[PATCH v2 13/23] NFSv4: Dispatch CB_NOTIFY_DEVICEID CHANGE to an in-place refresh

[Benjamin Coddington <ben.coddington@hammerspace.com>]

nfs4_callback_devicenotify() treated CHANGE identically to DELETE: both
only unhashed the cached device, so references pinned under live
layouts kept sending I/O to the old mapping until the layouts were
freed.  Per RFC 8881 Section 12.2.10, CHANGE exists precisely so a
server can change a mapping without recalling the layouts.

For CHANGE, after unhashing the stale cache entry (so re-resolution
cannot re-pin it), invoke the layout driver's re-resolve walker.  The
walker does not install the new mapping itself: it un-pins the stale
node so the next I/O to that stripe re-resolves and fetches the new
one, while I/O already in flight completes on the old node through its
own reference.  DELETE is unchanged, and drivers without a
reresolve_deviceid hook see no change.

Re-resolution is best effort.  A lookup that hit the cache just before
the unhash can still install that node after the walk has passed the
stripe, and the walk skips nothing else; such a stripe keeps the old
mapping until the next notification.  Nothing is left inconsistent by
that -- the old mapping is a valid address the server published -- so
the race is tolerated rather than serialised against.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
---
 fs/nfs/callback_proc.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/nfs/callback_proc.c b/fs/nfs/callback_proc.c
index 10f2354ba304..64c994790d3f 100644
--- a/fs/nfs/callback_proc.c
+++ b/fs/nfs/callback_proc.c
@@ -390,7 +390,16 @@ __be32 nfs4_callback_devicenotify(void *argp, void *resp,
 			if (!ld)
 				continue;
 		}
+		/*
+		 * Unhash the cached device first so re-resolution cannot
+		 * re-pin the stale node, then re-point any references
+		 * pinned under live layouts (RFC 8881 Section 12.2.10).
+		 */
 		nfs4_delete_deviceid(ld, cps->clp, &dev->cbd_dev_id);
+		if (dev->cbd_notify_type == NOTIFY_DEVICEID4_CHANGE)
+			pnfs_layout_reresolve_deviceid_byclid(cps->clp, ld,
+							&dev->cbd_dev_id,
+							dev->cbd_immediate);
 	}
 	pnfs_put_layoutdriver(ld);
 out:
-- 
2.53.0