Re: [PATCH] kho: add support for deferred struct page init

[Mike Rapoport <rppt@kernel.org>]

Hi Evangelos,

On Tue, Dec 16, 2025 at 08:49:12AM +0000, Evangelos Petrongonas wrote:
> When `CONFIG_DEFERRED_STRUCT_PAGE_INIT` is enabled, struct page

No need for markup formatting in the changelog.

> initialization is deferred to parallel kthreads that run later
> in the boot process.
> 
> During KHO restoration, `deserialize_bitmap()` writes metadata for
> each preserved memory region. However, if the struct page has not been
> initialized, this write targets uninitialized memory, potentially
> leading to errors like:
> ```
> BUG: unable to handle page fault for address: ...
> ```
> 
> Fix this by introducing `kho_get_preserved_page()`,  which ensures
> all struct pages in a preserved region are initialized by calling
> `init_deferred_page()` which is a no-op when deferred init is disabled
> or when the struct page is already initialized.
> 
> Fixes: 8b66ed2c3f42 ("kho: mm: don't allow deferred struct page with KHO")
> Signed-off-by: Evangelos Petrongonas <epetron@amazon.de>
> ---

...

> +static struct page *__init kho_get_preserved_page(phys_addr_t phys,
> +						  unsigned int order)
> +{
> +	unsigned long pfn = PHYS_PFN(phys);
> +	int nid = early_pfn_to_nid(pfn);
> +
> +	for (int i = 0; i < (1 << order); i++)
> +		init_deferred_page(pfn + i, nid);

This will skip pages below node->first_deferred_pfn, we need to use
__init_page_from_nid() here.

> +
> +	return pfn_to_page(pfn);
> +}
> +
>  static void __init deserialize_bitmap(unsigned int order,
>  				      struct khoser_mem_bitmap_ptr *elm)
>  {
> @@ -449,7 +466,7 @@ static void __init deserialize_bitmap(unsigned int order,
>  		int sz = 1 << (order + PAGE_SHIFT);
>  		phys_addr_t phys =
>  			elm->phys_start + (bit << (order + PAGE_SHIFT));
> -		struct page *page = phys_to_page(phys);
> +		struct page *page = kho_get_preserved_page(phys, order);

I think it's better to initialize deferred struct pages later in
kho_restore_page. deserialize_bitmap() runs before SMP and it already does
heavy lifting of memblock_reserve()s. Delaying struct page initialization
until restore makes it at least run in parallel with other initialization
tasks.

I started to work on this just before plumbers and I have something
untested here:

https://git.kernel.org/pub/scm/linux/kernel/git/rppt/linux.git/log/?h=kho/deferred-page/v0.1

>  		union kho_page_info info;
>  
>  		memblock_reserve(phys, sz);
> -- 
> 2.43.0
> 
> 
> 
> 
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-- 
Sincerely yours,
Mike.