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

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Message-ID: <mafs08qimjoez.fsf@yadavpratyush.com>
Date: Wed, 10 Sep 2025 17:52:04 +0200
From: Pratyush Yadav <me@...avpratyush.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Pratyush Yadav <pratyush@...nel.org>,  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, Matthew Wilcox wrote:

> 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?

Yes. Currently only x86_64 and ARM64. It is mainly for hypervisor live
update so there isn't much reason to support it on 32-bit platforms.

>
>> @@ -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.

Right, good point. I can send that as a follow up patch. But I think
this patch stands on its own without that fix too.

-- 
Regards,
Pratyush Yadav