[PATCH 0/6] thp/khugepaged improvements and CoW semantics

[PATCH 0/6] thp/khugepaged improvements and CoW semantics

[Kirill A. Shutemov]

The patchset adds khugepaged selftest (anon-THP only for now), expands
cases khugepaged can handle and switches anon-THP copy-on-write handling
to 4k.

Please review and consider applying.

v3:
 - Fix handling compound pages in swap cache;
 - Rework swaped vs. referenced check;
 - Fix refcounting for compound pages;
 - Drop Reviewed-by/Acked-by as patchset changed non-trivially;
 - Typos;
v2:
 - Fix race in compound page handling;
 - Add one more test-case for compound page case;
 - Rework LRU add cache draining;
 - Typos;

Kirill A. Shutemov (6):
  khugepaged: Add self test
  khugepaged: Do not stop collapse if less than half PTEs are referenced
  khugepaged: Drain all LRU caches before scanning pages
  khugepaged: Drain LRU add pagevec after swapin
  khugepaged: Allow to collapse a page shared across fork
  khugepaged: Allow to collapse PTE-mapped compound pages

 mm/khugepaged.c                         | 172 +++--
 tools/testing/selftests/vm/Makefile     |   1 +
 tools/testing/selftests/vm/khugepaged.c | 899 ++++++++++++++++++++++++
 3 files changed, 1012 insertions(+), 60 deletions(-)
 create mode 100644 tools/testing/selftests/vm/khugepaged.c

-- 
2.26.0

[PATCH 1/6] khugepaged: Add self test

[Kirill A. Shutemov]

The test checks if khugepaged is able to recover huge page where we
expect to do so. It only covers anon-THP for now.

Currently the test shows few failures. They are going to be addressed by
the following patches.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 tools/testing/selftests/vm/Makefile     |   1 +
 tools/testing/selftests/vm/khugepaged.c | 899 ++++++++++++++++++++++++
 2 files changed, 900 insertions(+)
 create mode 100644 tools/testing/selftests/vm/khugepaged.c

diff --git a/tools/testing/selftests/vm/Makefile b/tools/testing/selftests/vm/Makefile
index 7f9a8a8c31da..981d0dc21f9e 100644
--- a/tools/testing/selftests/vm/Makefile
+++ b/tools/testing/selftests/vm/Makefile
@@ -18,6 +18,7 @@ TEST_GEN_FILES += on-fault-limit
 TEST_GEN_FILES += thuge-gen
 TEST_GEN_FILES += transhuge-stress
 TEST_GEN_FILES += userfaultfd
+TEST_GEN_FILES += khugepaged
 
