Re: [PATCH v4] kho: kexec-metadata: track previous kernel chain

[Mike Rapoport <rppt@kernel.org>]

On Thu, Jan 22, 2026 at 04:04:55AM -0800, Breno Leitao wrote:
> Hello Mike,
> 
> On Thu, Jan 22, 2026 at 12:57:50PM +0200, Mike Rapoport wrote:
> > > +/**
> > > + * DOC: Kexec Metadata ABI
> > > + *
> > 
> > It would be nice to link it from Documentation/ as well ;-)
> 
> Ack! I am planning something as:
> 
> 	commit 90e098ca0d611b44594f08e50ba1cff3c932dd2b
> 	Author: Breno Leitao <leitao@debian.org>
> 	Date:   Thu Jan 22 03:47:23 2026 -0800
> 
> 	kho: document kexec-metadata tracking feature
> 	
> 	Add documentation for the kexec-metadata feature that tracks the
> 	previous kernel version and kexec boot count across kexec reboots.
> 	This helps diagnose bugs that only reproduce when kexecing from
> 	specific kernel versions.
> 	
> 	Suggested-by: Mike Rapoport <rppt@kernel.org>
> 	Signed-off-by: Breno Leitao <leitao@debian.org>
> 
> 	diff --git a/Documentation/admin-guide/mm/kho.rst b/Documentation/admin-guide/mm/kho.rst
> 	index 6dc18ed4b8861..1faf2c3ba4620 100644
> 	--- a/Documentation/admin-guide/mm/kho.rst
> 	+++ b/Documentation/admin-guide/mm/kho.rst
> 	@@ -113,3 +113,42 @@ stabilized.
> 	``/sys/kernel/debug/kho/in/sub_fdts/``
> 	Similar to ``kho/out/sub_fdts/``, but contains sub FDT blobs
> 	of KHO producers passed from the old kernel.
> 	+
> 	+Kexec Metadata
> 	+==============

I'd move this section before "debugfs Interfaces", other than that LGTM.

> 	+
> 	+KHO automatically tracks metadata about the kexec chain, passing information
> 	+about the previous kernel to the next kernel. This feature helps diagnose
> 	+bugs that only reproduce when kexecing from specific kernel versions.

...

> > >  static __init int kho_init(void)
> > >  {
> > >  	const void *fdt = kho_get_fdt();
> > > @@ -1357,6 +1413,15 @@ static __init int kho_init(void)
> > >  	if (err)
> > >  		goto err_free_fdt;
> > >  
> > > +	if (fdt)
> > > +		kho_process_kexec_metadata();
> > 
> > Can't we move it into the existing if (fdt) below?
> 
> Unfortunately, that won't work due to a data dependency between the two
> functions.
> 
> kho_process_kexec_metadata() reads from the FDT subtree and populates kho_in:
> 
> Basically:
> 
> 	kho_in.kexec_count = metadata->kexec_count;
> 
> While kho_populate_kexec_metadata() increments metadata->kexec_count:
> 
>           /* kho_in.kexec_count is set to 0 on cold boot */
>           metadata->kexec_count = kho_in.kexec_count + 1;
> 
> If kho_process_kexec_metadata() is moved after kho_populate_kexec_metadata(),
> the count would always increment from 0 to 1, ignoring whatever was passed in
> the FDT.
> 
> Restructuring to call kho_in_debugfs_init() earlier also doesn't work:
> 
> 
> 	if (fdt) {
> 		kho_in_debugfs_init(&kho_in.dbg, fdt);
> 		kho_process_kexec_metadata();
> 		return 0;
> 	}
> 
> 	/* Populate kexec metadata for the possible next kexec */
> 	err = kho_populate_kexec_metadata();
> 	if (err)
>                   pr_warn("failed to initialize kexec-metadata subtree: %d\n",
>                           err);
> 
> This would return early without populating the kexec metadata for the next
> kexec, breaking the chain on KHO boots.

How about we rename kho_process_kexec_metadata() to
kho_retreive_kexec_metadata() and add kho_process_kexec_metadata() that
will first call _retrieve and then _populate? Something like

static int __init kho_process_kexec_metadata(const void *fdt)
{
	int err;

	if (fdt)
		kho_retrieve_kexec_metadata();

	/* Populate kexec metadata for the possible next kexec */
	err = kho_populate_kexec_metadata();
	if (err)
		pr_warn("failed to initialize kexec-metadata subtree: %d\n",
			err);

	return err;
}

> --breno

-- 
Sincerely yours,
Mike.