linux-kernel - Re: [PATCH] kho: make sure folio being restored is actually from KHO

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Message-ID: <aMGc-ExhkqwAyY_C@casper.infradead.org>
Date: Wed, 10 Sep 2025 16:44:56 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Alexander Graf <graf@...zon.com>, Mike Rapoport <rppt@...nel.org>,
	Changyuan Lyu <changyuanl@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Baoquan He <bhe@...hat.com>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	Jason Gunthorpe <jgg@...dia.com>, Chris Li <chrisl@...nel.org>,
	Jason Miu <jasonmiu@...gle.com>, linux-kernel@...r.kernel.org,
	kexec@...ts.infradead.org, linux-mm@...ck.org
Subject: Re: [PATCH] kho: make sure folio being restored is actually from KHO

On Wed, Sep 10, 2025 at 05:34:40PM +0200, Pratyush Yadav wrote:
> +#define KHO_PAGE_MAGIC 0x4b484f50U /* ASCII for 'KHOP' */
> +
> +/*
> + * KHO uses page->private, which is an unsigned long, to store page metadata.
> + * Use it to store both the magic and the order.
> + */
> +union kho_page_info {
> +	unsigned long page_private;
> +	struct {
> +		unsigned int order;
> +		unsigned int magic;
> +	};

KHO is only supported on 64-bit?

> @@ -210,16 +226,16 @@ static void kho_restore_page(struct page *page, unsigned int order)
>  struct folio *kho_restore_folio(phys_addr_t phys)
>  {
>  	struct page *page = pfn_to_online_page(PHYS_PFN(phys));
> -	unsigned long order;
> +	union kho_page_info info;
>  
>  	if (!page)
>  		return NULL;
>  
> -	order = page->private;
> -	if (order > MAX_PAGE_ORDER)
> +	info.page_private = page->private;
> +	if (info.magic != KHO_PAGE_MAGIC || info.order > MAX_PAGE_ORDER)
>  		return NULL;
>  
> -	kho_restore_page(page, order);
> +	kho_restore_page(page, info.order);
>  	return page_folio(page);

This all looks very confused.  Before your patch as well as after it.
I don't see anything in the current KHO code that requires the
phys_addr_t to be order-aligned.