 ifneq (,$(filter $(ARCH),arm64 ia64 mips64 parisc64 ppc64 riscv64 s390x sh64 sparc64 x86_64))
 TEST_GEN_FILES += va_128TBswitch
diff --git a/tools/testing/selftests/vm/khugepaged.c b/tools/testing/selftests/vm/khugepaged.c
new file mode 100644
index 000000000000..34d945e71e2e
--- /dev/null
+++ b/tools/testing/selftests/vm/khugepaged.c
@@ -0,0 +1,899 @@
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <limits.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <string.h>
+#include <unistd.h>
+
+#include <sys/mman.h>
+#include <sys/wait.h>
+
+#ifndef MADV_PAGEOUT
+#define MADV_PAGEOUT 21
+#endif
+
+#define BASE_ADDR ((void *)(1UL << 30))
+static unsigned long hpage_pmd_size;
+static unsigned long page_size;
+static int hpage_pmd_nr;
+
+#define THP_SYSFS "/sys/kernel/mm/transparent_hugepage/"
+
+enum thp_enabled {
+	THP_ALWAYS,
+	THP_MADVISE,
+	THP_NEVER,
+};
+
+static const char *thp_enabled_strings[] = {
+	"always",
+	"madvise",
+	"never",
+	NULL
+};
+
+enum thp_defrag {
+	THP_DEFRAG_ALWAYS,
+	THP_DEFRAG_DEFER,
+	THP_DEFRAG_DEFER_MADVISE,
+	THP_DEFRAG_MADVISE,
+	THP_DEFRAG_NEVER,
+};
+
+static const char *thp_defrag_strings[] = {
+	"always",
+	"defer",
+	"defer+madvise",
+	"madvise",
+	"never",
+	NULL
+};
+
+enum shmem_enabled {
+	SHMEM_ALWAYS,
+	SHMEM_WITHIN_SIZE,
+	SHMEM_ADVISE,
+	SHMEM_NEVER,
+	SHMEM_DENY,
+	SHMEM_FORCE,
+};
+
+static const char *shmem_enabled_strings[] = {
+	"always",
+	"within_size",
+	"advise",
+	"never",
+	"deny",
+	"force",
+	NULL
+};
+
+struct khugepaged_settings {
+	bool defrag;
+	unsigned int alloc_sleep_millisecs;
+	unsigned int scan_sleep_millisecs;
+	unsigned int max_ptes_none;
+	unsigned int max_ptes_swap;
+	unsigned long pages_to_scan;
+};
+
+struct settings {
+	enum thp_enabled thp_enabled;
+	enum thp_defrag thp_defrag;
+	enum shmem_enabled shmem_enabled;
+	bool debug_cow;
+	bool use_zero_page;
+	struct khugepaged_settings khugepaged;
+};
+
+static struct settings default_settings = {
+	.thp_enabled = THP_MADVISE,
+	.thp_defrag = THP_DEFRAG_ALWAYS,
+	.shmem_enabled = SHMEM_NEVER,
+	.debug_cow = 0,
+	.use_zero_page = 0,
+	.khugepaged = {
+		.defrag = 1,
+		.alloc_sleep_millisecs = 10,
+		.scan_sleep_millisecs = 10,
+	},
+};
+
+static struct settings saved_settings;
+static bool skip_settings_restore;
+
+static int exit_status;
+
+static void success(const char *msg)
+{
+	printf(" \e[32m%s\e[0m\n", msg);
+}
+
+static void fail(const char *msg)
+{
+	printf(" \e[31m%s\e[0m\n", msg);
+	exit_status++;
+}
+
+static int read_file(const char *path, char *buf, size_t buflen)
+{
+	int fd;
+	ssize_t numread;
+
+	fd = open(path, O_RDONLY);
+	if (fd == -1)
+		return 0;
+
+	numread = read(fd, buf, buflen - 1);
+	if (numread < 1) {
+		close(fd);
+		return 0;
+	}
+
+	buf[numread] = '\0';
+	close(fd);
+
+	return (unsigned int) numread;
+}
+
+static int write_file(const char *path, const char *buf, size_t buflen)
+{
+	int fd;
+	ssize_t numwritten;
+
+	fd = open(path, O_WRONLY);
+	if (fd == -1)
+		return 0;
+
+	numwritten = write(fd, buf, buflen - 1);
+	close(fd);
+	if (numwritten < 1)
+		return 0;
+
+	return (unsigned int) numwritten;
+}
+
+static int read_string(const char *name, const char *strings[])
+{
+	char path[PATH_MAX];
+	char buf[256];
+	char *c;
+	int ret;
+
+	ret = snprintf(path, PATH_MAX, THP_SYSFS "%s", name);
+	if (ret >= PATH_MAX) {
+		printf("%s: Pathname is too long\n", __func__);
+		exit(EXIT_FAILURE);
+	}
+
+	if (!read_file(path, buf, sizeof(buf))) {
+		perror(path);
+		exit(EXIT_FAILURE);
+	}
+
+	c = strchr(buf, '[');
+	if (!c) {
+		printf("%s: Parse failure\n", __func__);
+		exit(EXIT_FAILURE);
+	}
+
+	c++;
+	memmove(buf, c, sizeof(buf) - (c - buf));
+
+	c = strchr(buf, ']');
+	if (!c) {
+		printf("%s: Parse failure\n", __func__);
+		exit(EXIT_FAILURE);
+	}
+	*c = '\0';
+
+	ret = 0;
+	while (strings[ret]) {
+		if (!strcmp(strings[ret], buf))
+			return ret;
+		ret++;
+	}
+
+	printf("Failed to parse %s\n", name);
+	exit(EXIT_FAILURE);
+}
+
+static void write_string(const char *name, const char *val)
+{
+	char path[PATH_MAX];
+	int ret;
+
+	ret = snprintf(path, PATH_MAX, THP_SYSFS "%s", name);
+	if (ret >= PATH_MAX) {
+		printf("%s: Pathname is too long\n", __func__);
+		exit(EXIT_FAILURE);
+	}
+
+	if (!write_file(path, val, strlen(val) + 1)) {
+		perror(path);
+		exit(EXIT_FAILURE);
+	}
+}
+
+static const unsigned long read_num(const char *name)
+{
+	char path[PATH_MAX];
+	char buf[21];
+	int ret;
+
+	ret = snprintf(path, PATH_MAX, THP_SYSFS "%s", name);
+	if (ret >= PATH_MAX) {
+		printf("%s: Pathname is too long\n", __func__);
+		exit(EXIT_FAILURE);
+	}
+
+	ret = read_file(path, buf, sizeof(buf));
+	if (ret < 0) {
+		perror("read_file(read_num)");
+		exit(EXIT_FAILURE);
+	}
+
+	return strtoul(buf, NULL, 10);
+}
+
+static void write_num(const char *name, unsigned long num)
+{
+	char path[PATH_MAX];
+	char buf[21];
+	int ret;
+
+	ret = snprintf(path, PATH_MAX, THP_SYSFS "%s", name);
+	if (ret >= PATH_MAX) {
+		printf("%s: Pathname is too long\n", __func__);
+		exit(EXIT_FAILURE);
+	}
+
+	sprintf(buf, "%ld", num);
