linux-kernel - Re: [PATCH v5 09/16] kexec: enable KHO support for memory preservation

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Message-ID: <Z-Gh-t9xB21UNGU2@kernel.org>
Date: Mon, 24 Mar 2025 14:18:34 -0400
From: Mike Rapoport <rppt@...nel.org>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Changyuan Lyu <changyuanl@...gle.com>, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v5 09/16] kexec: enable KHO support for memory
 preservation

On Sun, Mar 23, 2025 at 03:55:52PM -0300, Jason Gunthorpe wrote:
> On Sat, Mar 22, 2025 at 03:12:26PM -0400, Mike Rapoport wrote:
>  
> > > > +		page->private = order;
> > > 
> > > Can't just set the page order directly? Why use private?
> > 
> > Setting the order means recreating the folio the way prep_compound_page()
> > does. I think it's better to postpone it until the folio is requested. This
> > way it might run after SMP is enabled. 
> 
> I see, that makes sense, but also it could stil use page->order..

But there's no page->order :)
 
> > Besides, when we start allocating
> > folios separately from struct page, initializing it here would be a real
> > issue.
> 
> Yes, but also we wouldn't have page->private to make it work.. Somehow
> anything we want to carry over would have to become encoded in the
> memdesc directly.

This is a problem to solve in 2026 :)

The January update for State of Page [1] talks about 

	reasonable goal to shrink struct page to (approximately): 

	struct page {
	    unsigned long flags;
	    union {
	        struct list_head buddy_list;
	        struct list_head pcp_list;
	        struct {
	            unsigned long memdesc;
	            int _refcount;
	        };
	    };
	    union {
	        unsigned long private;
	        struct {
	            int _folio_mapcount;
	        };
	    };
	};
 
[1] https://lore.kernel.org/linux-mm/Z37pxbkHPbLYnDKn@casper.infradead.org/
 
> Jason

-- 
Sincerely yours,
Mike.