NFSv4.1/pnfs: remember when a server needs a larger LAYOUTGET reply buffer

When a LAYOUTGET only succeeds after escalating the reply buffer, every
layout fetched from that server is likely to need the larger buffer:
remember the escalated size on the nfs_server and use it as the floor
for subsequent LAYOUTGET reply buffers, skipping the doomed attempt at
the layout driver's default size.

This also lets the LAYOUTGET attached to OPEN benefit: the lgopen path
is best-effort with no retry of its own, so without the learned size
it would fail with NFS4ERR_TOOSMALL at every open against a server
handing out wide layouts, and layouts would only ever be acquired by
the I/O path's retry.

The field is a hint: reads and writes are racy by design, the value
only ever grows toward the session's maximum response size, and a
stale-low read merely costs one escalation round trip.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/nfs_fs_sb.h b/include/linux/nfs_fs_sb.h
index 34d294774f8c..3e6bae7e5221 100644
--- a/include/linux/nfs_fs_sb.h
+++ b/include/linux/nfs_fs_sb.h
@@ -248,6 +248,10 @@ struct nfs_server {
 						   that are supported on this
 						   filesystem */
 	struct pnfs_layoutdriver_type  *pnfs_curr_ld; /* Active layout driver */
+	unsigned int		lg_reply_sz;	/* Learned LAYOUTGET reply
+						   buffer size, when the layout
+						   driver's default has proved
+						   too small */
 	struct rpc_wait_queue	roc_rpcwaitq;
 
 	/* the following fields are protected by nfs_client->cl_lock */