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

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Message-ID: <mafs0zf8o5xjk.fsf@kernel.org>
Date: Fri, 14 Nov 2025 18:33:35 +0100
From: Pratyush Yadav <pratyush@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
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, 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().

>  	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