Re: [PATCH RESEND V12 3/8] fuse: Definitions and ioctl for passthrough

From: Amir Goldstein <amir73il@gmail.com>
To: Miklos Szeredi <miklos@szeredi.hu>,
	Daniel Borkmann <daniel@iogearbox.net>
Cc: Alessio Balsini <balsini@android.com>,
	Peng Tao <bergwolf@gmail.com>,
	Akilesh Kailash <akailash@google.com>,
	Antonio SJ Musumeci <trapexit@spawn.link>,
	David Anderson <dvander@google.com>,
	Giuseppe Scrivano <gscrivan@redhat.com>,
	Jann Horn <jannh@google.com>, Jens Axboe <axboe@kernel.dk>,
	Martijn Coenen <maco@android.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Lawrence <paullawrence@google.com>,
	Stefano Duo <duostefano93@gmail.com>,
	Zimuzo Ezeozue <zezeozue@google.com>, wuyan <wu-yan@tcl.com>,
	fuse-devel <fuse-devel@lists.sourceforge.net>,
	kernel-team <kernel-team@android.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH RESEND V12 3/8] fuse: Definitions and ioctl for passthrough
Date: Fri, 12 May 2023 15:37:47 -0400	[thread overview]
Message-ID: <CAOQ4uxg2k3DsTdiMKNm4ESZinjS513Pj2EeKGW4jQR_o5Mp3-Q@mail.gmail.com> (raw)
In-Reply-To: <CAOQ4uxgXhVOpF8NgAcJCeW67QMKBOytzMXwy-GjdmS=DGGZ0hA@mail.gmail.com>

On Mon, Sep 12, 2022 at 8:29 AM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Mon, Sep 12, 2022 at 12:29 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > On Sat, 10 Sept 2022 at 10:52, Amir Goldstein <amir73il@gmail.com> wrote:
> >
> > > I think we should accept the fact that just as any current FUSE
> > > passthrough (in userspace) implementation is limited to max number of
> > > open files as the server's process limitation, kernel passthrough implementation
> > > will be limited by inheriting the mounter's process limitation.
> > >
> > > There is no reason that the server should need to keep more
> > > passthrough fd's open than client open fds.
> >
> > Maybe you're right.
> >
> > > If we only support FOPEN_PASSTHROUGH_AUTOCLOSE as v12
> > > patches implicitly do, then the memory overhead is not much different
> > > than the extra overlayfs pseudo realfiles.
> >
> > How exactly would this work?
> >
> > ioctl(F_D_I_P_OPEN) - create passthrough fd with ref 1
> > open/FOPEN_PASSTHOUGH -  inc refcount in passthrough fd
> > release - put refcount in passthrough fd
> > ioctl(F_D_I_P_CLOSE) - put ref in passthrough fd
> >
> > Due to being refcounted the F_D_I_P_CLOSE can come at any point past
> > the finished open request.
> >
> > Or did you have something else in mind?
> >
>
> What I had in mind is that FOPEN_PASSTHROUGH_AUTOCLOSE
> "transfers" the server's refcount to the kernel and server does
> not need to call explicit F_D_I_P_CLOSE.
>
> This is useful for servers that don't care about reusing mappings.
>

Hi Daniel,

I was waiting for LSFMM to see if and how FUSE-BPF intends to
address the highest value use case of read/write passthrough.

From what I've seen, you are still taking a very broad approach of
all-or-nothing which still has a lot of core design issues to address,
while these old patches already address the most important use case
of read/write passthrough of fd without any of the core issues
(credentials, hidden fds).

As far as I can tell, this old implementation is mostly independent of your
lookup based approach - they share the low level read/write passthrough
functions but not much more than that, so merging them should not be
a blocker to your efforts in the longer run.
Please correct me if I am wrong.

As things stand, I intend to re-post these old patches with mandatory
FOPEN_PASSTHROUGH_AUTOCLOSE to eliminate the open
questions about managing mappings.

Miklos, please stop me if I missed something and if you do not
think that these two approaches are independent.

Thanks,
Amir.