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

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Message-ID: <20240805143223.GA1110778@mit.edu>
Date: Mon, 5 Aug 2024 10:32:23 -0400
From: "Theodore Ts'o" <tytso@....edu>
To: James Gowans <jgowans@...zon.com>
Cc: linux-kernel@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.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 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.

Why does the memory have to be (a) contiguous, and (b) carved out of
*host* system memory early in boot?  This seems to be very inflexible;
it means that you have to know how much memory will be needed for
guestmemfs in early boot.

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, 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.

Cheers,

						- Ted