[PATCH 6.6 151/283] smb: client: correct value for smbd_max_fragmented_recv_size

Sasha Levin
2026-02-28T18:09:16Z

From: Stefan Metzmacher <metze@samba.org>

[ Upstream commit 4a93d1ee2d0206970b6eb13fbffe07938cd95948 ]

When we download a file without rdma offload or get
a large directly enumeration from the server,
the server might want to send up to smbd_max_fragmented_recv_size
bytes, but if it is too large all our recv buffers
might already be moved to the recv_io.reassembly.list
and we're no longer able to grant recv credits.

The maximum fragmented upper-layer payload receive size supported

Assume max_payload_per_credit is
smbd_max_receive_size - 24 = 1340

The maximum number would be
smbd_receive_credit_max * max_payload_per_credit

1340 * 255 = 341700 (0x536C4)

The minimum value from the spec is 131072 (0x20000)

For now we use the logic we used in ksmbd before:
(1364 * 255) / 2 = 173910 (0x2A756)

Fixes: 03bee01d6215 ("CIFS: SMBD: Add SMB Direct protocol initial values and constants")
Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Long Li <longli@microsoft.com>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/smb/client/smbdirect.c | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/fs/smb/client/smbdirect.c b/fs/smb/client/smbdirect.c
index be9be8f363319..359ab64e17d99 100644
--- a/fs/smb/client/smbdirect.c
+++ b/fs/smb/client/smbdirect.c
@@ -91,8 +91,23 @@ int smbd_send_credit_target = 255;
/* The maximum single message size can be sent to remote peer */
int smbd_max_send_size = 1364;

-/* The maximum fragmented upper-layer payload receive size supported */
-int smbd_max_fragmented_recv_size = 1024 * 1024;
+/*
+ * The maximum fragmented upper-layer payload receive size supported
+ *
+ * Assume max_payload_per_credit is
+ * smbd_max_receive_size - 24 = 1340
+ *
+ * The maximum number would be
+ * smbd_receive_credit_max * max_payload_per_credit
+ *
+ * 1340 * 255 = 341700 (0x536C4)
+ *
+ * The minimum value from the spec is 131072 (0x20000)
+ *
+ * For now we use the logic we used in ksmbd before:
+ * (1364 * 255) / 2 = 173910 (0x2A756)
+ */
+int smbd_max_fragmented_recv_size = (1364 * 255) / 2;

/* The maximum single-message size which can be received */
int smbd_max_receive_size = 1364;
--
2.51.0