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

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Message-ID: <Z_f3Jyac_o308ws-@casper.infradead.org>
Date: Thu, 10 Apr 2025 17:51:51 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Mike Rapoport <rppt@...nel.org>, Pratyush Yadav <ptyadav@...zon.de>,
	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 Wed, Apr 09, 2025 at 01:28:37PM -0300, Jason Gunthorpe wrote:
> On Wed, Apr 09, 2025 at 07:19:30PM +0300, Mike Rapoport wrote:
> > But we have memdesc today, it's struct page.
> 
> No, I don't think it is. struct page seems to be turning into
> something legacy that indicates the code has not been converted to the
> new stuff yet.

No, struct page will be with us for a while.  Possibly forever.  I have
started reluctantly talking about a future in which there aren't struct
pages, but it's really premature at this point.  That's a 2030 kind
of future.

For 2025-2029, we will still have alloc_page(s)().  It's just that
the size of struct page will be gradually shrinking over that time.

> > And when the data structure that memdesc points to will be allocated
> > separately folios won't make sense for order-0 allocations.
> 
> At that point the lowest level allocator function will be allocating
> the memdesc along with the struct page. Then folio will become
> restricted to only actual folio memdescs and alot of the type punning
> should go away. We are not there yet.

We'll have a few allocator functions.  There'll be a slab_alloc(),
folio_alloc(), pt_alloc() and so on.  I sketched out how these might
work last year:

https://kernelnewbies.org/MatthewWilcox/FolioAlloc

> > > The lowest allocator primitive returns folios, which can represent any
> > > order, and the caller casts to their own memdesc.
> > 
> > The lowest allocation primitive returns pages. 
> 
> Yes, but as I understand things, we should not be calling that
> interface in new code because we are trying to make 'struct page' go
> away.
> 
> Instead you should use the folio interfaces and cast to your own
> memdesc, or use an allocator interface that returns void * (ie slab)
> and never touch the struct page area.
> 
> AFAICT, and I just wrote one of these..

Casting is the best you can do today because I haven't provided a better
interface yet.

> > And I don't think folio will be a lowest primitive buddy returns anytime
> > soon if ever.
> 
> Maybe not internally, but driver facing, I think it should be true.
> 
> Like I just completely purged all struct page from the iommu code:
> 
> https://lore.kernel.org/linux-iommu/0-v4-c8663abbb606+3f7-iommu_pages_jgg@nvidia.com/
> 
> I don't want some weird KHO interface that doesn't align with using
> __folio_alloc_node() and folio_put() as the lowest level allocator
> interface.

I think it's fine to say "the KHO interface doesn't support bare pages;
you must have a memdesc".  But I'm not sure that's the right approach.