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

From: Nikolaus Rath <Nikolaus@rath.org>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>, Jens Axboe <axboe@kernel.dk>,
	Giuseppe Scrivano <gscrivan@redhat.com>,
	fuse-devel <fuse-devel@lists.sourceforge.net>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Jann Horn <jannh@google.com>,
	Zimuzo Ezeozue <zezeozue@google.com>,
	kernel-team <kernel-team@android.com>,
	Peng Tao <bergwolf@gmail.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Stefano Duo <duostefano93@gmail.com>,
	David Anderson <dvander@google.com>, wuyan <wu-yan@tcl.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	Akilesh Kailash <akailash@google.com>,
	Martijn Coenen <maco@android.com>
Subject: Re: [fuse-devel] [PATCH RESEND V12 3/8] fuse: Definitions and ioctl for passthrough
Date: Mon, 15 May 2023 21:16:44 +0100	[thread overview]
Message-ID: <87353xjqoj.fsf@vostro.rath.org> (raw)
In-Reply-To: <CAOQ4uxjrhf8D081Z8aG71=Kjjub28MwR3xsaAHD4cK48-FzjNA@mail.gmail.com> (Amir Goldstein's message of "Mon, 15 May 2023 17:00:16 +0300")

On May 15 2023, Amir Goldstein <amir73il@gmail.com> wrote:
> On Mon, May 15, 2023 at 10:29 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
>> On Fri, 12 May 2023 at 21:37, Amir Goldstein <amir73il@gmail.com> wrote:
>>
>> > 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.
>>
>> Do you mean that the BPF patches should use their own passthrough mechanism?
>>
>> I think it would be better if we could agree on a common interface for
>> passthough (or per Paul's suggestion: backing) mechanism.
>
> Well, not exactly different.
> With BFP patches, if you have a backing inode that was established during
> LOOKUP with rules to do passthrough for open(), you'd get a backing file and
> that backing file would be used to passthrough read/write.
>
> FOPEN_PASSTHROUGH is another way to configure passthrough read/write
> to a backing file that is controlled by the server per open fd instead of by BFP
> for every open of the backing inode.
>
> Obviously, both methods would use the same backing_file field and
> same read/write passthrough methods regardless of how the backing file
> was setup.
>
> Obviously, the BFP patches will not use the same ioctl to setup passthrough
> (and/or BPF program) to a backing inode, but I don't think that matters much.
> When we settle on ioctls for setting up backing inodes, we can also add new
> ioctls for setting up backing file with optional BPF program.

> I don't see any reason to make the first ioctl more complicated than this:
>
> struct fuse_passthrough_out {
>         uint32_t        fd;
>         /* For future implementation */
>         uint32_t        len;
>         void            *vec;
> };
>
> One advantage with starting with FOPEN_PASSTHROUGH, besides
> dealing with the highest priority performance issue, is how it deals with
> resource limits on open files.

One thing that struck me when we discussed FUSE-BPF at LSF was that from
a userspace point of view, FUSE-BPF presents an almost completely
different API than traditional FUSE (at least in its current form).

As long as there is no support for falling back to standard FUSE
callbacks, using FUSE-BPF means that most of the existing API no longer
works, and instead there is a large new API surface that doesn't work in
standard FUSE (the pre-filter and post-filter callbacks for each
operation).

I think this means that FUSE-BPF file systems won't work with FUSE, and
FUSE filesystems won't work with FUSE-BPF.

Would it be worth thinking about FUSE-BPF as a completely separate
approach that stands next to FUSE, as opposed to considering it an
extension?

In that case, we wouldn't need to worry about a FUSE-passthrough
implementation being forward compatible with FUSE-BPF or not.

Best,
-Nikolaus

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