mm: numa: do not trap faults on shared data section pages.

Workloads consisting of a large number of processes running the same
program with a very large shared data segment may experience performance
problems when numa balancing attempts to migrate the shared cow pages.
This manifests itself with many processes or tasks in
TASK_UNINTERRUPTIBLE state waiting for the shared pages to be migrated.

The program listed below simulates the conditions with these results
when run with 288 processes on a 144 core/8 socket machine.

Average throughput 	Average throughput     Average throughput
with numa_balancing=0	with numa_balancing=1  with numa_balancing=1
     			without the patch      with the patch
---------------------	---------------------  ---------------------
2118782			2021534		       2107979

Complex production environments show less variability and fewer poorly
performing outliers accompanied with a smaller number of processes
waiting on NUMA page migration with this patch applied.  In some cases,
%iowait drops from 16%-26% to 0.

  // SPDX-License-Identifier: GPL-2.0
  /*
   * Copyright (c) 2017 Oracle and/or its affiliates. All rights reserved.
   */
  #include <sys/time.h>
  #include <stdio.h>
  #include <wait.h>
  #include <sys/mman.h>

  int a[1000000] = {13};

  int  main(int argc, const char **argv)
  {
	int n = 0;
	int i;
	pid_t pid;
	int stat;
	int *count_array;
	int cpu_count = 288;
	long total = 0;

	struct timeval t1, t2 = {(argc > 1 ? atoi(argv[1]) : 10), 0};

	if (argc > 2)
		cpu_count = atoi(argv[2]);

	count_array = mmap(NULL, cpu_count * sizeof(int),
			   (PROT_READ|PROT_WRITE),
			   (MAP_SHARED|MAP_ANONYMOUS), 0, 0);

	if (count_array == MAP_FAILED) {
		perror("mmap:");
		return 0;
	}

	for (i = 0; i < cpu_count; ++i) {
		pid = fork();
		if (pid <= 0)
			break;
		if ((i & 0xf) == 0)
			usleep(2);
	}

	if (pid != 0) {
		if (i == 0) {
			perror("fork:");
			return 0;
		}

		for (;;) {
			pid = wait(&stat);
			if (pid < 0)
				break;
		}

		for (i = 0; i < cpu_count; ++i)
			total += count_array[i];

		printf("Total %ld\n", total);
		munmap(count_array, cpu_count * sizeof(int));
		return 0;
	}

	gettimeofday(&t1, 0);
	timeradd(&t1, &t2, &t1);
	while (timercmp(&t2, &t1, <)) {
		int b = 0;
		int j;

		for (j = 0; j < 1000000; j++)
			b += a[j];
		gettimeofday(&t2, 0);
		n++;
	}
	count_array[i] = n;
	return 0;
  }

This patch changes change_pte_range() to skip shared copy-on-write pages
when called from change_prot_numa().

NOTE: change_prot_numa() is nominally called from task_numa_work() and
queue_pages_test_walk().  task_numa_work() is the auto NUMA balancing
path, and queue_pages_test_walk() is part of explicit NUMA policy
management.  However, queue_pages_test_walk() only calls
change_prot_numa() when MPOL_MF_LAZY is specified and currently that is
not allowed, so change_prot_numa() is only called from auto NUMA
balancing.

In the case of explicit NUMA policy management, shared pages are not
migrated unless MPOL_MF_MOVE_ALL is specified, and MPOL_MF_MOVE_ALL
depends on CAP_SYS_NICE.  Currently, there is no way to pass information
about MPOL_MF_MOVE_ALL to change_pte_range.  This will have to be fixed
if MPOL_MF_LAZY is enabled and MPOL_MF_MOVE_ALL is to be honored in lazy
migration mode.

task_numa_work() skips the read-only VMAs of programs and shared
libraries.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Henry Willard <[email protected]>
Reviewed-by: Håkon Bugge <[email protected]>
Reviewed-by: Steve Sistare <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Cc: Kate Stewart <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: Philippe Ombredanne <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: "Jérôme Glisse" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Signed-off-by: Linus Torvalds <[email protected]>

mm/mprotect.c

@@ -84,6 +84,11 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 				if (!page || PageKsm(page))
 					continue;
 
+				/* Also skip shared copy-on-write pages */
+				if (is_cow_mapping(vma->vm_flags) &&
+				    page_mapcount(page) != 1)
+					continue;
+
 				/* Avoid TLB flush if possible */
 				if (pte_protnone(oldpte))
 					continue;