alloc_tag: prevent enabling memory profiling if it was shut down

Memory profiling can be shut down due to reasons like a failure during
initialization. When this happens, the user should not be able to
re-enable it. Current sysctrl interface does not handle this properly and
will allow re-enabling memory profiling. Fix this by checking for this
condition during sysctrl write operation.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Suren Baghdasaryan <[e-mail]>
Acked-by: Shakeel Butt <[e-mail]>
Acked-by: Usama Arif <[e-mail]>
Cc: David Wang <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Kent Overstreet <[e-mail]>
Cc: Pasha Tatashin <[e-mail]>
Cc: Sourav Panda <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

lib/alloc_tag.c

···
 EXPORT_SYMBOL(page_alloc_tagging_ops);
 
 #ifdef CONFIG_SYSCTL
+/*
+ * Not using proc_do_static_key() directly to prevent enabling profiling
+ * after it was shut down.
+ */
+static int proc_mem_profiling_handler(const struct ctl_table *table, int write,
+				      void *buffer, size_t *lenp, loff_t *ppos)
+{
+	if (!mem_profiling_support && write)
+		return -EINVAL;
+
+	return proc_do_static_key(table, write, buffer, lenp, ppos);
+}
+
+
 static struct ctl_table memory_allocation_profiling_sysctls[] = {
 	{
 		.procname	= "mem_profiling",
···
 #else
 		.mode		= 0644,
 #endif
-		.proc_handler	= proc_do_static_key,
+		.proc_handler	= proc_mem_profiling_handler,
 	},
 };