+	if (!write_file(path, buf, strlen(buf) + 1)) {
+		perror(path);
+		exit(EXIT_FAILURE);
+	}
+}
+
+static void write_settings(struct settings *settings)
+{
+	struct khugepaged_settings *khugepaged = &settings->khugepaged;
+
+	write_string("enabled", thp_enabled_strings[settings->thp_enabled]);
+	write_string("defrag", thp_defrag_strings[settings->thp_defrag]);
+	write_string("shmem_enabled",
+			shmem_enabled_strings[settings->shmem_enabled]);
+	write_num("debug_cow", settings->debug_cow);
+	write_num("use_zero_page", settings->use_zero_page);
+
+	write_num("khugepaged/defrag", khugepaged->defrag);
+	write_num("khugepaged/alloc_sleep_millisecs",
+			khugepaged->alloc_sleep_millisecs);
+	write_num("khugepaged/scan_sleep_millisecs",
+			khugepaged->scan_sleep_millisecs);
+	write_num("khugepaged/max_ptes_none", khugepaged->max_ptes_none);
+	write_num("khugepaged/max_ptes_swap", khugepaged->max_ptes_swap);
+	write_num("khugepaged/pages_to_scan", khugepaged->pages_to_scan);
+}
+
+static void restore_settings(int sig)
+{
+	if (skip_settings_restore)
+		goto out;
+
+	printf("Restore THP and khugepaged settings...");
+	write_settings(&saved_settings);
+	success("OK");
+	if (sig)
+		exit(EXIT_FAILURE);
+out:
+	exit(exit_status);
+}
+
+static void save_settings(void)
+{
+	printf("Save THP and khugepaged settings...");
+	saved_settings = (struct settings) {
+		.thp_enabled = read_string("enabled", thp_enabled_strings),
+		.thp_defrag = read_string("defrag", thp_defrag_strings),
+		.shmem_enabled =
+			read_string("shmem_enabled", shmem_enabled_strings),
+		.debug_cow = read_num("debug_cow"),
+		.use_zero_page = read_num("use_zero_page"),
+	};
+	saved_settings.khugepaged = (struct khugepaged_settings) {
+		.defrag = read_num("khugepaged/defrag"),
+		.alloc_sleep_millisecs =
+			read_num("khugepaged/alloc_sleep_millisecs"),
+		.scan_sleep_millisecs =
+			read_num("khugepaged/scan_sleep_millisecs"),
+		.max_ptes_none = read_num("khugepaged/max_ptes_none"),
+		.max_ptes_swap = read_num("khugepaged/max_ptes_swap"),
+		.pages_to_scan = read_num("khugepaged/pages_to_scan"),
+	};
+	success("OK");
+
+	signal(SIGTERM, restore_settings);
+	signal(SIGINT, restore_settings);
+	signal(SIGHUP, restore_settings);
+	signal(SIGQUIT, restore_settings);
+}
+
+static void adjust_settings(void)
+{
+
+	printf("Adjust settings...");
+	write_settings(&default_settings);
+	success("OK");
+}
+
+#define CHECK_HUGE_FMT "sed -ne " \
+	"'/^%lx/,/^AnonHugePages/{/^AnonHugePages:\\s*%ld kB/ q1}' " \
+	"/proc/%d/smaps"
+
+static bool check_huge(void *p)
+{
+	char *cmd;
+	int ret;
+
+	ret = asprintf(&cmd, CHECK_HUGE_FMT,
+			(unsigned long)p, hpage_pmd_size >> 10, getpid());
+	if (ret < 0) {
+		perror("asprintf(CHECK_FMT)");
+		exit(EXIT_FAILURE);
+	}
+
+	ret = system(cmd);
+	free(cmd);
+	if (ret < 0 || !WIFEXITED(ret)) {
+		perror("system(check_huge)");
+		exit(EXIT_FAILURE);
+	}
+
+	return WEXITSTATUS(ret);
+}
+
+#define CHECK_SWAP_FMT "sed -ne " \
+	"'/^%lx/,/^Swap:/{/^Swap:\\s*%ld kB/ q1}' " \
+	"/proc/%d/smaps"
+
+static bool check_swap(void *p, unsigned long size)
+{
+	char *cmd;
+	int ret;
+
+	ret = asprintf(&cmd, CHECK_SWAP_FMT,
+			(unsigned long)p, size >> 10, getpid());
+	if (ret < 0) {
+		perror("asprintf(CHECK_SWAP)");
+		exit(EXIT_FAILURE);
+	}
+
+	ret = system(cmd);
+	free(cmd);
+	if (ret < 0 || !WIFEXITED(ret)) {
+		perror("system(check_swap)");
+		exit(EXIT_FAILURE);
+	}
+
+	return WEXITSTATUS(ret);
+}
+
+static void *alloc_mapping(void)
+{
+	void *p;
+
+	p = mmap(BASE_ADDR, hpage_pmd_size, PROT_READ | PROT_WRITE,
+			MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+	if (p != BASE_ADDR) {
+		printf("Failed to allocate VMA at %p\n", BASE_ADDR);
+		exit(EXIT_FAILURE);
+	}
+
+	return p;
+}
+
+static void fill_memory(int *p, unsigned long start, unsigned long end)
+{
+	int i;
+
+	for (i = start / page_size; i < end / page_size; i++)
+		p[i * page_size / sizeof(*p)] = i + 0xdead0000;
+}
+
+static void validate_memory(int *p, unsigned long start, unsigned long end)
+{
+	int i;
+
+	for (i = start / page_size; i < end / page_size; i++) {
+		if (p[i * page_size / sizeof(*p)] != i + 0xdead0000) {
+			printf("Page %d is corrupted: %#x\n",
+					i, p[i * page_size / sizeof(*p)]);
+			exit(EXIT_FAILURE);
+		}
+	}
+}
+
+#define TICK 500000
+static bool wait_for_scan(const char *msg, char *p)
+{
+	int full_scans;
+	int timeout = 6; /* 3 seconds */
+
+	/* Sanity check */
+	if (check_huge(p)) {
+		printf("Unexpected huge page\n");
+		exit(EXIT_FAILURE);
+	}
+
+	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
+
+	/* Wait until the second full_scan completed */
+	full_scans = read_num("khugepaged/full_scans") + 2;
+
+	printf("%s...", msg);
+	while (timeout--) {
+		if (check_huge(p))
+			break;
+		if (read_num("khugepaged/full_scans") >= full_scans)
+			break;
+		printf(".");
+		usleep(TICK);
+	}
+
+	madvise(p, hpage_pmd_size, MADV_NOHUGEPAGE);
+
+	return !timeout;
+}
+
+static void alloc_at_fault(void)
+{
+	struct settings settings = default_settings;
+	char *p;
+
+	settings.thp_enabled = THP_ALWAYS;
+	write_settings(&settings);
