mm: huge_memory: a new debugfs interface for splitting THP tests

We did not have a direct user interface of splitting the compound page
backing a THP and there is no need unless we want to expose the THP
implementation details to users.  Make <debugfs>/split_huge_pages accept a
new command to do that.

By writing "<pid>,<vaddr_start>,<vaddr_end>" to
<debugfs>/split_huge_pages, THPs within the given virtual address range
from the process with the given pid are split. It is used to test
split_huge_page function. In addition, a selftest program is added to
tools/testing/selftests/vm to utilize the interface by splitting
PMD THPs and PTE-mapped THPs.

This does not change the old behavior, i.e., writing 1 to the interface
to split all THPs in the system.

Link: https://lkml.kernel.org/r/20210331235309.332292-1-zi.yan@sent.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mika Penttila <mika.penttila@nextfour.com>
Cc: Sandipan Das <sandipan@linux.ibm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -7,6 +7,7 @@
 
 #include <linux/mm.h>
 #include <linux/sched.h>
+#include <linux/sched/mm.h>
 #include <linux/sched/coredump.h>
 #include <linux/sched/numa_balancing.h>
 #include <linux/highmem.h>
@@ -2915,16 +2916,14 @@ static struct shrinker deferred_split_shrinker = {
 };
 
 #ifdef CONFIG_DEBUG_FS
-static int split_huge_pages_set(void *data, u64 val)
+static void split_huge_pages_all(void)
 {
 	struct zone *zone;
 	struct page *page;
 	unsigned long pfn, max_zone_pfn;
 	unsigned long total = 0, split = 0;
 
-	if (val != 1)
+	pr_debug("Split all THPs\n");
-		return -EINVAL;
-
 	for_each_populated_zone(zone) {
 		max_zone_pfn = zone_end_pfn(zone);
 		for (pfn = zone->zone_start_pfn; pfn < max_zone_pfn; pfn++) {
@@ -2948,15 +2947,155 @@ static int split_huge_pages_set(void *data, u64 val)
 			unlock_page(page);
 next:
 			put_page(page);
+			cond_resched();
 		}
 	}
 
