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

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Message-ID: <CA+CK2bDBU4Bb83tUyzdiTa+kjhKK7fvObzpqH1gOPwKqoOnO8Q@mail.gmail.com>
Date: Sun, 21 Sep 2025 19:07:01 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Jason Gunthorpe <jgg@...dia.com>, 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>, 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 Sun, Sep 21, 2025 at 6:26 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> 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?

Normally, I would agree, but in this case, this has to be a simple
data structure that, in the long run, is going to be stable between
different kernel versions: the old and the next kernel must understand
it. Therefore, relying on any external data structure would require
the maintainers and other developers to be aware of this rather
unusual kernel requirement. So, I think it is much better to keep this
implementation private to KHO, whose only responsibility is reliably
passing memory pages from the old kernel to the next kernel.