+
+	p = alloc_mapping();
+	*p = 1;
+	printf("Allocate huge page on fault...");
+	if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+
+	write_settings(&default_settings);
+
+	madvise(p, page_size, MADV_DONTNEED);
+	printf("Split huge PMD on MADV_DONTNEED...");
+	if (!check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+	munmap(p, hpage_pmd_size);
+}
+
+static void collapse_full(void)
+{
+	void *p;
+
+	p = alloc_mapping();
+	fill_memory(p, 0, hpage_pmd_size);
+	if (wait_for_scan("Collapse fully populated PTE table", p))
+		fail("Timeout");
+	else if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+	validate_memory(p, 0, hpage_pmd_size);
+	munmap(p, hpage_pmd_size);
+}
+
+static void collapse_empty(void)
+{
+	void *p;
+
+	p = alloc_mapping();
+	if (wait_for_scan("Do not collapse empty PTE table", p))
+		fail("Timeout");
+	else if (check_huge(p))
+		fail("Fail");
+	else
+		success("OK");
+	munmap(p, hpage_pmd_size);
+}
+
+static void collapse_single_pte_entry(void)
+{
+	void *p;
+
+	p = alloc_mapping();
+	fill_memory(p, 0, page_size);
+	if (wait_for_scan("Collapse PTE table with single PTE entry present", p))
+		fail("Timeout");
+	else if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+	validate_memory(p, 0, page_size);
+	munmap(p, hpage_pmd_size);
+}
+
+static void collapse_max_ptes_none(void)
+{
+	int max_ptes_none = hpage_pmd_nr / 2;
+	struct settings settings = default_settings;
+	void *p;
+
+	settings.khugepaged.max_ptes_none = max_ptes_none;
+	write_settings(&settings);
+
+	p = alloc_mapping();
+
+	fill_memory(p, 0, (hpage_pmd_nr - max_ptes_none - 1) * page_size);
+	if (wait_for_scan("Do not collapse with max_ptes_none exeeded", p))
+		fail("Timeout");
+	else if (check_huge(p))
+		fail("Fail");
+	else
+		success("OK");
+	validate_memory(p, 0, (hpage_pmd_nr - max_ptes_none - 1) * page_size);
+
+	fill_memory(p, 0, (hpage_pmd_nr - max_ptes_none) * page_size);
+	if (wait_for_scan("Collapse with max_ptes_none PTEs empty", p))
+		fail("Timeout");
+	else if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+	validate_memory(p, 0, (hpage_pmd_nr - max_ptes_none) * page_size);
+
+	munmap(p, hpage_pmd_size);
+	write_settings(&default_settings);
+}
+
+static void collapse_swapin_single_pte(void)
+{
+	void *p;
+	p = alloc_mapping();
+	fill_memory(p, 0, hpage_pmd_size);
+
+	printf("Swapout one page...");
+	if (madvise(p, page_size, MADV_PAGEOUT)) {
+		perror("madvise(MADV_PAGEOUT)");
+		exit(EXIT_FAILURE);
+	}
+	if (check_swap(p, page_size)) {
+		success("OK");
+	} else {
+		fail("Fail");
+		goto out;
+	}
+
+	if (wait_for_scan("Collapse with swaping in single PTE entry", p))
+		fail("Timeout");
+	else if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+	validate_memory(p, 0, hpage_pmd_size);
+out:
+	munmap(p, hpage_pmd_size);
+}
+
+static void collapse_max_ptes_swap(void)
+{
+	int max_ptes_swap = read_num("khugepaged/max_ptes_swap");
+	void *p;
+
+	p = alloc_mapping();
+
+	fill_memory(p, 0, hpage_pmd_size);
+	printf("Swapout %d of %d pages...", max_ptes_swap + 1, hpage_pmd_nr);
+	if (madvise(p, (max_ptes_swap + 1) * page_size, MADV_PAGEOUT)) {
+		perror("madvise(MADV_PAGEOUT)");
+		exit(EXIT_FAILURE);
+	}
+	if (check_swap(p, (max_ptes_swap + 1) * page_size)) {
+		success("OK");
+	} else {
+		fail("Fail");
+		goto out;
+	}
+
+	if (wait_for_scan("Do not collapse with max_ptes_swap exeeded", p))
+		fail("Timeout");
+	else if (check_huge(p))
+		fail("Fail");
+	else
+		success("OK");
+	validate_memory(p, 0, hpage_pmd_size);
+
+	fill_memory(p, 0, hpage_pmd_size);
+	printf("Swapout %d of %d pages...", max_ptes_swap, hpage_pmd_nr);
+	if (madvise(p, max_ptes_swap * page_size, MADV_PAGEOUT)) {
+		perror("madvise(MADV_PAGEOUT)");
+		exit(EXIT_FAILURE);
+	}
+	if (check_swap(p, max_ptes_swap * page_size)) {
+		success("OK");
+	} else {
+		fail("Fail");
+		goto out;
+	}
+
+	if (wait_for_scan("Collapse with max_ptes_swap pages swapped out", p))
+		fail("Timeout");
+	else if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+	validate_memory(p, 0, hpage_pmd_size);
+out:
+	munmap(p, hpage_pmd_size);
+}
+
+static void collapse_single_pte_entry_compound(void)
+{
+	void *p;
+
+	p = alloc_mapping();
+
+	printf("Allocate huge page...");
+	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
+	fill_memory(p, 0, hpage_pmd_size);
+	if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+	madvise(p, hpage_pmd_size, MADV_NOHUGEPAGE);
+
+	printf("Split huge page leaving single PTE mapping compount page...");
+	madvise(p + page_size, hpage_pmd_size - page_size, MADV_DONTNEED);
+	if (!check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+
+	if (wait_for_scan("Collapse PTE table with single PTE mapping compount page", p))
+		fail("Timeout");
+	else if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+	validate_memory(p, 0, page_size);
+	munmap(p, hpage_pmd_size);
+}
+
+static void collapse_full_of_compound(void)
+{
+	void *p;
+
+	p = alloc_mapping();
+
+	printf("Allocate huge page...");
+	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
+	fill_memory(p, 0, hpage_pmd_size);
