patches/mm-compaction-Disable-compact_unevictable_allowed-on.patch

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Fri, 8 Nov 2019 12:55:47 +0100
Subject: [PATCH] mm/compaction: Disable compact_unevictable_allowed on RT

Since commit
    5bbe3547aa3ba ("mm: allow compaction of unevictable pages")

it is allowed to examine mlocked pages and compact them by default.
On -RT even minor pagefaults are problematic because it may take a few
100us to resolve them and until then the task is blocked.

Make compact_unevictable_allowed = 0 default and issue a warning on RT
if it is changed.

Link: https://lore.kernel.org/linux-mm/20190710144138.qyn4tuttdq6h7kqx@linutronix.de/
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 Documentation/admin-guide/sysctl/vm.rst |    3 +++
 kernel/sysctl.c                         |   29 ++++++++++++++++++++++++++++-
 mm/compaction.c                         |    4 ++++
 3 files changed, 35 insertions(+), 1 deletion(-)

--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -128,6 +128,9 @@ allowed to examine the unevictable lru (
 This should be used on systems where stalls for minor page faults are an
 acceptable trade for large contiguous free memory.  Set to 0 to prevent
 compaction from moving pages that are unevictable.  Default value is 1.
+On CONFIG_PREEMPT_RT the default value is 0 in order to avoid a page fault, due
+to compaction, which would block the task from becomming active until the fault
+is resolved.
 
 
 dirty_background_bytes
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -212,6 +212,11 @@ static int proc_do_cad_pid(struct ctl_ta
 		  void __user *buffer, size_t *lenp, loff_t *ppos);
 static int proc_taint(struct ctl_table *table, int write,
 			       void __user *buffer, size_t *lenp, loff_t *ppos);
+#ifdef CONFIG_COMPACTION
+static int proc_dointvec_minmax_warn_RT_change(struct ctl_table *table,
+					       int write, void __user *buffer,
+					       size_t *lenp, loff_t *ppos);
+#endif
 #endif
 
 #ifdef CONFIG_PRINTK
@@ -1484,7 +1489,7 @@ static struct ctl_table vm_table[] = {
 		.data		= &sysctl_compact_unevictable_allowed,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec_minmax,
+		.proc_handler	= proc_dointvec_minmax_warn_RT_change,
 		.extra1		= SYSCTL_ZERO,
 		.extra2		= SYSCTL_ONE,
 	},
@@ -2572,6 +2577,28 @@ int proc_dointvec(struct ctl_table *tabl
 	return do_proc_dointvec(table, write, buffer, lenp, ppos, NULL, NULL);
 }
 
+#ifdef CONFIG_COMPACTION
+static int proc_dointvec_minmax_warn_RT_change(struct ctl_table *table,
+					       int write, void __user *buffer,
+					       size_t *lenp, loff_t *ppos)
+{
+	int ret, old;
+
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || !write)
+		return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+
+	old = *(int *)table->data;
+	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+	if (ret)
+		return ret;
+	if (old != *(int *)table->data)
+		pr_warn_once("sysctl attribute %s changed by %s[%d]\n",
+			     table->procname, current->comm,
+			     task_pid_nr(current));
+	return ret;
+}
+#endif
+
 /**
  * proc_douintvec - read a vector of unsigned integers
  * @table: the sysctl table
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1590,7 +1590,11 @@ typedef enum {
  * Allow userspace to control policy on scanning the unevictable LRU for
  * compactable pages.
  */
+#ifdef CONFIG_PREEMPT_RT
+int sysctl_compact_unevictable_allowed __read_mostly = 0;
+#else
 int sysctl_compact_unevictable_allowed __read_mostly = 1;
+#endif
 
 static inline void
 update_fast_start_pfn(struct compact_control *cc, unsigned long pfn)