mm/mempolicy: Add override to force numa policy

Some apps like linpack call numa_setpolicy to disable numa,
but that tends to have a significant performance hit for us.

Add an option to force the default numa mode (and so ignore
policy changes) for now.

Consider making this the default behaviour in the future.

Use mempolicy.force_numa=1 in cmdline.txt or
echo 1 | sudo tee  /sys/module/mempolicy/parameters/force_numa

to force the default numa policy.

Signed-off-by: Dom Cobley <[email protected]>

Author: popcornmix

mm/mempolicy.c

@@ -119,6 +119,10 @@
 
 #include "internal.h"
 
+/* When true, this forces the default numa policy and igores requests to change */
+static bool force_numa;
+module_param(force_numa, bool, 0644);
+
 /* Internal flags */
 #define MPOL_MF_DISCONTIG_OK (MPOL_MF_INTERNAL << 0)	/* Skip checks for continuous vmas */
 #define MPOL_MF_INVERT       (MPOL_MF_INTERNAL << 1)	/* Invert check for nodemask */
@@ -1629,6 +1633,13 @@ static long kernel_set_mempolicy(int mode, const unsigned long __user *nmask,
 	int err;
 
 	err = sanitize_mpol_flags(&lmode, &mode_flags);
+
+	if (force_numa) {
+		pr_info("Request to set policy ignored (mode:%x lmode:%x flags:%x err:%d)\n",
+				mode, lmode, mode_flags, err);
+		return 0;
+	}
+
 	if (err)
 		return err;