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Date:   Tue, 29 Sep 2020 15:30:11 +0100
From:   Alessio Balsini <balsini@android.com>
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     Alessio Balsini <balsini@android.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Akilesh Kailash <akailash@google.com>,
        David Anderson <dvander@google.com>,
        Eric Yan <eric.yan@oneplus.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 <stefanoduo@google.com>,
        Zimuzo Ezeozue <zezeozue@google.com>,
        fuse-devel <fuse-devel@lists.sourceforge.net>,
        kernel-team <kernel-team@android.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH V8 1/3] fuse: Definitions and ioctl() for passthrough
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On Tue, Sep 22, 2020 at 07:08:48PM +0300, Amir Goldstein wrote:
> 
> I am hearing about a lot of these projects.
> I think that FUSE_PASSTHROUGH is a very useful feature.
> I have an intention to explore passthrough to directory fd for
> directory modifications. I sure hope you will beat me to it ;-)


It's awesome that you mentioned this, something similar is already in my
TODO list, suggested by Paul (in CC). I'll start working on this and will
be glad to discuss as soon as this FUSE_PASSTHROUGH extension will
hopefully get accepted.


> 
> > I'm not directly involved in the Incremental FS project, but, as far as I
> > remember, only for the first PoC was actually developed as a FUSE file
> > system. Because of the overhead introduced by the user space round-trips,
> > that design was left behind and the whole Incremental FS infrastructure
> > switched to becoming a kernel module.
> > In general, the FUSE passthrough patch set proposed in this series wouldn't
> > be helpful for that use case because, for example, Incremental FS requires
> > live (de)compression of data, that can only be performed by the FUSE
> > daemon.
> >
> 
> Ext4 supports inline encryption. Btrfs supports encrypted/compressed extents.
> No reason for FUSE not to support the same.
> Is it trivial? No.
> Is it an excuse for not using FUSE and writing a new userspace fs. Not
> in my option.


I started exploring the FUSE internals only in the last months and had the
feeling this compression feature was a bit out of context or at least very
use-case specific. But I'll start thinking about this.


> 
> > The specific use case I'm trying to improve with this FUSE passthrough
> > series is instead related to the scoped storage feature that we introduced
> > in Android 11, that is based on FUSE, and affects those folders that are
> > shared among all the Apps (e.g., DCIM, Downloads, etc). More details here:
> >
> 
> sdcard fs has had a lot of reincarnations :-)
> 
> I for one am happy with the return to FUSE.
> Instead of saying "FUSE is too slow" and implementing a kernel sdcardfs,
> improve FUSE to be faster for everyone - that's the way to go ;-)


Yes! This is exactly what we are trying to achieve and this patch-set is
the first piece of a bigger picture which, among others, includes the
direct directory access that you mentioned before.
I hope the community appreciates these improvement attempts :)

As you may have noticed, I recently shared the v9 of the patch-set.
Thanks to the feedback I received, what we have now has a completely
different than the initial proposal.

Thanks again everyone for the suggestions!

Alessio