linux-kernel - Re: [PATCH v1 11/13] kho: Allow memory preservation state updates after finalization

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Message-ID: <CA+CK2bC99-VwzuyB-0D=z9H_zF6gnH-EX1USf-Ox-fwGd1s5dg@mail.gmail.com>
Date: Fri, 14 Nov 2025 12:47:24 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: akpm@...ux-foundation.org, bhe@...hat.com, rppt@...nel.org, 
	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 11/13] kho: Allow memory preservation state updates
 after finalization

On Fri, Nov 14, 2025 at 12:33 PM Pratyush Yadav <pratyush@...nel.org> wrote:
>
> On Fri, Nov 14 2025, Pasha Tatashin wrote:
>
> > Currently, kho_preserve_* and kho_unpreserve_* return -EBUSY if
> > KHO is finalized. This enforces a rigid "freeze" on the KHO memory
> > state.
> >
> > With the introduction of re-entrant finalization, this restriction is
> > no longer necessary. Users should be allowed to modify the preservation
> > set (e.g., adding new pages or freeing old ones) even after an initial
> > finalization.
> >
> > The intended workflow for updates is now:
> > 1. Modify state (preserve/unpreserve).
> > 2. Call kho_finalize() again to refresh the serialized metadata.
> >
> > Remove the kho_out.finalized checks to enable this dynamic behavior.
> >
> > Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> > ---
> >  kernel/liveupdate/kexec_handover.c | 13 -------------
> >  1 file changed, 13 deletions(-)
> >
> > diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> > index 27ef20565a5f..87e9b488237d 100644
> > --- a/kernel/liveupdate/kexec_handover.c
> > +++ b/kernel/liveupdate/kexec_handover.c
> > @@ -183,10 +183,6 @@ static int __kho_preserve_order(struct kho_mem_track *track, unsigned long pfn,
> >       const unsigned long pfn_high = pfn >> order;
> >
> >       might_sleep();
> > -
> > -     if (kho_out.finalized)
> > -             return -EBUSY;
> > -
> >       physxa = xa_load(&track->orders, order);
> >       if (!physxa) {
> >               int err;
> > @@ -815,9 +811,6 @@ int kho_unpreserve_folio(struct folio *folio)
>
> This can be void now. This would make consumers a bit simpler, since
> right now, the memfd preservation logic does a WARN_ON() if this
> function fails. That can be dropped now that the function can never
> fail.
>
> Same for kho_unpreserve_pages() and kho_unpreserve_vmalloc().

Oh, this is a very good suggestion, really disliked those
kho_unpreserve_* errors.

>
> >       const unsigned int order = folio_order(folio);
> >       struct kho_mem_track *track = &kho_out.track;
> >
> > -     if (kho_out.finalized)
> > -             return -EBUSY;
> > -
> >       __kho_unpreserve_order(track, pfn, order);
> >       return 0;
> >  }
> > @@ -885,9 +878,6 @@ int kho_unpreserve_pages(struct page *page, unsigned int nr_pages)
> >       const unsigned long start_pfn = page_to_pfn(page);
> >       const unsigned long end_pfn = start_pfn + nr_pages;
> >
> > -     if (kho_out.finalized)
> > -             return -EBUSY;
> > -
> >       __kho_unpreserve(track, start_pfn, end_pfn);
> >
> >       return 0;
> > @@ -1066,9 +1056,6 @@ EXPORT_SYMBOL_GPL(kho_preserve_vmalloc);
> >   */
> >  int kho_unpreserve_vmalloc(struct kho_vmalloc *preservation)
> >  {
> > -     if (kho_out.finalized)
> > -             return -EBUSY;
> > -
> >       kho_vmalloc_free_chunks(preservation);
> >
> >       return 0;
>
> --
> Regards,
> Pratyush Yadav