[PATCH v6 15/37] lib: introduce early boot parameter to avoid page_ext memory overhead

[Suren Baghdasaryan]

The highest memory overhead from memory allocation profiling comes from
page_ext objects. This overhead exists even if the feature is disabled
but compiled-in. To avoid it, introduce an early boot parameter that
prevents page_ext object creation. The new boot parameter is a tri-state
with possible values of 0|1|never. When it is set to "never" the
memory allocation profiling support is disabled, and overhead is minimized
(currently no page_ext objects are allocated, in the future more overhead
might be eliminated). As a result we also lose ability to enable memory
allocation profiling at runtime (because there is no space to store
alloctag references). Runtime sysctrl becomes read-only if the early boot
parameter was set to "never". Note that the default value of this boot
parameter depends on the CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT
configuration. When CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=n
the boot parameter is set to "never", therefore eliminating any overhead.
CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=y results in boot parameter
being set to 1 (enabled). This allows distributions to avoid any overhead
by setting CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=n config and
with no changes to the kernel command line.
We reuse sysctl.vm.mem_profiling boot parameter name in order to avoid
introducing yet another control. This change turns it into a tri-state
early boot parameter.

Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 lib/alloc_tag.c | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
index cb5adec4b2e2..617c2fbb6673 100644
--- a/lib/alloc_tag.c
+++ b/lib/alloc_tag.c
@@ -116,9 +116,46 @@ static bool alloc_tag_module_unload(struct codetag_type *cttype,
 	return module_unused;
 }
 
+#ifdef CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT
+static bool mem_profiling_support __meminitdata = true;
+#else
+static bool mem_profiling_support __meminitdata;
+#endif
+
+static int __init setup_early_mem_profiling(char *str)
+{
+	bool enable;
+
+	if (!str || !str[0])
+		return -EINVAL;
+
+	if (!strncmp(str, "never", 5)) {
+		enable = false;
+		mem_profiling_support = false;
+	} else {
+		int res;
+
+		res = kstrtobool(str, &enable);
+		if (res)
+			return res;
+
+		mem_profiling_support = true;
+	}
+
+	if (enable != static_key_enabled(&mem_alloc_profiling_key)) {
+		if (enable)
+			static_branch_enable(&mem_alloc_profiling_key);
+		else
+			static_branch_disable(&mem_alloc_profiling_key);
+	}
+
+	return 0;
+}
+early_param("sysctl.vm.mem_profiling", setup_early_mem_profiling);
+
 static __init bool need_page_alloc_tagging(void)
 {
-	return true;
+	return mem_profiling_support;
 }
 
 static __init void init_page_alloc_tagging(void)
@@ -158,6 +195,8 @@ static int __init alloc_tag_init(void)
 	if (IS_ERR_OR_NULL(alloc_tag_cttype))
 		return PTR_ERR(alloc_tag_cttype);
 
+	if (!mem_profiling_support)
+		memory_allocation_profiling_sysctls[0].mode = 0444;
 	register_sysctl_init("vm", memory_allocation_profiling_sysctls);
 	procfs_init();
 
-- 
2.44.0.291.gc1ea87d7ee-goog