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Message-ID: <CAJnrk1bJ3VbZCYJet1eDPy0V=_3cPxz6kDbgcxwtirk2yA9P0w@mail.gmail.com>
Date: Fri, 9 Jan 2026 11:22:54 -0800
From: Joanne Koong <joannelkoong@...il.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>, 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>, David Hildenbrand <david@...nel.org>,
Christian Brauner <brauner@...nel.org>, "Darrick J . Wong" <djwong@...nel.org>,
Randy Dunlap <rdunlap@...radead.org>, Jeff Layton <jlayton@...nel.org>,
Amir Goldstein <amir73il@...il.com>, Jonathan Cameron <Jonathan.Cameron@...wei.com>,
Stefan Hajnoczi <shajnocz@...hat.com>, Josef Bacik <josef@...icpanda.com>,
Bagas Sanjaya <bagasdotme@...il.com>, Chen Linxuan <chenlinxuan@...ontech.com>,
James Morse <james.morse@....com>, Fuad Tabba <tabba@...gle.com>,
Sean Christopherson <seanjc@...gle.com>, Shivank Garg <shivankg@....com>,
Ackerley Tng <ackerleytng@...gle.com>, Gregory Price <gourry@...rry.net>,
Aravind Ramesh <arramesh@...ron.com>, Ajay Joshi <ajayjoshi@...ron.com>, venkataravis@...ron.com,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org,
linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH V3 13/21] famfs_fuse: Famfs mount opt: -o shadow=<shadowpath>
On Wed, Jan 7, 2026 at 7:34 AM John Groves <John@...ves.net> wrote:
>
> The shadow path is a (usually in tmpfs) file system area used by the
> famfs user space to communicate with the famfs fuse server. There is a
> minor dilemma that the user space tools must be able to resolve from a
> mount point path to a shadow path. Passing in the 'shadow=<path>'
> argument at mount time causes the shadow path to be exposed via
> /proc/mounts, Solving this dilemma. The shadow path is not otherwise
> used in the kernel.
Instead of using mount options to pass the userspace metadata, could
/sys/fs be used instead? The client is able to get the connection id
by stat-ing the famfs mount path. There could be a
/sys/fs/fuse/connections/{id}/metadata file that the server fills out
with whatever metadata needs to be read by the client. Having
something like this would be useful to non-famfs servers as well.
>
> Signed-off-by: John Groves <john@...ves.net>
> ---
> fs/fuse/fuse_i.h | 25 ++++++++++++++++++++++++-
> fs/fuse/inode.c | 28 +++++++++++++++++++++++++++-
> 2 files changed, 51 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index ec2446099010..84d0ee2a501d 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -620,9 +620,11 @@ struct fuse_fs_context {
> unsigned int blksize;
> const char *subtype;
>
> - /* DAX device, may be NULL */
> + /* DAX device for virtiofs, may be NULL */
> struct dax_device *dax_dev;
>
> + const char *shadow; /* famfs - null if not famfs */
> +
> /* fuse_dev pointer to fill in, should contain NULL on entry */
> void **fudptr;
> };
> @@ -998,6 +1000,18 @@ struct fuse_conn {
> /* Request timeout (in jiffies). 0 = no timeout */
> unsigned int req_timeout;
> } timeout;
> +
> + /*
> + * This is a workaround until fuse uses iomap for reads.
> + * For fuseblk servers, this represents the blocksize passed in at
> + * mount time and for regular fuse servers, this is equivalent to
> + * inode->i_blkbits.
> + */
> + u8 blkbits;
> +
I think you meant to remove these lines?
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> + char *shadow;
Should this be const char * too?
> +#endif
> };
>
> /*
> @@ -1631,4 +1645,13 @@ extern void fuse_sysctl_unregister(void);
> #define fuse_sysctl_unregister() do { } while (0)
> #endif /* CONFIG_SYSCTL */
>
> +/* famfs.c */
> +
> +static inline void famfs_teardown(struct fuse_conn *fc)
> +{
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> + kfree(fc->shadow);
> +#endif
> +}
> +
> #endif /* _FS_FUSE_I_H */
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index acabf92a11f8..2e0844aabbae 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -783,6 +783,9 @@ enum {
> OPT_ALLOW_OTHER,
> OPT_MAX_READ,
> OPT_BLKSIZE,
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> + OPT_SHADOW,
> +#endif
> OPT_ERR
> };
>
> @@ -797,6 +800,9 @@ static const struct fs_parameter_spec fuse_fs_parameters[] = {
> fsparam_u32 ("max_read", OPT_MAX_READ),
> fsparam_u32 ("blksize", OPT_BLKSIZE),
> fsparam_string ("subtype", OPT_SUBTYPE),
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> + fsparam_string("shadow", OPT_SHADOW),
nit: having the spacing for ("shadow", align with the lines above
would be aesthetically nice
> +#endif
> {}
> };
>
> @@ -892,6 +898,15 @@ static int fuse_parse_param(struct fs_context *fsc, struct fs_parameter *param)
> ctx->blksize = result.uint_32;
> break;
>
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> + case OPT_SHADOW:
> + if (ctx->shadow)
> + return invalfc(fsc, "Multiple shadows specified");
> + ctx->shadow = param->string;
> + param->string = NULL;
> + break;
> +#endif
> +
> default:
> return -EINVAL;
> }
> @@ -905,6 +920,7 @@ static void fuse_free_fsc(struct fs_context *fsc)
>
> if (ctx) {
> kfree(ctx->subtype);
> + kfree(ctx->shadow);
> kfree(ctx);
> }
> }
> @@ -936,7 +952,10 @@ static int fuse_show_options(struct seq_file *m, struct dentry *root)
> else if (fc->dax_mode == FUSE_DAX_INODE_USER)
> seq_puts(m, ",dax=inode");
> #endif
> -
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> + if (fc->shadow)
> + seq_printf(m, ",shadow=%s", fc->shadow);
> +#endif
> return 0;
> }
>
> @@ -1041,6 +1060,8 @@ void fuse_conn_put(struct fuse_conn *fc)
> WARN_ON(atomic_read(&bucket->count) != 1);
> kfree(bucket);
> }
> + famfs_teardown(fc);
imo it looks a bit cleaner with
if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX))
famfs_teardown(fc);
which also matches the pattern the passthrough config below uses
> +
> if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
> fuse_backing_files_free(fc);
> call_rcu(&fc->rcu, delayed_release);
> @@ -1916,6 +1937,11 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
> *ctx->fudptr = fud;
> wake_up_all(&fuse_dev_waitq);
> }
> +
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> + fc->shadow = kstrdup(ctx->shadow, GFP_KERNEL);
Is a shadow path a must-have for a famfs mount? if so, then should the
mount fail if the allocation here fails?
Thanks,
Joanne
> +#endif
> +
> mutex_unlock(&fuse_mutex);
> return 0;
>
> --
> 2.49.0
>
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