linux-kernel - Re: [PATCH 05/10] guestmemfs: add file mmap callback

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Message-ID: <20241104143933.GF35848@ziepe.ca>
Date: Mon, 4 Nov 2024 10:39:33 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Gowans, James" <jgowans@...zon.com>
Cc: "quic_eberman@...cinc.com" <quic_eberman@...cinc.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"rppt@...nel.org" <rppt@...nel.org>,
	"brauner@...nel.org" <brauner@...nel.org>,
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	"steven.sistare@...cle.com" <steven.sistare@...cle.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Durrant, Paul" <pdurrant@...zon.co.uk>,
	"Woodhouse, David" <dwmw@...zon.co.uk>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"Saenz Julienne, Nicolas" <nsaenz@...zon.es>,
	"Graf (AWS), Alexander" <graf@...zon.de>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"jack@...e.cz" <jack@...e.cz>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 05/10] guestmemfs: add file mmap callback

On Sat, Nov 02, 2024 at 08:24:15AM +0000, Gowans, James wrote:

> KHO can persist any memory ranges which are not MOVABLE. Provided that
> guest_memfd does non-movable allocations then serialising and persisting
> should be possible.
> 
> There are other requirements here, specifically the ability to be
> *guaranteed* GiB-level allocations, have the guest memory out of the
> direct map for secret hiding, and remove the struct page overhead.
> Struct page overhead could be handled via HVO. 

IMHO this should all be handled as part of normal guestmemfd operation
because it has nothing to do with KHO. Many others have asked for the
same things in guest memfd already.

So I would start by assuming guest memfd will get those things
eventually and design around a 'freeze and record' model for KHO of a
guestmemfd, instead of yet another special memory allocator..

Jason