linux-kernel - Re: [PATCH] Fuse: Add backing file support for uring

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Message-ID: <CAOQ4uxieuFTN4Ni4HoBsEvTPW_odWSo78-5shJTh3T2A-vzP=g@mail.gmail.com>
Date: Fri, 21 Feb 2025 19:21:22 +0100
From: Amir Goldstein <amir73il@...il.com>
To: Moinak Bhattacharyya <moinakb001@...il.com>
Cc: Bernd Schubert <bernd@...ernd.com>, Miklos Szeredi <miklos@...redi.hu>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, io-uring@...r.kernel.org
Subject: Re: [PATCH] Fuse: Add backing file support for uring_cmd

On Fri, Feb 21, 2025 at 7:13 PM Moinak Bhattacharyya
<moinakb001@...il.com> wrote:
>
> I don't have the modifications to libfuse. What tree are you using for
> the uring modifications? I dont see any uring patches on the latest
> master liburing.
> >> It is possible, for example set FOPEN_PASSTHROUGH_FD to
> >> interpret backing_id as backing_fd, but note that in the current
> >> implementation of passthrough_hp, not every open does
> >> fuse_passthrough_open().
> >> The non-first open of an inode uses a backing_id stashed in inode,
> >> from the first open so we'd need different server logic depending on
> >> the commands channel, which is not nice.
> I wonder if we can just require URING registered FDs (using
> IORING_REGISTER_FILES). I think io_uring does checks on the file
> permissions when the FD is registered.

That's an interesting idea.
There are definitely similarities between IORING_REGISTER_FILES
and registering backing ids.

There is however one difference, which is going to be even more
emphasised when backing files are setup during LOOKUP -
The backing fd setup during BACKING_OPEN does not need to
be open for write - it could even be an O_PATH fd.

So fc->backing_files_map are not really fds registered for IO,
they are essential references to backing inodes.

Thanks,
Amir.