-	pr_info("%lu of %lu THP split\n", split, total);
+	pr_debug("%lu of %lu THP split\n", split, total);
-
-	return 0;
 }
-DEFINE_DEBUGFS_ATTRIBUTE(split_huge_pages_fops, NULL, split_huge_pages_set,
+
-		"%llu\n");
+static inline bool vma_not_suitable_for_thp_split(struct vm_area_struct *vma)
+{
+	return vma_is_special_huge(vma) || (vma->vm_flags & VM_IO) ||
+		    is_vm_hugetlb_page(vma);
+}
+
+static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
+				unsigned long vaddr_end)
+{
+	int ret = 0;
+	struct task_struct *task;
+	struct mm_struct *mm;
+	unsigned long total = 0, split = 0;
+	unsigned long addr;
+
+	vaddr_start &= PAGE_MASK;
+	vaddr_end &= PAGE_MASK;
+
+	/* Find the task_struct from pid */
+	rcu_read_lock();
+	task = find_task_by_vpid(pid);
+	if (!task) {
+		rcu_read_unlock();
+		ret = -ESRCH;
+		goto out;
+	}
+	get_task_struct(task);
+	rcu_read_unlock();
+
+	/* Find the mm_struct */
+	mm = get_task_mm(task);
+	put_task_struct(task);
+
+	if (!mm) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	pr_debug("Split huge pages in pid: %d, vaddr: [0x%lx - 0x%lx]\n",
+		 pid, vaddr_start, vaddr_end);
+
+	mmap_read_lock(mm);
+	/*
+	 * always increase addr by PAGE_SIZE, since we could have a PTE page
+	 * table filled with PTE-mapped THPs, each of which is distinct.
+	 */
+	for (addr = vaddr_start; addr < vaddr_end; addr += PAGE_SIZE) {
+		struct vm_area_struct *vma = find_vma(mm, addr);
+		unsigned int follflags;
+		struct page *page;
+
+		if (!vma || addr < vma->vm_start)
+			break;
+
+		/* skip special VMA and hugetlb VMA */
+		if (vma_not_suitable_for_thp_split(vma)) {
+			addr = vma->vm_end;
+			continue;
+		}
+
+		/* FOLL_DUMP to ignore special (like zero) pages */
+		follflags = FOLL_GET | FOLL_DUMP;
+		page = follow_page(vma, addr, follflags);
+
+		if (IS_ERR(page))
+			continue;
+		if (!page)
+			continue;
+
+		if (!is_transparent_hugepage(page))
+			goto next;
+
+		total++;
+		if (!can_split_huge_page(compound_head(page), NULL))
+			goto next;
+
+		if (!trylock_page(page))
+			goto next;
+
+		if (!split_huge_page(page))
+			split++;
+
+		unlock_page(page);
+next:
+		put_page(page);
+		cond_resched();
+	}
+	mmap_read_unlock(mm);
+	mmput(mm);
+
+	pr_debug("%lu of %lu THP split\n", split, total);
+
+out:
+	return ret;
+}
+
+#define MAX_INPUT_BUF_SZ 255
+
+static ssize_t split_huge_pages_write(struct file *file, const char __user *buf,
+				size_t count, loff_t *ppops)
+{
+	static DEFINE_MUTEX(split_debug_mutex);
+	ssize_t ret;
+	char input_buf[MAX_INPUT_BUF_SZ]; /* hold pid, start_vaddr, end_vaddr */
+	int pid;
+	unsigned long vaddr_start, vaddr_end;
+
+	ret = mutex_lock_interruptible(&split_debug_mutex);
+	if (ret)
+		return ret;
+
+	ret = -EFAULT;
+
+	memset(input_buf, 0, MAX_INPUT_BUF_SZ);
+	if (copy_from_user(input_buf, buf, min_t(size_t, count, MAX_INPUT_BUF_SZ)))
+		goto out;
+
+	input_buf[MAX_INPUT_BUF_SZ - 1] = '\0';
+	ret = sscanf(input_buf, "%d,0x%lx,0x%lx", &pid, &vaddr_start, &vaddr_end);
+	if (ret == 1 && pid == 1) {
+		split_huge_pages_all();
+		ret = strlen(input_buf);
+		goto out;
+	} else if (ret != 3) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	ret = split_huge_pages_pid(pid, vaddr_start, vaddr_end);
+	if (!ret)
+		ret = strlen(input_buf);
+out:
+	mutex_unlock(&split_debug_mutex);
+	return ret;
+
+}
+
+static const struct file_operations split_huge_pages_fops = {
+	.owner	 = THIS_MODULE,
+	.write	 = split_huge_pages_write,
+	.llseek  = no_llseek,
+};
 
 static int __init split_huge_pages_debugfs(void)
 {

tools/testing/selftests/vm/.gitignore

@@ -22,3 +22,4 @@ map_fixed_noreplace
 write_to_hugetlbfs
 hmm-tests
 local_config.*
+split_huge_page_test

tools/testing/selftests/vm/Makefile

@@ -42,6 +42,7 @@ TEST_GEN_FILES += on-fault-limit
 TEST_GEN_FILES += thuge-gen
 TEST_GEN_FILES += transhuge-stress
 TEST_GEN_FILES += userfaultfd
+TEST_GEN_FILES += split_huge_page_test
 
 ifeq ($(MACHINE),x86_64)
 CAN_BUILD_I386 := $(shell ./../x86/check_cc.sh $(CC) ../x86/trivial_32bit_program.c -m32)

