Re: [PATCH 1/9] kho: Reclaim the metadata page left reserved after kexec

[Pasha Tatashin]

> After a KHO kexec, the new kernel keeps the page holding the previous
> kernel's kexec metadata reserved for as long as it runs. It only needs
> the page once, since kho_in_kexec_metadata() copies the previous release
> and the kexec count out of it at boot, and nothing reads it again.
> 
> Free it with kho_restore_free() once copying is complete.
> 
> Fixes: 76aa46b9e404 ("kho: kexec-metadata: track previous kernel chain")

Do we really need to apply this to stable? It behaves correctly, and 
your patch performs a minor optimization by releasing a single page back 
to the system.

> Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>
>
> diff --git a/Documentation/admin-guide/mm/kho.rst b/Documentation/admin-guide/mm/kho.rst
> index 902d171a780a..f9d2cc5999ee 100644
> --- a/Documentation/admin-guide/mm/kho.rst
> +++ b/Documentation/admin-guide/mm/kho.rst
> @@ -120,4 +120,6 @@ stabilized.
>  
>  ``/sys/kernel/debug/kho/in/sub_fdts/``
>      Similar to ``kho/out/sub_fdts/``, but contains sub blobs
> -    of KHO producers passed from the old kernel.
> +    of KHO producers passed from the old kernel. A blob disappears
> +    once the kernel has read and freed it, as ``kexec-metadata`` does
> +    during boot.
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 6f1f55fe3181..8a07783326a4 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -1782,6 +1782,9 @@ static void __init kho_in_kexec_metadata(void)
>  
>  	pr_info("exec from: %s (count %u)\n",
>  		kho_in.previous_release, kho_in.kexec_count);
> +
> +	kho_debugfs_blob_remove(&kho_in.dbg, metadata);
> +	kho_restore_free(metadata);
>  }
>  
>  /*
> @@ -1859,14 +1862,15 @@ static __init int kho_init(void)
>  	if (err)
>  		goto err_free_fdt;
>  
> +	if (fdt)
> +		kho_in_debugfs_init(&kho_in.dbg, fdt);
> +

so add debugfs node here.

>  	err = kho_kexec_metadata_init(fdt);

Remove the kexec metadata node here. While this is somewhat unintuitive, 
I do not see a better alternative.

>  	if (err)
>  		goto err_free_fdt;
>  
> -	if (fdt) {
> -		kho_in_debugfs_init(&kho_in.dbg, fdt);
> +	if (fdt)
>  		return 0;
> -	}
>  
>  	for (int i = 0; i < kho_bootmem_cnt; i++) {
>  		unsigned long base_pfn = PHYS_PFN(kho_bootmem[i].addr);

Reviewed-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>

-- 
Pasha