platform: fix handling of fq_codel's memory limit default value

The memory-limit is an unsigned integer. It is ugly (if not wrong) to compare unsigned
values with "-1". When comparing with the default value we must also use an u32 type.
Instead add a define NM_PLATFORM_FQ_CODEL_MEMORY_LIMIT_UNSET.

Note that like iproute2 we treat NM_PLATFORM_FQ_CODEL_MEMORY_LIMIT_UNSET
to indicate to not set TCA_FQ_CODEL_MEMORY_LIMIT in RTM_NEWQDISC. This
special value is entirely internal to NetworkManager (or iproute2) and
kernel will then choose a default memory limit (of 32MB). So setting
NM_PLATFORM_FQ_CODEL_MEMORY_LIMIT_UNSET means to leave it to kernel to
choose a value (which then chooses 32MB).

See kernel's net/sched/sch_fq_codel.c:

    static int fq_codel_init(struct Qdisc *sch, struct nlattr *opt,
                             struct netlink_ext_ack *extack)
    {
    ...
            q->memory_limit = 32 << 20; /* 32 MBytes */

    static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt,
                               struct netlink_ext_ack *extack)
    ...
            if (tb[TCA_FQ_CODEL_MEMORY_LIMIT])
                    q->memory_limit = min(1U << 31, nla_get_u32(tb[TCA_FQ_CODEL_MEMORY_LIMIT]));

Note that not having zero as default value is problematic. In fields like
"NMPlatformIP4Route.table_coerced" and "NMPlatformRoutingRule.suppress_prefixlen_inverse"
we avoid this problem by storing a coerced value in the structure so that zero is still
the default. We don't do that here for memory-limit, so the caller must always explicitly
set the value.

libnm-core/nm-utils.c

@@ -2333,7 +2333,12 @@ static const NMVariantAttributeSpec *const tc_qdisc_fq_codel_spec[] = {
 	NM_VARIANT_ATTRIBUTE_SPEC_DEFINE ("interval",     G_VARIANT_TYPE_UINT32,                    ),
 	NM_VARIANT_ATTRIBUTE_SPEC_DEFINE ("quantum",      G_VARIANT_TYPE_UINT32,                    ),
 	NM_VARIANT_ATTRIBUTE_SPEC_DEFINE ("ce_threshold", G_VARIANT_TYPE_UINT32,                    ),
+
+	/* kernel clamps the value at 2^31. Possibly such values should be rejected from configuration
+	 * as they cannot be configured. Leaving the attribute unspecified causes kernel to choose
+	 * a default (currently 32MB). */
 	NM_VARIANT_ATTRIBUTE_SPEC_DEFINE ("memory",       G_VARIANT_TYPE_UINT32,                    ),
+
 	NM_VARIANT_ATTRIBUTE_SPEC_DEFINE ("ecn",          G_VARIANT_TYPE_BOOLEAN, .no_value = TRUE, ),
 	NULL,
 };

src/devices/nm-device.c

@@ -6543,7 +6543,7 @@ tc_commit (NMDevice *self)
 				GET_ATTR("interval", qdisc->fq_codel.interval, UINT32, uint32, 0);
 				GET_ATTR("quantum", qdisc->fq_codel.quantum, UINT32, uint32, 0);
 				GET_ATTR("ce_threshold", qdisc->fq_codel.ce_threshold, UINT32, uint32, -1);
-				GET_ATTR("memory", qdisc->fq_codel.memory, UINT32, uint32, -1);
+				GET_ATTR("memory", qdisc->fq_codel.memory, UINT32, uint32, NM_PLATFORM_FQ_CODEL_MEMORY_LIMIT_UNSET);
 				GET_ATTR("ecn", qdisc->fq_codel.ecn, BOOLEAN, boolean, FALSE);
 			}
 

src/platform/nm-linux-platform.c

@@ -3515,6 +3515,9 @@ _new_from_nl_qdisc (struct nlmsghdr *nlh, gboolean id_only)
 	obj->qdisc.parent = tcm->tcm_parent;
 	obj->qdisc.info = tcm->tcm_info;
 
+	if (nm_streq0 (obj->qdisc.kind, "fq_codel"))
+		obj->qdisc.fq_codel.memory = NM_PLATFORM_FQ_CODEL_MEMORY_LIMIT_UNSET;
+
 	if (tb[TCA_OPTIONS]) {
 		struct nlattr *options_attr;
 		int remaining;
@@ -4241,7 +4244,7 @@ _nl_msg_new_qdisc (int nlmsg_type,
 			NLA_PUT_U32 (msg, TCA_FQ_CODEL_QUANTUM, qdisc->fq_codel.quantum);
 		if (qdisc->fq_codel.ce_threshold != -1)
 			NLA_PUT_U32 (msg, TCA_FQ_CODEL_CE_THRESHOLD, qdisc->fq_codel.ce_threshold);
-		if (qdisc->fq_codel.memory != -1)
+		if (qdisc->fq_codel.memory != NM_PLATFORM_FQ_CODEL_MEMORY_LIMIT_UNSET)
 			NLA_PUT_U32 (msg, TCA_FQ_CODEL_MEMORY_LIMIT, qdisc->fq_codel.memory);
 		if (qdisc->fq_codel.ecn)
 			NLA_PUT_U32 (msg, TCA_FQ_CODEL_ECN, qdisc->fq_codel.ecn);

src/platform/nm-platform.c

@@ -6455,7 +6455,7 @@ nm_platform_qdisc_to_string (const NMPlatformQdisc *qdisc, char *buf, gsize len)
 			nm_utils_strbuf_append (&buf, &len, " quantum %u", qdisc->fq_codel.quantum);
 		if (qdisc->fq_codel.ce_threshold != -1)
 			nm_utils_strbuf_append (&buf, &len, " ce_threshold %u", qdisc->fq_codel.ce_threshold);
-		if (qdisc->fq_codel.memory != -1)
+		if (qdisc->fq_codel.memory != NM_PLATFORM_FQ_CODEL_MEMORY_LIMIT_UNSET)
 			nm_utils_strbuf_append (&buf, &len, " memory %u", qdisc->fq_codel.memory);
 		if (qdisc->fq_codel.ecn)
 			nm_utils_strbuf_append (&buf, &len, " ecn");

src/platform/nm-platform.h

@@ -596,6 +596,8 @@ typedef struct {
 	bool     uid_range_has:1;            /* has(FRA_UID_RANGE) */
 } NMPlatformRoutingRule;
 
+#define NM_PLATFORM_FQ_CODEL_MEMORY_LIMIT_UNSET   (~((guint32) 0))
+
 typedef struct {
 	guint32 limit;
 	guint32 flows;
@@ -603,7 +605,12 @@ typedef struct {
 	guint32 interval;
 	guint32 quantum;
 	guint32 ce_threshold;
-	guint32 memory;
+	guint32 memory;       /* TCA_FQ_CODEL_MEMORY_LIMIT: note that only values <= 2^31 are accepted by kernel
+	                       *   and kernel defaults to 32MB.
+	                       *   Note that we use the special value NM_PLATFORM_FQ_CODEL_MEMORY_LIMIT_UNSET
+	                       *   to indicate that no explicit limit is set (when we send a RTM_NEWQDISC request).
+	                       *   This will cause kernel to choose the default (32MB).
+	                       *   Beware: zero is not the default you must always explicitly set this value. */
 	bool ecn:1;
 } NMPlatformQdiscFqCodel;