linux-kernel - Re: [RFC PATCH 01/10] kho: drop restriction on maximum page order

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Message-ID: <CA+CK2bDvaGmfkCPCMWM6gPcd4FfUyD6e5yWE+kNcma1vT3Jw3g@mail.gmail.com>
Date: Tue, 23 Dec 2025 12:59:05 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Mike Rapoport <rppt@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...nel.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, Jonathan Corbet <corbet@....net>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Muchun Song <muchun.song@...ux.dev>, 
	Oscar Salvador <osalvador@...e.de>, Alexander Graf <graf@...zon.com>, David Matlack <dmatlack@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, Jason Gunthorpe <jgg@...dia.com>, 
	Samiullah Khawaja <skhawaja@...gle.com>, Vipin Sharma <vipinsh@...gle.com>, 
	Zhu Yanjun <yanjun.zhu@...ux.dev>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	linux-doc@...r.kernel.org, kexec@...ts.infradead.org
Subject: Re: [RFC PATCH 01/10] kho: drop restriction on maximum page order

On Sat, Dec 6, 2025 at 6:03 PM Pratyush Yadav <pratyush@...nel.org> wrote:
>
> KHO currently restricts the maximum order of a restored page to the
> maximum order supported by the buddy allocator. While this works fine
> for much of the data passed across kexec, it is possible to have pages
> larger than MAX_PAGE_ORDER.
>
> For one, it is possible to get a larger order when using
> kho_preserve_pages() if the number of pages is large enough, since it
> tries to combine multiple aligned 0-order preservations into one higher
> order preservation.
>
> For another, upcoming support for hugepages can have gigantic hugepages
> being preserved over KHO.
>
> There is no real reason for this limit. The KHO preservation machinery
> can handle any page order. Remove this artificial restriction on max
> page order.

Before removing this restriction, please also add a patch that makes
sure that the calculated order does not intermix memory from different
topologies.

I suspect we need something like this in kho_preserve_pages():

order = min(count_trailing_zeros(pfn), ilog2(end_pfn - pfn));
/* Reduce order if the block spans across NUMA nodes. */
while (order > 0) {
   size_pfn = 1UL << order;
   if (pfn_to_nid(pfn) == pfn_to_nid(pfn + size_pfn - 1))
     break;
   order--;
}

Pasha