[PATCH v3] ceph: reverse MDSMap dencoding of max_xattr_size/bal_rank_mask

[xiubli@redhat.com]

From: Xiubo Li <xiubli@redhat.com>

Ceph added the bal_rank_mask with encoded (ev) version 17.  This
was merged into main Oct 2022 and made it into the reef release
normally. While a latter commit added the max_xattr_size also
with encoded (ev) version 17 but places it before bal_rank_mask.

And this will breaks some usages, for example when upgrading old
cephs to newer versions.

URL: https://tracker.ceph.com/issues/64440
Reported-by: Patrick Donnelly <pdonnell@redhat.com>
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Patrick Donnelly <pdonnell@ibm.com>
Reviewed-by: Venky Shankar <vshankar@redhat.com>
---

V3:
- Fix the comment suggested by Patrick in V2.


 fs/ceph/mdsmap.c | 7 ++++---
 fs/ceph/mdsmap.h | 6 +++++-
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/ceph/mdsmap.c b/fs/ceph/mdsmap.c
index fae97c25ce58..8109aba66e02 100644
--- a/fs/ceph/mdsmap.c
+++ b/fs/ceph/mdsmap.c
@@ -380,10 +380,11 @@ struct ceph_mdsmap *ceph_mdsmap_decode(struct ceph_mds_client *mdsc, void **p,
 		ceph_decode_skip_8(p, end, bad_ext);
 		/* required_client_features */
 		ceph_decode_skip_set(p, end, 64, bad_ext);
+		/* bal_rank_mask */
+		ceph_decode_skip_string(p, end, bad_ext);
+	}
+	if (mdsmap_ev >= 18) {
 		ceph_decode_64_safe(p, end, m->m_max_xattr_size, bad_ext);
-	} else {
-		/* This forces the usage of the (sync) SETXATTR Op */
-		m->m_max_xattr_size = 0;
 	}
 bad_ext:
 	doutc(cl, "m_enabled: %d, m_damaged: %d, m_num_laggy: %d\n",
diff --git a/fs/ceph/mdsmap.h b/fs/ceph/mdsmap.h
index 89f1931f1ba6..1f2171dd01bf 100644
--- a/fs/ceph/mdsmap.h
+++ b/fs/ceph/mdsmap.h
@@ -27,7 +27,11 @@ struct ceph_mdsmap {
 	u32 m_session_timeout;          /* seconds */
 	u32 m_session_autoclose;        /* seconds */
 	u64 m_max_file_size;
-	u64 m_max_xattr_size;		/* maximum size for xattrs blob */
+	/*
+	 * maximum size for xattrs blob.
+	 * Zeroed by default to force the usage of the (sync) SETXATTR Op.
+	 */
+	u64 m_max_xattr_size;
 	u32 m_max_mds;			/* expected up:active mds number */
 	u32 m_num_active_mds;		/* actual up:active mds number */
 	u32 possible_max_rank;		/* possible max rank index */
-- 
2.43.0