linux-kernel - Re: [PATCH] kho: add support for deferred struct page init

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Message-ID: <20260205093948.GA96044@dev-dsk-epetron-1c-1d4d9719.eu-west-1.amazon.com>
Date: Thu, 5 Feb 2026 09:39:48 +0000
From: Evangelos Petrongonas <epetron@...zon.de>
To: Mike Rapoport <rppt@...nel.org>
CC: Pasha Tatashin <pasha.tatashin@...een.com>, Pratyush Yadav
	<pratyush@...nel.org>, Alexander Graf <graf@...zon.com>, Andrew Morton
	<akpm@...ux-foundation.org>, Jason Miu <jasonmiu@...gle.com>,
	<linux-kernel@...r.kernel.org>, <kexec@...ts.infradead.org>,
	<linux-mm@...ck.org>, <nh-open-source@...zon.com>
Subject: Re: [PATCH] kho: add support for deferred struct page init

On Wed, Feb 04, 2026 at 08:44:48PM +0200 Mike Rapoport wrote:
> Hi Evangelos,
> 
> Are you planning to send a v2?
> 
> On Tue, Dec 16, 2025 at 08:49:12AM +0000, Evangelos Petrongonas wrote:

Hi Mike,
I am leaving soon for some AFK time, therefore I don't believe I will
post a v2 anytime before the beginning of March. Feel free (you or
anyone else) to send a patch. If a patch is sent until then I am happy,
to test it when I am back.

> > When `CONFIG_DEFERRED_STRUCT_PAGE_INIT` is enabled, struct page
> > initialization is deferred to parallel kthreads that run later
> > in the boot process.
> > 
> > During KHO restoration, `deserialize_bitmap()` writes metadata for
> > each preserved memory region. However, if the struct page has not been
> > initialized, this write targets uninitialized memory, potentially
> > leading to errors like:
> > ```
> > BUG: unable to handle page fault for address: ...
> > ```
> > 
> > Fix this by introducing `kho_get_preserved_page()`,  which ensures
> > all struct pages in a preserved region are initialized by calling
> > `init_deferred_page()` which is a no-op when deferred init is disabled
> > or when the struct page is already initialized.
> > 
> > Fixes: 8b66ed2c3f42 ("kho: mm: don't allow deferred struct page with KHO")
> > Signed-off-by: Evangelos Petrongonas <epetron@...zon.de>
> > ---
> > ### Notes
> > @Jason, this patch should act as a temporary fix to make KHO play nice
> > with deferred struct page init until you post your ideas about splitting
> > "Physical Reservation" from "Metadata Restoration".
> > 
> > ### Testing
> > In order to test the fix, I modified the KHO selftest, to allocate more
> > memory and do so from higher memory to trigger the incompatibility. The
> > branch with those changes can be found in:
> > https://git.infradead.org/?p=users/vpetrog/linux.git;a=shortlog;h=refs/heads/kho-deferred-struct-page-init
> > 
> > In future patches, we might want to enhance the selftest to cover
> > this case as well. However, properly adopting the test for this
> > is much more work than the actual fix, therefore it can be deferred to a
> > follow-up series.
> > 
> > In addition attempting to run the selftest for arm (without my changes)
> > fails with:
> > ```
> > ERROR:target/arm/internals.h:767:regime_is_user: code should not be reached
> > Bail out! ERROR:target/arm/internals.h:767:regime_is_user: code should not be reached
> > ./tools/testing/selftests/kho/vmtest.sh: line 113: 61609 Aborted
> > ```
> > I have not looked it up further, but can also do so as part of a
> > selftest follow-up.
> > 
> >  kernel/liveupdate/Kconfig          |  2 --
> >  kernel/liveupdate/kexec_handover.c | 19 ++++++++++++++++++-
> >  2 files changed, 18 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/liveupdate/Kconfig b/kernel/liveupdate/Kconfig
> > index d2aeaf13c3ac..9394a608f939 100644
> > --- a/kernel/liveupdate/Kconfig
> > +++ b/kernel/liveupdate/Kconfig
> > @@ -1,12 +1,10 @@
> >  # SPDX-License-Identifier: GPL-2.0-only
> >  
> >  menu "Live Update and Kexec HandOver"
> > -	depends on !DEFERRED_STRUCT_PAGE_INIT
> >  
> >  config KEXEC_HANDOVER
> >  	bool "kexec handover"
> >  	depends on ARCH_SUPPORTS_KEXEC_HANDOVER && ARCH_SUPPORTS_KEXEC_FILE
> > -	depends on !DEFERRED_STRUCT_PAGE_INIT
> >  	select MEMBLOCK_KHO_SCRATCH
> >  	select KEXEC_FILE
> >  	select LIBFDT
> > diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> > index 9dc51fab604f..78cfe71e6107 100644
> > --- a/kernel/liveupdate/kexec_handover.c
> > +++ b/kernel/liveupdate/kexec_handover.c
> > @@ -439,6 +439,23 @@ static int kho_mem_serialize(struct kho_out *kho_out)
> >  	return err;
> >  }
> >  
> > +/*
> > + * With CONFIG_DEFERRED_STRUCT_PAGE_INIT, struct pages in higher memory
> > + * regions may not be initialized yet at the time KHO deserializes preserved
> > + * memory. This function ensures all struct pages in the region are initialized.
> > + */
> > +static struct page *__init kho_get_preserved_page(phys_addr_t phys,
> > +						  unsigned int order)
> > +{
> > +	unsigned long pfn = PHYS_PFN(phys);
> > +	int nid = early_pfn_to_nid(pfn);
> > +
> > +	for (int i = 0; i < (1 << order); i++)
> > +		init_deferred_page(pfn + i, nid);
> > +
> > +	return pfn_to_page(pfn);
> > +}
> > +
> >  static void __init deserialize_bitmap(unsigned int order,
> >  				      struct khoser_mem_bitmap_ptr *elm)
> >  {
> > @@ -449,7 +466,7 @@ static void __init deserialize_bitmap(unsigned int order,
> >  		int sz = 1 << (order + PAGE_SHIFT);
> >  		phys_addr_t phys =
> >  			elm->phys_start + (bit << (order + PAGE_SHIFT));
> > -		struct page *page = phys_to_page(phys);
> > +		struct page *page = kho_get_preserved_page(phys, order);
> >  		union kho_page_info info;
> >  
> >  		memblock_reserve(phys, sz);
> > -- 
> > 2.43.0
> > 
> > 
> > 
> > 
> > Amazon Web Services Development Center Germany GmbH
> > Tamara-Danz-Str. 13
> > 10243 Berlin
> > Geschaeftsfuehrung: Christof Hellmis, Andreas Stieger
> > Eingetragen am Amtsgericht Charlottenburg unter HRB 257764 B
> > Sitz: Berlin
> > Ust-ID: DE 365 538 597
> > 
> 
> -- 
> Sincerely yours,
> Mike.

--
Kind Regards,
Evangelos




Amazon Web Services Development Center Germany GmbH
Tamara-Danz-Str. 13
10243 Berlin
Geschaeftsfuehrung: Christof Hellmis, Andreas Stieger
Eingetragen am Amtsgericht Charlottenburg unter HRB 257764 B
Sitz: Berlin
Ust-ID: DE 365 538 597