+	if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+
+	printf("Split huge page leaving single PTE page table full of compount pages...");
+	madvise(p, page_size, MADV_NOHUGEPAGE);
+	madvise(p, hpage_pmd_size, MADV_NOHUGEPAGE);
+	if (!check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+
+	if (wait_for_scan("Collapse PTE table full of compound pages", p))
+		fail("Timeout");
+	else if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+	validate_memory(p, 0, hpage_pmd_size);
+	munmap(p, hpage_pmd_size);
+}
+
+static void collapse_compound_extreme(void)
+{
+	void *p;
+	int i;
+
+	p = alloc_mapping();
+	for (i = 0; i < hpage_pmd_nr; i++) {
+		printf("\rConstruct PTE page table full of different PTE-mapped compound pages %3d/%d...",
+				i + 1, hpage_pmd_nr);
+
+		madvise(BASE_ADDR, hpage_pmd_size, MADV_HUGEPAGE);
+		fill_memory(BASE_ADDR, 0, hpage_pmd_size);
+		if (!check_huge(BASE_ADDR)) {
+			printf("Failed to allocate huge page\n");
+			exit(EXIT_FAILURE);
+		}
+		madvise(BASE_ADDR, hpage_pmd_size, MADV_NOHUGEPAGE);
+
+		p = mremap(BASE_ADDR - i * page_size,
+				i * page_size + hpage_pmd_size,
+				(i + 1) * page_size,
+				MREMAP_MAYMOVE | MREMAP_FIXED,
+				BASE_ADDR + 2 * hpage_pmd_size);
+		if (p == MAP_FAILED) {
+			perror("mremap+unmap");
+			exit(EXIT_FAILURE);
+		}
+
+		p = mremap(BASE_ADDR + 2 * hpage_pmd_size,
+				(i + 1) * page_size,
+				(i + 1) * page_size + hpage_pmd_size,
+				MREMAP_MAYMOVE | MREMAP_FIXED,
+				BASE_ADDR - (i + 1) * page_size);
+		if (p == MAP_FAILED) {
+			perror("mremap+alloc");
+			exit(EXIT_FAILURE);
+		}
+	}
+
+	munmap(BASE_ADDR, hpage_pmd_size);
+	fill_memory(p, 0, hpage_pmd_size);
+	if (!check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+
+	if (wait_for_scan("Collapse PTE table full of different compound pages", p))
+		fail("Timeout");
+	else if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+
+	validate_memory(p, 0, hpage_pmd_size);
+	munmap(p, hpage_pmd_size);
+}
+
+static void collapse_fork(void)
+{
+	int wstatus;
+	void *p;
+
+	p = alloc_mapping();
+
+	printf("Allocate small page...");
+	fill_memory(p, 0, page_size);
+	if (!check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+
+	printf("Share small page over fork()...");
+	if (!fork()) {
+		/* Do not touch settings on child exit */
+		skip_settings_restore = true;
+		exit_status = 0;
+
+		if (!check_huge(p))
+			success("OK");
+		else
+			fail("Fail");
+
+		fill_memory(p, page_size, 2 * page_size);
+
+		if (wait_for_scan("Collapse PTE table with single page shared with parent process", p))
+			fail("Timeout");
+		else if (check_huge(p))
+			success("OK");
+		else
+			fail("Fail");
+
+		validate_memory(p, 0, page_size);
+		munmap(p, hpage_pmd_size);
+		exit(exit_status);
+	}
+
+	wait(&wstatus);
+	exit_status += WEXITSTATUS(wstatus);
+
+	printf("Check if parent still has small page...");
+	if (!check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+	validate_memory(p, 0, page_size);
+	munmap(p, hpage_pmd_size);
+}
+
+static void collapse_fork_compound(void)
+{
+	int wstatus;
+	void *p;
+
+	p = alloc_mapping();
+
+	printf("Allocate huge page...");
+	madvise(p, hpage_pmd_size, MADV_HUGEPAGE);
+	fill_memory(p, 0, hpage_pmd_size);
+	if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+
+	printf("Share huge page over fork()...");
+	if (!fork()) {
+		/* Do not touch settings on child exit */
+		skip_settings_restore = true;
+		exit_status = 0;
+
+		if (check_huge(p))
+			success("OK");
+		else
+			fail("Fail");
+
+		printf("Split huge page PMD in child process...");
+		madvise(p, page_size, MADV_NOHUGEPAGE);
+		madvise(p, hpage_pmd_size, MADV_NOHUGEPAGE);
+		if (!check_huge(p))
+			success("OK");
+		else
+			fail("Fail");
+		fill_memory(p, 0, page_size);
+
+		if (wait_for_scan("Collapse PTE table full of compound pages in child", p))
+			fail("Timeout");
+		else if (check_huge(p))
+			success("OK");
+		else
+			fail("Fail");
+
+		validate_memory(p, 0, hpage_pmd_size);
+		munmap(p, hpage_pmd_size);
+		exit(exit_status);
+	}
+
+	wait(&wstatus);
+	exit_status += WEXITSTATUS(wstatus);
+
+	printf("Check if parent still has huge page...");
+	if (check_huge(p))
+		success("OK");
+	else
+		fail("Fail");
+	validate_memory(p, 0, hpage_pmd_size);
+	munmap(p, hpage_pmd_size);
+}
+
+int main(void)
+{
+	setbuf(stdout, NULL);
+
+	page_size = getpagesize();
+	hpage_pmd_size = read_num("hpage_pmd_size");
+	hpage_pmd_nr = hpage_pmd_size / page_size;
+
+	default_settings.khugepaged.max_ptes_none = hpage_pmd_nr - 1;
+	default_settings.khugepaged.max_ptes_swap = hpage_pmd_nr / 8;
+	default_settings.khugepaged.pages_to_scan = hpage_pmd_nr * 8;
+
+	save_settings();
+	adjust_settings();
+
+	alloc_at_fault();
+	collapse_full();
+	collapse_empty();
+	collapse_single_pte_entry();
+	collapse_max_ptes_none();
+	collapse_swapin_single_pte();
+	collapse_max_ptes_swap();
+	collapse_single_pte_entry_compound();
+	collapse_full_of_compound();
+	collapse_compound_extreme();
+	collapse_fork();
+	collapse_fork_compound();
+
+	restore_settings(0);
+}
-- 
2.26.0