tools/testing/selftests/vm/split_huge_page_test.c

@@ -0,0 +1,318 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A test of splitting PMD THPs and PTE-mapped THPs from a specified virtual
+ * address range in a process via <debugfs>/split_huge_pages interface.
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <inttypes.h>
+#include <string.h>
+#include <fcntl.h>
+#include <sys/mman.h>
+#include <malloc.h>
+#include <stdbool.h>
+
+uint64_t pagesize;
+unsigned int pageshift;
+uint64_t pmd_pagesize;
+
+#define PMD_SIZE_PATH "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"
+#define SPLIT_DEBUGFS "/sys/kernel/debug/split_huge_pages"
+#define SMAP_PATH "/proc/self/smaps"
+#define INPUT_MAX 80
+
+#define PFN_MASK     ((1UL<<55)-1)
+#define KPF_THP      (1UL<<22)
+
+int is_backed_by_thp(char *vaddr, int pagemap_file, int kpageflags_file)
+{
+	uint64_t paddr;
+	uint64_t page_flags;
+
+	if (pagemap_file) {
+		pread(pagemap_file, &paddr, sizeof(paddr),
+			((long)vaddr >> pageshift) * sizeof(paddr));
+
+		if (kpageflags_file) {
+			pread(kpageflags_file, &page_flags, sizeof(page_flags),
+				(paddr & PFN_MASK) * sizeof(page_flags));
+
+			return !!(page_flags & KPF_THP);
+		}
+	}
+	return 0;
+}
+
+
+static uint64_t read_pmd_pagesize(void)
+{
+	int fd;
+	char buf[20];
+	ssize_t num_read;
+
+	fd = open(PMD_SIZE_PATH, O_RDONLY);
+	if (fd == -1) {
+		perror("Open hpage_pmd_size failed");
+		exit(EXIT_FAILURE);
+	}
+	num_read = read(fd, buf, 19);
+	if (num_read < 1) {
+		close(fd);
+		perror("Read hpage_pmd_size failed");
+		exit(EXIT_FAILURE);
+	}
+	buf[num_read] = '\0';
+	close(fd);
+
+	return strtoul(buf, NULL, 10);
+}
+
+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 void write_debugfs(int pid, uint64_t vaddr_start, uint64_t vaddr_end)
+{
+	char input[INPUT_MAX];
+	int ret;
+
+	ret = snprintf(input, INPUT_MAX, "%d,0x%lx,0x%lx", pid, vaddr_start,
+			vaddr_end);
+	if (ret >= INPUT_MAX) {
+		printf("%s: Debugfs input is too long\n", __func__);
+		exit(EXIT_FAILURE);
+	}
+
+	if (!write_file(SPLIT_DEBUGFS, input, ret + 1)) {
+		perror(SPLIT_DEBUGFS);
+		exit(EXIT_FAILURE);
+	}
+}
+
+#define MAX_LINE_LENGTH 500
+
+static bool check_for_pattern(FILE *fp, const char *pattern, char *buf)
+{
+	while (fgets(buf, MAX_LINE_LENGTH, fp) != NULL) {
+		if (!strncmp(buf, pattern, strlen(pattern)))
+			return true;
+	}
+	return false;
+}
+
+static uint64_t check_huge(void *addr)
+{
+	uint64_t thp = 0;
+	int ret;
+	FILE *fp;
+	char buffer[MAX_LINE_LENGTH];
+	char addr_pattern[MAX_LINE_LENGTH];
+
+	ret = snprintf(addr_pattern, MAX_LINE_LENGTH, "%08lx-",
+		       (unsigned long) addr);
+	if (ret >= MAX_LINE_LENGTH) {
+		printf("%s: Pattern is too long\n", __func__);
+		exit(EXIT_FAILURE);
+	}
+
+
+	fp = fopen(SMAP_PATH, "r");
+	if (!fp) {
+		printf("%s: Failed to open file %s\n", __func__, SMAP_PATH);
