linux-kernel - Re: [PATCH V3 4/4] fuse: add famfs DAX fmap support

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Message-ID: <20260108153148.00001e63@huawei.com>
Date: Thu, 8 Jan 2026 15:31:48 +0000
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: John Groves <John@...ves.net>
CC: Miklos Szeredi <miklos@...redi.hu>, Dan Williams
	<dan.j.williams@...el.com>, Bernd Schubert <bschubert@....com>, "Alison
 Schofield" <alison.schofield@...el.com>, John Groves <jgroves@...ron.com>,
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Subject: Re: [PATCH V3 4/4] fuse: add famfs DAX fmap support

On Wed,  7 Jan 2026 09:34:43 -0600
John Groves <John@...ves.net> wrote:

> Add new FUSE operations and capability for famfs DAX file mapping:
> 
> - FUSE_CAP_DAX_FMAP: New capability flag at bit 32 (using want_ext/capable_ext
>   fields) to indicate kernel and userspace support for DAX fmaps
> 
> - GET_FMAP: New operation to retrieve a file map for DAX-mapped files.
>   Returns a fuse_famfs_fmap_header followed by simple or interleaved
>   extent descriptors. The kernel passes the file size as an argument.
> 
> - GET_DAXDEV: New operation to retrieve DAX device info by index.
>   Called when GET_FMAP returns an fmap referencing a previously
>   unknown DAX device.
> 
> These operations enable FUSE filesystems to provide direct access
> mappings to persistent memory, allowing the kernel to map files
> directly to DAX devices without page cache intermediation.
> 
> Signed-off-by: John Groves <john@...ves.net>


> ---
>  include/fuse_common.h   |  5 +++++
>  include/fuse_lowlevel.h | 37 +++++++++++++++++++++++++++++++++++++
>  lib/fuse_lowlevel.c     | 31 ++++++++++++++++++++++++++++++-
>  3 files changed, 72 insertions(+), 1 deletion(-)
> 
> diff --git a/include/fuse_common.h b/include/fuse_common.h
> index 041188e..e428ddb 100644
> --- a/include/fuse_common.h
> +++ b/include/fuse_common.h
> @@ -512,6 +512,11 @@ struct fuse_loop_config_v1 {
>   */
>  #define FUSE_CAP_OVER_IO_URING (1UL << 31)
>  
> +/**
> + * handle files that use famfs dax fmaps
> + */
> +#define FUSE_CAP_DAX_FMAP (1UL<<32)

From the context above, looks like local style is spaces around <<

That's about the level of my understanding of the fuse code ;)

> +
>  /**
>   * Ioctl flags
>   *