linux-kernel - Re: [PATCH 00/10] Introduce guestmemfs: persistent in-memory filesystem

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Message-ID: <CABgObfYhg6uoR7cQN4wf3bNLZbHfXv6fr35aKsKbqMvuv20Xrg@mail.gmail.com>
Date: Mon, 5 Aug 2024 16:41:25 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: "Theodore Ts'o" <tytso@....edu>
Cc: James Gowans <jgowans@...zon.com>, linux-kernel@...r.kernel.org, 
	Sean Christopherson <seanjc@...gle.com>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Steve Sistare <steven.sistare@...cle.com>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Anthony Yznaga <anthony.yznaga@...cle.com>, Mike Rapoport <rppt@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	Jason Gunthorpe <jgg@...pe.ca>, linux-fsdevel@...r.kernel.org, 
	Usama Arif <usama.arif@...edance.com>, kvm@...r.kernel.org, 
	Alexander Graf <graf@...zon.com>, David Woodhouse <dwmw@...zon.co.uk>, 
	Paul Durrant <pdurrant@...zon.co.uk>, Nicolas Saenz Julienne <nsaenz@...zon.es>
Subject: Re: [PATCH 00/10] Introduce guestmemfs: persistent in-memory filesystem

On Mon, Aug 5, 2024 at 4:35 PM Theodore Ts'o <tytso@....edu> wrote:
> On Mon, Aug 05, 2024 at 11:32:35AM +0200, James Gowans wrote:
> > Guestmemfs implements preservation acrosss kexec by carving out a
> > large contiguous block of host system RAM early in boot which is
> > then used as the data for the guestmemfs files.
>
> Also, the VMM update process is not a common case thing, so we don't
> need to optimize for performance.  If we need to temporarily use
> swap/zswap to allocate memory at VMM update time, and if the pages
> aren't contiguous when they are copied out before doing the VMM
> update

I'm not sure I understand, where would this temporary allocation happen?

> that might be very well worth the vast of of memory needed to
> pay for reserving memory on the host for the VMM update that only
> might happen once every few days/weeks/months (depending on whether
> you are doing update just for high severity security fixes, or for
> random VMM updates).
>
> Even if you are updating the VMM every few days, it still doesn't seem
> that permanently reserving contiguous memory on the host can be
> justified from a TCO perspective.

As far as I understand, this is intended for use in systems that do
not do anything except hosting VMs, where anyway you'd devote 90%+ of
host memory to hugetlbfs gigapages.

Paolo