linux-kernel - Re: [RFC PATCH 04/10] liveupdate: flb: allow getting FLB data in early boot

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Message-ID: <CA+CK2bAV1y_LySjyj-wcn1cdSuVBdC+r+zQL7AQTY64nk3OxuQ@mail.gmail.com>
Date: Thu, 18 Dec 2025 13:25:32 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Mike Rapoport <rppt@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...nel.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, Jonathan Corbet <corbet@....net>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Muchun Song <muchun.song@...ux.dev>, 
	Oscar Salvador <osalvador@...e.de>, Alexander Graf <graf@...zon.com>, David Matlack <dmatlack@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, Jason Gunthorpe <jgg@...dia.com>, 
	Samiullah Khawaja <skhawaja@...gle.com>, Vipin Sharma <vipinsh@...gle.com>, 
	Zhu Yanjun <yanjun.zhu@...ux.dev>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	linux-doc@...r.kernel.org, kexec@...ts.infradead.org
Subject: Re: [RFC PATCH 04/10] liveupdate: flb: allow getting FLB data in
 early boot

On Sat, Dec 6, 2025 at 6:03 PM Pratyush Yadav <pratyush@...nel.org> wrote:
>
> To support hugepage preservation using LUO, the hugetlb subsystem needs
> to get liveupdate data when it allocates the hugepages to find out how
> many pages are coming from live update. This data is preserved via LUO
> FLB.
>
> Since gigantic hugepage allocations happen before LUO (and much of the
> rest of the system) is initialized, the usual
> liveupdate_flb_get_incoming() can not work.
>
> Add a read-only variant that fetches the FLB data but does not trigger
> its retrieve or do any locking or reference counting. It is the caller's
> responsibility to make sure there are no side effects of using this data
> to the proper retrieve call that would happen later.
>
> Refactor the logic to find the right FLB in the serialized data in a
> helper that can be used from both luo_flb_retrieve_one() (called from
> luo_flb_get_incoming()), and from luo_flb_get_incoming_early().
>
> Signed-off-by: Pratyush Yadav <pratyush@...nel.org>
> ---
>  include/linux/liveupdate.h  |  6 ++++
>  kernel/liveupdate/luo_flb.c | 69 +++++++++++++++++++++++++++++--------
>  2 files changed, 60 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/liveupdate.h b/include/linux/liveupdate.h
> index 78e8c529e4e7..39b429d2c62c 100644
> --- a/include/linux/liveupdate.h
> +++ b/include/linux/liveupdate.h
> @@ -232,6 +232,7 @@ int liveupdate_unregister_flb(struct liveupdate_file_handler *fh,
>
>  int liveupdate_flb_get_incoming(struct liveupdate_flb *flb, void **objp);
>  int liveupdate_flb_get_outgoing(struct liveupdate_flb *flb, void **objp);
> +int liveupdate_flb_incoming_early(struct liveupdate_flb *flb, u64 *datap);

Hi Pratyush,

[Follow-up from LPC discussion]

This patch is not needed, you can use liveupdate_flb_get_incoming()
directly in early boot. The main concern is that we take mutex in that
function, but that I think is safe. The might_sleep() has the proper
handling to be called early in boot, it has "system_state ==
SYSTEM_BOOTING" check to silence warning during boot.

Pasha