linux-kernel - Re: 9p caching with cache=loose and cache=fscache

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Date:   Tue, 28 Mar 2023 11:31:50 +0900
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Christian Schoenebeck <linux_oss@...debyte.com>,
        Eric Van Hensbergen <ericvh@...il.com>,
        Josef Bacik <josef@...icpanda.com>,
        Jeff Layton <jlayton@...nel.org>, lucho@...kov.net,
        v9fs-developer@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        Amir Goldstein <amir73il@...il.com>,
        Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: 9p caching with cache=loose and cache=fscache

Luis Chamberlain wrote on Mon, Mar 27, 2023 at 10:39:54AM -0700:
> > I have fixed what
> > I hope to be my last bug with the new patch series so it should be
> > going into linux-next today.
> 
> Nice, thanks, since kdevops relies on a host kernel though and we strive
> to have stability for that, I personally like to recommend distro
> kernels and so they're a few kernel releases out of date. So debian-testing
> is on 6.1 as of today for example.
> [...]
> -    opts: "ro,trans=virtio,version=9p2000.L,posixacl,cache=loose"
> +    opts: "ro,trans=virtio,version=9p2000.L,posixacl,cache=none"

Yes, if you want something mostly coherent with the host, cache=none (or
cache=mmap if you need mmap, iirc linux build does for linking? if you
want to do that on guest...) is what you'll want to use on current
kernels.

> BTW the qemu wiki seems to suggest cache=loose and its why I used it on
> kdevops as a default. What about the following so to avoid folks running
> into similar issues? I can go and update the wiki too.

I've added Christian in Cc for this point, he's more active on the qemu
side
(thread started here:
https://lkml.kernel.org/r/ZA0FEyOtRBvpIXbi@bombadil.infradead.org
)

I have no opinion on the current wording, the default is there for a
reason and it's a safe default (none), and cache=loose is clearly
described with "no attempts are made at consistency, intended for
exclusive, read-only mounts" which I think ought to be clear enough
(exclusive means not shared with the host), but if you think it's not
clear enough it probably isn't.

A word on the qemu wiki "if you want to share with host..." would
probably be good though.

-- 
Dominique