[PATCH] Bluetooth: L2CAP: Increase default socket buffer to hold large SDUs

[PATCH] Bluetooth: L2CAP: Increase default socket buffer to hold large SDUs

[Amir Abudubai]

The L2CAP send path requires a full SDU to be assembled in the socket
buffer before transmission can begin. Therefore, the socket buffer must
be large enough to hold a worst-case SDU.

Derive the required buffer space from the worst-case parameters:
Maximum SDU plus length field: 65535 + 2 = 65537 bytes
At MPS 24:                    2730 full PDUs + 17-byte tail
With 4-byte L2CAP headers:    28-byte full PDUs, 21-byte tail
At HCI MTU 27:                2730 * 2 + 1 = 5461 fragments

MPS 24 and MTU 27 are chosen because they produce the most fragmentation.
Apply a 4x safety factor because SKB_TRUESIZE can underestimate actual
usage because of class-size rounding and minimum allocations.

Signed-off-by: Amir Abudubai <amirabudubai@gmail.com>
---
The L2CAP send path deadlocks waiting for buffer space if it cannot fit
a full SDU in the send buffer. Using the system default typically causes
that to happen before the max allowed SDU. I didn't address the deadlock.
I just scaled the buffer so it is unlikely to happen unless the socket
gets resized after initialization.

 include/net/bluetooth/l2cap.h | 4 ++++
 net/bluetooth/l2cap_sock.c    | 1 +
 2 files changed, 5 insertions(+)

diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
index 54eddcdd341f..0cf1ca4ec3cd 100644
--- a/include/net/bluetooth/l2cap.h
+++ b/include/net/bluetooth/l2cap.h
@@ -26,6 +26,7 @@
 
 #include <linux/unaligned.h>
 #include <linux/atomic.h>
+#include <linux/skbuff.h>
 
 /* L2CAP defaults */
 #define L2CAP_DEFAULT_MTU		672
@@ -41,6 +42,9 @@
 #define L2CAP_DEFAULT_MAX_PDU_SIZE	1492    /* Sized for AMP packet */
 #define L2CAP_DEFAULT_ACK_TO		200
 #define L2CAP_DEFAULT_MAX_SDU_SIZE	0xFFFF
+#define L2CAP_MAX_SDU_FRAGS		5461    /* Sized for worst-case CoC fragmentation */
+#define L2CAP_DEFAULT_SNDBUF \
+	(4 * L2CAP_MAX_SDU_FRAGS * SKB_TRUESIZE(SKB_DATA_ALIGN(HCI_MIN_LE_MTU + BT_SKB_RESERVE)))
 #define L2CAP_DEFAULT_SDU_ITIME		0xFFFFFFFF
 #define L2CAP_DEFAULT_ACC_LAT		0xFFFFFFFF
 #define L2CAP_BREDR_MAX_PAYLOAD		1019    /* 3-DH5 packet */
diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index 7a2551ef7f50..5f4fde8b4a6d 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c
@@ -2046,6 +2046,7 @@ static struct sock *l2cap_sock_alloc(struct net *net, struct socket *sock,
 
 	sk->sk_destruct = l2cap_sock_destruct;
 	sk->sk_sndtimeo = L2CAP_CONN_TIMEOUT;
+	sk->sk_sndbuf   = L2CAP_DEFAULT_SNDBUF;
 
 	INIT_LIST_HEAD(&l2cap_pi(sk)->rx_busy);
 
-- 
2.43.0

RE: Bluetooth: L2CAP: Increase default socket buffer to hold large SDUs

[bluez.test.bot]

[-- Attachment #1: Type: text/plain, Size: 1074 bytes --]

This is automated email and please do not reply to this email!

Dear submitter,

Thank you for submitting the patches to the linux bluetooth mailing list.
This is a CI test results with your patch series:
PW Link:https://patchwork.kernel.org/series/1207660/

---Test result---

Test Summary:
CheckPatch                    PASS      0.67 seconds
VerifyFixes                   PASS      0.07 seconds
VerifySignedoff               PASS      0.06 seconds
GitLint                       PASS      0.21 seconds
SubjectPrefix                 PASS      0.06 seconds
BuildKernel                   PASS      23.46 seconds
CheckAllWarning               PASS      28.77 seconds
CheckSparse                   PASS      31.59 seconds
BuildKernel32                 PASS      24.75 seconds
CheckKernelLLVM               PASS      26.53 seconds
TestRunnerSetup               PASS      574.85 seconds
TestRunner_l2cap-tester       PASS      13.55 seconds
IncrementalBuild              PASS      24.59 seconds



https://github.com/bluez/bluetooth-next/pull/1106

---
Regards,
Linux Bluetooth