >> In practice, sure. I still don't see a good reason to _not_ initialize
> >> the page properly. It's not like it costs us much in terms of
> >> performance or code complexity.
> >>
> >> Since kho_restore_folio() makes sure the folio was _actually_ preserved
> >> from KHO, you have a safety check against previous kernel having a bug
> >> and not preserving the FDT properly. And I get that the FDT has already
> >> been used by this point, but at least you would have some known point to
> >> catch this.
> >
> > The kho_alloc_preserve() API is different from kho_preserve_folio().
> > With kho_preserve_folio(), memory is allocated and some time later is
> > preserved, so there is a possibility for that memory to exist and be
> > used where it is not preserved, therefore it is a crucial step for
> > such memory to also do kho_restore_folio() before used. With
> > kho_alloc_preserve(), when the memory exists it is always preserved;
> > it is gurantee of this API. There is no reason to do
> > kho_restore_folio() on such memory at all. It can be released back to
> > the system via kho_free_restore()/kho_free_unpreserve().
> 
> Even for those I think there should be a kho_restore_mem() or something
> similar (naming things is hard :/), so they go through the restore,
> their struct page is properly initialized and accounted for, and
> make sure the pages were actually preserved.
> 
> Using the memory without restoring it first should be the exception IMO.
 
Base KHO and LUO FTDs are such exceptions for sure :)
We have to use them way before we can even think about restoring.

> -- 
> Regards,
> Pratyush Yadav

-- 
Sincerely yours,
Mike.