linux-kernel - Re: [PATCH v1 05/13] kho: Always expose output FDT in debugfs

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Message-ID: <mafs0pl9k7dor.fsf@kernel.org>
Date: Fri, 14 Nov 2025 17:59:32 +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 05/13] kho: Always expose output FDT in debugfs

On Fri, Nov 14 2025, Pasha Tatashin wrote:

> Currently, the output FDT is added to debugfs only when KHO is
> finalized and removed when aborted.
>
> There is no need to hide the FDT based on the state. Always expose it
> starting from initialization. This aids the transition toward removing
> the explicit abort functionality and converting KHO to be fully
> stateless.
>
> Also, pre-zero the FDT tree so we do not expose random bits to the
> user and to the next kernel.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> ---
>  kernel/liveupdate/kexec_handover.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 83aca3b4af15..cd8641725343 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -1147,8 +1147,6 @@ int kho_abort(void)
>  	__kho_abort();
>  	kho_out.finalized = false;
>  
> -	kho_debugfs_fdt_remove(&kho_out.dbg, kho_out.fdt);
> -
>  	return 0;
>  }
>  
> @@ -1219,9 +1217,6 @@ int kho_finalize(void)
>  
>  	kho_out.finalized = true;
>  
> -	WARN_ON_ONCE(kho_debugfs_fdt_add(&kho_out.dbg, "fdt",
> -					 kho_out.fdt, true));
> -
>  	return 0;
>  }
>  
> @@ -1310,7 +1305,7 @@ static __init int kho_init(void)
>  	if (!kho_enable)
>  		return 0;
>  
> -	fdt_page = alloc_page(GFP_KERNEL);
> +	fdt_page = alloc_page(GFP_KERNEL | __GFP_ZERO);

If I read the series right, patch 9 will make this a full FDT with no
subnodes. That makes a lot more sense than a zero page. Thinking out
loud.

For this patch,

Reviewed-by: Pratyush Yadav <pratyush@...nel.org>

>  	if (!fdt_page) {
>  		err = -ENOMEM;
>  		goto err_free_scratch;
> @@ -1344,6 +1339,9 @@ static __init int kho_init(void)
>  			init_cma_reserved_pageblock(pfn_to_page(pfn));
>  	}
>  
> +	WARN_ON_ONCE(kho_debugfs_fdt_add(&kho_out.dbg, "fdt",
> +					 kho_out.fdt, true));
> +
>  	return 0;
>  
>  err_free_fdt:

-- 
Regards,
Pratyush Yadav