[PATCH 2/6] khugepaged: Do not stop collapse if less than half PTEs are referenced

[Kirill A. Shutemov]

__collapse_huge_page_swapin() checks the number of referenced PTE to
decide if the memory range is hot enough to justify swapin.

We have few problems with the approach:

 - It is way too late: we can do the check much earlier and safe time.
   khugepaged_scan_pmd() already knows if we have any pages to swap in
   and number of referenced page.

 - It stops collapse altogether if there's not enough referenced pages,
   not only swappingin.

Fix it by making the right check early. We also can avoid additional
page table scanning if khugepaged_scan_pmd() haven't found any swap
entries.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Fixes: 0db501f7a34c ("mm, thp: convert from optimistic swapin collapsing to conservative")
---
 mm/khugepaged.c | 25 ++++++++++---------------
 1 file changed, 10 insertions(+), 15 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 99bab7e4d05b..5968ec5ddd6b 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -902,11 +902,6 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
 		.pgoff = linear_page_index(vma, address),
 	};
 
-	/* we only decide to swapin, if there is enough young ptes */
-	if (referenced < HPAGE_PMD_NR/2) {
-		trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
-		return false;
-	}
 	vmf.pte = pte_offset_map(pmd, address);
 	for (; vmf.address < address + HPAGE_PMD_NR*PAGE_SIZE;
 			vmf.pte++, vmf.address += PAGE_SIZE) {
@@ -946,7 +941,7 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
 static void collapse_huge_page(struct mm_struct *mm,
 				   unsigned long address,
 				   struct page **hpage,
-				   int node, int referenced)
+				   int node, int referenced, int unmapped)
 {
 	pmd_t *pmd, _pmd;
 	pte_t *pte;
@@ -1003,7 +998,8 @@ static void collapse_huge_page(struct mm_struct *mm,
 	 * If it fails, we release mmap_sem and jump out_nolock.
 	 * Continuing to collapse causes inconsistency.
 	 */
-	if (!__collapse_huge_page_swapin(mm, vma, address, pmd, referenced)) {
+	if (unmapped && !__collapse_huge_page_swapin(mm, vma, address,
+				pmd, referenced)) {
 		mem_cgroup_cancel_charge(new_page, memcg, true);
 		up_read(&mm->mmap_sem);
 		goto out_nolock;
@@ -1214,22 +1210,21 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 		    mmu_notifier_test_young(vma->vm_mm, address))
 			referenced++;
 	}
-	if (writable) {
-		if (referenced) {
+	if (!writable) {
+		result = SCAN_PAGE_RO;
+	} else if (!referenced || (unmapped && referenced < HPAGE_PMD_NR/2)) {
+		result = SCAN_LACK_REFERENCED_PAGE;
+	} else {
 			result = SCAN_SUCCEED;
 			ret = 1;
-		} else {
-			result = SCAN_LACK_REFERENCED_PAGE;
-		}
-	} else {
-		result = SCAN_PAGE_RO;
 	}
 out_unmap:
 	pte_unmap_unlock(pte, ptl);
 	if (ret) {
 		node = khugepaged_find_target_node();
 		/* collapse_huge_page will return with the mmap_sem released */
-		collapse_huge_page(mm, address, hpage, node, referenced);
+		collapse_huge_page(mm, address, hpage, node,
+				referenced, unmapped);
 	}
 out:
 	trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,
-- 
2.26.0

[PATCH 3/6] khugepaged: Drain all LRU caches before scanning pages

[Kirill A. Shutemov]

Having a page in LRU add cache offsets page refcount and gives
false-negative on PageLRU(). It reduces collapse success rate.

Drain all LRU add caches before scanning. It happens relatively
rare and should not disturb the system too much.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/khugepaged.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 5968ec5ddd6b..ee66c140c2d6 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2059,6 +2059,8 @@ static void khugepaged_do_scan(void)
 
 	barrier(); /* write khugepaged_pages_to_scan to local stack */
 
+	lru_add_drain_all();
+
 	while (progress < pages) {
 		if (!khugepaged_prealloc_page(&hpage, &wait))
 			break;
-- 
2.26.0

[PATCH 4/6] khugepaged: Drain LRU add pagevec after swapin

[Kirill A. Shutemov]

__collapse_huge_page_isolate() may fail due to extra pin in the LRU add
pagevec. It's pretty common for swapin case: we swap in pages just to
fail due to the extra pin.

Drain LRU add pagevec on successful swapin.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/khugepaged.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index ee66c140c2d6..e3e41c2768d8 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -934,6 +934,11 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
 	}
 	vmf.pte--;
 	pte_unmap(vmf.pte);
+
+	/* Drain LRU add pagevec to remove extra pin on the swapped in pages */
+	if (swapped_in)
+		lru_add_drain();
+
 	trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 1);
 	return true;
 }
-- 
2.26.0

[PATCH 5/6] khugepaged: Allow to collapse a page shared across fork

[Kirill A. Shutemov]

The page can be included into collapse as long as it doesn't have extra
pins (from GUP or otherwise).

Logic to check the refcound is moved to a separate function.
Note that the function is ready to deal with compound pages. It's
preparation for the following patch.

VM_BUG_ON_PAGE() was removed from __collapse_huge_page_copy() as the
invariant it checks is no longer valid: the source can be mapped
multiple times now.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/khugepaged.c | 41 ++++++++++++++++++++++++++++++-----------
 1 file changed, 30 insertions(+), 11 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e3e41c2768d8..f9864644c3b7 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -529,6 +529,24 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte)
 	}
 }
 
+static bool is_refcount_suitable(struct page *page)
+{
+	int expected_refcount, refcount;
+
+	refcount = page_count(page);
+	expected_refcount = total_mapcount(page);
+	if (PageSwapCache(page))
+		expected_refcount += compound_nr(page);
+
+	if (IS_ENABLED(CONFIG_DEBUG_VM) && expected_refcount > refcount) {
+		pr_err("expected_refcount: %d, refcount: %d\n",
+				expected_refcount, refcount);
+		dump_page(page, "Unexpected refcount");
+	}
+
+	return page_count(page) == expected_refcount;
+}
+
 static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 					unsigned long address,
 					pte_t *pte)
@@ -581,11 +599,17 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		}
 
