dm: treat alloc_dax() -EOPNOTSUPP failure as non-fatal

In preparation for checking whether the architecture has data cache
aliasing within alloc_dax(), modify the error handling of dm alloc_dev()
to treat alloc_dax() -EOPNOTSUPP failure as non-fatal.

Link: https://lkml.kernel.org/r/20240215144633.96437-5-mathieu.desnoyers@efficios.com
Fixes: d92576f1 ("dax: does not work correctly with virtual aliasing caches")
Suggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Cc: Alasdair Kergon <agk@redhat.com>
Cc: Mike Snitzer <snitzer@kernel.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: kernel test robot <lkp@intel.com>
Cc: Michael Sclafani <dm-devel@lists.linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/md/dm.c

@@ -2054,6 +2054,7 @@ static void cleanup_mapped_device(struct mapped_device *md)
 static struct mapped_device *alloc_dev(int minor)
 {
 	int r, numa_node_id = dm_get_numa_node();
+	struct dax_device *dax_dev;
 	struct mapped_device *md;
 	void *old_md;
 
@@ -2122,15 +2123,15 @@ static struct mapped_device *alloc_dev(int minor)
 	md->disk->private_data = md;
 	sprintf(md->disk->disk_name, "dm-%d", minor);
 
-	if (IS_ENABLED(CONFIG_FS_DAX)) {
-		md->dax_dev = alloc_dax(md, &dm_dax_ops);
-		if (IS_ERR(md->dax_dev)) {
-			md->dax_dev = NULL;
+	dax_dev = alloc_dax(md, &dm_dax_ops);
+	if (IS_ERR(dax_dev)) {
+		if (PTR_ERR(dax_dev) != -EOPNOTSUPP)
 			goto bad;
-		}
-		set_dax_nocache(md->dax_dev);
-		set_dax_nomc(md->dax_dev);
-		if (dax_add_host(md->dax_dev, md->disk))
+	} else {
+		set_dax_nocache(dax_dev);
+		set_dax_nomc(dax_dev);
+		md->dax_dev = dax_dev;
+		if (dax_add_host(dax_dev, md->disk))
 			goto bad;
 	}