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

[Mike Rapoport <rppt@kernel.org>]

On Wed, Apr 09, 2025 at 12:37:14PM -0300, Jason Gunthorpe wrote:
> On Wed, Apr 09, 2025 at 04:58:16PM +0300, Mike Rapoport wrote:
> > >
> > > I think we still don't really know what will be needed, so I'd stick
> > > with folio only as that allows building the memfd and a potential slab
> > > preservation system.
> > 
> > void * seems to me much more reasonable than folio one as the starting
> > point because it allows preserving folios with the right order but it's not
> > limited to it. 
> 
> It would just call kho_preserve_folio() under the covers though.

How that will work for memblock and 1G pages?
 
> > I don't mind having kho_preserve_folio() from day 1 and even stretching the
> > use case we have right now to use it to preserve FDT memory.
> > 
> > But kho_preserve_folio() does not make sense for reserve_mem and it won't
> > make sense for vmalloc.
> 
> It does for vmalloc too, just stop thinking about it as a
> folio-for-pagecache and instead as an arbitary order handle to buddy
> allocator memory that will someday be changed to a memdesc :|

But we have memdesc today, it's struct page. It will be shrinked and maybe
renamed, it will contain a pointer rather than data, but that's what basic
memdesc is.
And when the data structure that memdesc points to will be allocated
separately folios won't make sense for order-0 allocations.
 
> > The weird games slab does with casting back and forth to folio also seem to
> > me like transitional and there won't be that folios in slab later.
> 
> Yes transitional, but we are at the transitional point and KHO should
> fit in.
> 
> 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. 

struct folio *__folio_alloc_noprof(gfp_t gfp, unsigned int order, int preferred_nid,
		nodemask_t *nodemask)
{
	struct page *page = __alloc_pages_noprof(gfp | __GFP_COMP, order,
					preferred_nid, nodemask);
	return page_rmappable_folio(page);
}
EXPORT_SYMBOL(__folio_alloc_noprof);

And page_rmappable_folio() clues about folio-for-pagecache very clearly.

And I don't think folio will be a lowest primitive buddy returns anytime
soon if ever.
 
> Jason
> 

-- 
Sincerely yours,
Mike.