linux-kernel - Re: [RFC v1 0/4] Make KHO Stateless

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Message-ID: <aNB7fTesaVCXHB0E@casper.infradead.org>
Date: Sun, 21 Sep 2025 23:26:05 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Jason Miu <jasonmiu@...gle.com>, Alexander Graf <graf@...zon.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Baoquan He <bhe@...hat.com>, Changyuan Lyu <changyuanl@...gle.com>,
	David Matlack <dmatlack@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Joel Granados <joel.granados@...nel.org>,
	Marcos Paulo de Souza <mpdesouza@...e.com>,
	Mario Limonciello <mario.limonciello@....com>,
	Mike Rapoport <rppt@...nel.org>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	Petr Mladek <pmladek@...e.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Steven Chen <chenste@...ux.microsoft.com>,
	Yan Zhao <yan.y.zhao@...el.com>, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC v1 0/4] Make KHO Stateless

On Wed, Sep 17, 2025 at 08:36:09AM -0300, Jason Gunthorpe wrote:
> On Tue, Sep 16, 2025 at 07:50:15PM -0700, Jason Miu wrote:
> > This series transitions KHO from an xarray-based metadata tracking
> > system with serialization to using page table like data structures
> > that can be passed directly to the next kernel.
> > 
> > The key motivations for this change are to:
> > - Eliminate the need for data serialization before kexec.
> > - Remove the former KHO state machine by deprecating the finalize
> >   and abort states.
> > - Pass preservation metadata more directly to the next kernel via the FDT.
> > 
> > The new approach uses a per-order page table structure (kho_order_table,
> > kho_page_table, kho_bitmap_table) to mark preserved pages. The physical
> > address of the root `kho_order_table` is passed in the FDT, allowing the
> > next kernel to reconstruct the preserved memory map.
> 
> It is not a "page table" structure, it is just a radix tree with bits
> as the leaf.

Sounds like the IDA data structure.  Maybe that API needs to be enhanced
for this use case, but surely using the same data structure would be a
good thing?