linux-kernel - [PATCH V3 13/21] famfs_fuse: Famfs mount opt: -o shadow=<shadowpath>

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Message-ID: <20260107153332.64727-14-john@groves.net>
Date: Wed,  7 Jan 2026 09:33:22 -0600
From: John Groves <John@...ves.net>
To: John Groves <John@...ves.net>,
	Miklos Szeredi <miklos@...redi.hu>,
	Dan Williams <dan.j.williams@...el.com>,
	Bernd Schubert <bschubert@....com>,
	Alison Schofield <alison.schofield@...el.com>
Cc: 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>,
	Joanne Koong <joannelkoong@...il.com>,
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	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,
	John Groves <john@...ves.net>
Subject: [PATCH V3 13/21] famfs_fuse: Famfs mount opt: -o shadow=<shadowpath>

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.

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;
+
+#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
+	char *shadow;
+#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),
+#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);
+
 	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);
+#endif
+
 	mutex_unlock(&fuse_mutex);
 	return 0;
 
-- 
2.49.0