 		/*
-		 * cannot use mapcount: can't collapse if there's a gup pin.
-		 * The page must only be referenced by the scanned process
-		 * and page swap cache.
+		 * Check if the page has any GUP (or other external) pins.
+		 *
+		 * The page table that maps the page has been already unlinked
+		 * from the page table tree and this process cannot get
+		 * an additinal pin on the page.
+		 *
+		 * New pins can come later if the page is shared across fork,
+		 * but not for the this process. It is fine. The other process
+		 * cannot write to the page, only trigger CoW.
 		 */
-		if (page_count(page) != 1 + PageSwapCache(page)) {
+		if (!is_refcount_suitable(page)) {
 			unlock_page(page);
 			result = SCAN_PAGE_COUNT;
 			goto out;
@@ -672,7 +696,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
 		} else {
 			src_page = pte_page(pteval);
 			copy_user_highpage(page, src_page, address, vma);
-			VM_BUG_ON_PAGE(page_mapcount(src_page) != 1, src_page);
 			release_pte_page(src_page);
 			/*
 			 * ptl mostly unnecessary, but preempt has to
@@ -1201,12 +1224,8 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 			goto out_unmap;
 		}
 
-		/*
-		 * cannot use mapcount: can't collapse if there's a gup pin.
-		 * The page must only be referenced by the scanned process
-		 * and page swap cache.
-		 */
-		if (page_count(page) != 1 + PageSwapCache(page)) {
+		/* Check if the page has any GUP (or other external) pins */
+		if (!is_refcount_suitable(page)) {
 			result = SCAN_PAGE_COUNT;
 			goto out_unmap;
 		}
-- 
2.26.0

[PATCH 6/6] khugepaged: Allow to collapse PTE-mapped compound pages

[Kirill A. Shutemov]

We can collapse PTE-mapped compound pages. We only need to avoid
handling them more than once: lock/unlock page only once if it's present
in the PMD range multiple times as it handled on compound level. The
same goes for LRU isolation and putback.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/khugepaged.c | 99 ++++++++++++++++++++++++++++++++-----------------
 1 file changed, 65 insertions(+), 34 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f9864644c3b7..11d500396d85 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -515,17 +515,30 @@ void __khugepaged_exit(struct mm_struct *mm)
 
 static void release_pte_page(struct page *page)
 {
-	dec_node_page_state(page, NR_ISOLATED_ANON + page_is_file_cache(page));
+	mod_node_page_state(page_pgdat(page),
+			NR_ISOLATED_ANON + page_is_file_cache(page),
+			-compound_nr(page));
 	unlock_page(page);
 	putback_lru_page(page);
 }
 
-static void release_pte_pages(pte_t *pte, pte_t *_pte)
+static void release_pte_pages(pte_t *pte, pte_t *_pte,
+		struct list_head *compound_pagelist)
 {
+	struct page *page, *tmp;
+
 	while (--_pte >= pte) {
 		pte_t pteval = *_pte;
-		if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)))
-			release_pte_page(pte_page(pteval));
+
+		page = pte_page(pteval);
+		if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)) &&
+				!PageCompound(page))
+			release_pte_page(page);
+	}
+
+	list_for_each_entry_safe(page, tmp, compound_pagelist, lru) {
+		list_del(&page->lru);
+		release_pte_page(page);
 	}
 }
 
@@ -549,7 +562,8 @@ static bool is_refcount_suitable(struct page *page)
 
 static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 					unsigned long address,
-					pte_t *pte)
+					pte_t *pte,
+					struct list_head *compound_pagelist)
 {
 	struct page *page = NULL;
 	pte_t *_pte;
@@ -579,13 +593,21 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 			goto out;
 		}
 
-		/* TODO: teach khugepaged to collapse THP mapped with pte */
+		VM_BUG_ON_PAGE(!PageAnon(page), page);
+
 		if (PageCompound(page)) {
-			result = SCAN_PAGE_COMPOUND;
-			goto out;
-		}
+			struct page *p;
+			page = compound_head(page);
 
-		VM_BUG_ON_PAGE(!PageAnon(page), page);
+			/*
+			 * Check if we have dealt with the compound page
+			 * already
+			 */
+			list_for_each_entry(p, compound_pagelist, lru) {
+				if (page == p)
+					goto next;
+			}
+		}
 
 		/*
 		 * We can do it before isolate_lru_page because the
@@ -614,19 +636,15 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 			result = SCAN_PAGE_COUNT;
 			goto out;
 		}
-		if (pte_write(pteval)) {
-			writable = true;
-		} else {
-			if (PageSwapCache(page) &&
-			    !reuse_swap_page(page, NULL)) {
-				unlock_page(page);
-				result = SCAN_SWAP_CACHE_PAGE;
-				goto out;
-			}
+		if (!pte_write(pteval) && PageSwapCache(page) &&
+				!reuse_swap_page(page, NULL)) {
 			/*
-			 * Page is not in the swap cache. It can be collapsed
-			 * into a THP.
+			 * Page is in the swap cache and cannot be re-used.
+			 * It cannot be collapsed into a THP.
 			 */
+			unlock_page(page);
+			result = SCAN_SWAP_CACHE_PAGE;
+			goto out;
 		}
 
 		/*
@@ -638,16 +656,23 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 			result = SCAN_DEL_PAGE_LRU;
 			goto out;
 		}
-		inc_node_page_state(page,
-				NR_ISOLATED_ANON + page_is_file_cache(page));
+		mod_node_page_state(page_pgdat(page),
+				NR_ISOLATED_ANON + page_is_file_cache(page),
+				compound_nr(page));
 		VM_BUG_ON_PAGE(!PageLocked(page), page);
 		VM_BUG_ON_PAGE(PageLRU(page), page);
 
+		if (PageCompound(page))
+			list_add_tail(&page->lru, compound_pagelist);
+next:
 		/* There should be enough young pte to collapse the page */
 		if (pte_young(pteval) ||
 		    page_is_young(page) || PageReferenced(page) ||
 		    mmu_notifier_test_young(vma->vm_mm, address))
 			referenced++;
+
+		if (pte_write(pteval))
+			writable = true;
 	}
 	if (likely(writable)) {
 		if (likely(referenced)) {
@@ -661,7 +686,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	}
 
 out:
-	release_pte_pages(pte, _pte);
+	release_pte_pages(pte, _pte, compound_pagelist);
 	trace_mm_collapse_huge_page_isolate(page, none_or_zero,
 					    referenced, writable, result);
 	return 0;
@@ -670,13 +695,14 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
 				      struct vm_area_struct *vma,
 				      unsigned long address,
-				      spinlock_t *ptl)
+				      spinlock_t *ptl,
+				      struct list_head *compound_pagelist)
 {
+	struct page *src_page, *tmp;
 	pte_t *_pte;
 	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
 				_pte++, page++, address += PAGE_SIZE) {
 		pte_t pteval = *_pte;
-		struct page *src_page;
 
