linux-kernel - Re: [RFC V2 10/18] famfs_fuse: Basic fuse kernel ABI enablement for famfs

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Message-ID: <CAOQ4uxi7fvMgYqe1M3_vD3+YXm7x1c4YjA=eKSGLuCz2Dsk0TQ@mail.gmail.com>
Date: Fri, 4 Jul 2025 09:54:57 +0200
From: Amir Goldstein <amir73il@...il.com>
To: John Groves <John@...ves.net>, "Darrick J . Wong" <djwong@...nel.org>
Cc: Dan Williams <dan.j.williams@...el.com>, Miklos Szeredi <miklos@...redb.hu>, 
	Bernd Schubert <bschubert@....com>, John Groves <jgroves@...ron.com>, Jonathan Corbet <corbet@....net>, 
	Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>, 
	Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, 
	Randy Dunlap <rdunlap@...radead.org>, Jeff Layton <jlayton@...nel.org>, 
	Kent Overstreet <kent.overstreet@...ux.dev>, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev, 
	linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	Jonathan Cameron <Jonathan.Cameron@...wei.com>, Stefan Hajnoczi <shajnocz@...hat.com>, 
	Joanne Koong <joannelkoong@...il.com>, Josef Bacik <josef@...icpanda.com>, 
	Aravind Ramesh <arramesh@...ron.com>, Ajay Joshi <ajayjoshi@...ron.com>
Subject: Re: [RFC V2 10/18] famfs_fuse: Basic fuse kernel ABI enablement for famfs

On Thu, Jul 3, 2025 at 8:51 PM John Groves <John@...ves.net> wrote:
>
> * FUSE_DAX_FMAP flag in INIT request/reply
>
> * fuse_conn->famfs_iomap (enable famfs-mapped files) to denote a
>   famfs-enabled connection
>
> Signed-off-by: John Groves <john@...ves.net>
> ---
>  fs/fuse/fuse_i.h          |  3 +++
>  fs/fuse/inode.c           | 14 ++++++++++++++
>  include/uapi/linux/fuse.h |  4 ++++
>  3 files changed, 21 insertions(+)
>
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 9d87ac48d724..a592c1002861 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -873,6 +873,9 @@ struct fuse_conn {
>         /* Use io_uring for communication */
>         unsigned int io_uring;
>
> +       /* dev_dax_iomap support for famfs */
> +       unsigned int famfs_iomap:1;
> +

pls move up to the bit fields members.

>         /** Maximum stack depth for passthrough backing files */
>         int max_stack_depth;
>
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 29147657a99f..e48e11c3f9f3 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -1392,6 +1392,18 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
>                         }
>                         if (flags & FUSE_OVER_IO_URING && fuse_uring_enabled())
>                                 fc->io_uring = 1;
> +                       if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX) &&
> +                           flags & FUSE_DAX_FMAP) {
> +                               /* XXX: Should also check that fuse server
> +                                * has CAP_SYS_RAWIO and/or CAP_SYS_ADMIN,
> +                                * since it is directing the kernel to access
> +                                * dax memory directly - but this function
> +                                * appears not to be called in fuse server
> +                                * process context (b/c even if it drops
> +                                * those capabilities, they are held here).
> +                                */
> +                               fc->famfs_iomap = 1;
> +                       }

1. As long as the mapping requests are checking capabilities we should be ok
    Right?
2. What's the deal with capable(CAP_SYS_ADMIN) in process_init_limits then?
3. Darrick mentioned the need for a synchronic INIT variant for his work on
    blockdev iomap support [1]

I also wonder how much of your patches and Darrick's patches end up
being an overlap?

Thanks,
Amir.

[1] https://lore.kernel.org/linux-fsdevel/20250613174413.GM6138@frogsfrogsfrogs/