 		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
 			clear_user_highpage(page, address);
@@ -696,7 +722,8 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
 		} else {
 			src_page = pte_page(pteval);
 			copy_user_highpage(page, src_page, address, vma);
-			release_pte_page(src_page);
+			if (!PageCompound(src_page))
+				release_pte_page(src_page);
 			/*
 			 * ptl mostly unnecessary, but preempt has to
 			 * be disabled to update the per-cpu stats
@@ -713,6 +740,11 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
 			free_page_and_swap_cache(src_page);
 		}
 	}
+
+	list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
+		list_del(&src_page->lru);
+		release_pte_page(src_page);
+	}
 }
 
 static void khugepaged_alloc_sleep(void)
@@ -971,6 +1003,7 @@ static void collapse_huge_page(struct mm_struct *mm,
 				   struct page **hpage,
 				   int node, int referenced, int unmapped)
 {
+	LIST_HEAD(compound_pagelist);
 	pmd_t *pmd, _pmd;
 	pte_t *pte;
 	pgtable_t pgtable;
@@ -1071,7 +1104,8 @@ static void collapse_huge_page(struct mm_struct *mm,
 	mmu_notifier_invalidate_range_end(&range);
 
 	spin_lock(pte_ptl);
-	isolated = __collapse_huge_page_isolate(vma, address, pte);
+	isolated = __collapse_huge_page_isolate(vma, address, pte,
+			&compound_pagelist);
 	spin_unlock(pte_ptl);
 
 	if (unlikely(!isolated)) {
@@ -1096,7 +1130,8 @@ static void collapse_huge_page(struct mm_struct *mm,
 	 */
 	anon_vma_unlock_write(vma->anon_vma);
 
-	__collapse_huge_page_copy(pte, new_page, vma, address, pte_ptl);
+	__collapse_huge_page_copy(pte, new_page, vma, address, pte_ptl,
+			&compound_pagelist);
 	pte_unmap(pte);
 	__SetPageUptodate(new_page);
 	pgtable = pmd_pgtable(_pmd);
@@ -1193,11 +1228,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 			goto out_unmap;
 		}
 
-		/* TODO: teach khugepaged to collapse THP mapped with pte */
-		if (PageCompound(page)) {
-			result = SCAN_PAGE_COMPOUND;
-			goto out_unmap;
-		}
+		page = compound_head(page);
 
 		/*
 		 * Record which node the original page is from and save this
-- 
2.26.0

Re: [PATCH 0/6] thp/khugepaged improvements and CoW semantics

[Kirill A. Shutemov]

On Mon, Apr 13, 2020 at 01:04:41PM +0300, Kirill A. Shutemov wrote:
> The patchset adds khugepaged selftest (anon-THP only for now), expands
> cases khugepaged can handle and switches anon-THP copy-on-write handling
> to 4k.
> 
> Please review and consider applying.

Please ignore, I've missed last two patches by mistake. I'll resend.

-- 
 Kirill A. Shutemov

Re: [PATCHv3, RESEND 0/8] thp/khugepaged improvements and CoW semantics

[William Kucharski]

Quick comments, mostly nits:

[PATCH 5/6] khugepaged: Allow to collapse a page shared across fork


> +static bool is_refcount_suitable(struct page *page)
> +{
> +	int expected_refcount, refcount;
> +
> +	refcount = page_count(page);
> +	expected_refcount = total_mapcount(page);
> +	if (PageSwapCache(page))
> +		expected_refcount += compound_nr(page);
> +
> +	if (IS_ENABLED(CONFIG_DEBUG_VM) && expected_refcount > refcount) {
> +		pr_err("expected_refcount: %d, refcount: %d\n",
> +				expected_refcount, refcount);

I'd rather see this message reworded slightly as I prefer to know WHY a value
was unexpected directly from the error message:

		pr_err("expected_refcount (%d) > refcount (%d)\n",

[PATCHv3, RESEND 7/8] thp: Change CoW semantics for anon-THP

I really like the simplifications here.

[PATCHv3, RESEND 8/8] khugepaged: Introduce 'max_ptes_shared' tunable

As above, I'd like to see a tiny extra bit of specificity in the printf()s:

+static void collapse_max_ptes_shared()
+{

[ ... ]

+		if (check_huge(p))
+			success("OK");
+		else
+			fail("Fail");
+
+		printf("Trigger CoW in %d of %d...",

		printf("Trigger CoW on page %d of %d...",

+				hpage_pmd_nr - max_ptes_shared - 1, hpage_pmd_nr);
+		fill_memory(p, 0, (hpage_pmd_nr - max_ptes_shared - 1) * page_size);
+		if (!check_huge(p))
+			success("OK");
+		else
+			fail("Fail");
+
+		if (wait_for_scan("Do not collapse with max_ptes_shared exeeded", p))
+			fail("Timeout");
+		else if (!check_huge(p))
+			success("OK");
+		else
+			fail("Fail");
+
+		printf("Trigger CoW in %d of %d...",

		printf("Trigger CoW on page %d of %d...",

+				hpage_pmd_nr - max_ptes_shared, hpage_pmd_nr);
+		fill_memory(p, 0, (hpage_pmd_nr - max_ptes_shared) * page_size);
+		if (!check_huge(p))
+			success("OK");
+		else
+			fail("Fail");

Otherwise, for the khugepaged/THP series:

Reviewed-by: William Kucharski <william.kucharski@oracle.com>