+		exit(EXIT_FAILURE);
+	}
+	if (!check_for_pattern(fp, addr_pattern, buffer))
+		goto err_out;
+
+	/*
+	 * Fetch the AnonHugePages: in the same block and check the number of
+	 * hugepages.
+	 */
+	if (!check_for_pattern(fp, "AnonHugePages:", buffer))
+		goto err_out;
+
+	if (sscanf(buffer, "AnonHugePages:%10ld kB", &thp) != 1) {
+		printf("Reading smap error\n");
+		exit(EXIT_FAILURE);
+	}
+
+err_out:
+	fclose(fp);
+	return thp;
+}
+
+void split_pmd_thp(void)
+{
+	char *one_page;
+	size_t len = 4 * pmd_pagesize;
+	uint64_t thp_size;
+	size_t i;
+
+	one_page = memalign(pmd_pagesize, len);
+
+	if (!one_page) {
+		printf("Fail to allocate memory\n");
+		exit(EXIT_FAILURE);
+	}
+
+	madvise(one_page, len, MADV_HUGEPAGE);
+
+	for (i = 0; i < len; i++)
+		one_page[i] = (char)i;
+
+	thp_size = check_huge(one_page);
+	if (!thp_size) {
+		printf("No THP is allocated\n");
+		exit(EXIT_FAILURE);
+	}
+
+	/* split all THPs */
+	write_debugfs(getpid(), (uint64_t)one_page, (uint64_t)one_page + len);
+
+	for (i = 0; i < len; i++)
+		if (one_page[i] != (char)i) {
+			printf("%ld byte corrupted\n", i);
+			exit(EXIT_FAILURE);
+		}
+
+
+	thp_size = check_huge(one_page);
+	if (thp_size) {
+		printf("Still %ld kB AnonHugePages not split\n", thp_size);
+		exit(EXIT_FAILURE);
+	}
+
+	printf("Split huge pages successful\n");
+	free(one_page);
+}
+
+void split_pte_mapped_thp(void)
+{
+	char *one_page, *pte_mapped, *pte_mapped2;
+	size_t len = 4 * pmd_pagesize;
+	uint64_t thp_size;
+	size_t i;
+	const char *pagemap_template = "/proc/%d/pagemap";
+	const char *kpageflags_proc = "/proc/kpageflags";
+	char pagemap_proc[255];
+	int pagemap_fd;
+	int kpageflags_fd;
+
+	if (snprintf(pagemap_proc, 255, pagemap_template, getpid()) < 0) {
+		perror("get pagemap proc error");
+		exit(EXIT_FAILURE);
+	}
+	pagemap_fd = open(pagemap_proc, O_RDONLY);
+
+	if (pagemap_fd == -1) {
+		perror("read pagemap:");
+		exit(EXIT_FAILURE);
+	}
+
+	kpageflags_fd = open(kpageflags_proc, O_RDONLY);
+
+	if (kpageflags_fd == -1) {
+		perror("read kpageflags:");
+		exit(EXIT_FAILURE);
+	}
+
+	one_page = mmap((void *)(1UL << 30), len, PROT_READ | PROT_WRITE,
+			MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+
+	madvise(one_page, len, MADV_HUGEPAGE);
+
+	for (i = 0; i < len; i++)
+		one_page[i] = (char)i;
+
+	thp_size = check_huge(one_page);
+	if (!thp_size) {
+		printf("No THP is allocated\n");
+		exit(EXIT_FAILURE);
+	}
+
+	/* remap the first pagesize of first THP */
+	pte_mapped = mremap(one_page, pagesize, pagesize, MREMAP_MAYMOVE);
+
+	/* remap the Nth pagesize of Nth THP */
+	for (i = 1; i < 4; i++) {
+		pte_mapped2 = mremap(one_page + pmd_pagesize * i + pagesize * i,
+				     pagesize, pagesize,
+				     MREMAP_MAYMOVE|MREMAP_FIXED,
+				     pte_mapped + pagesize * i);
+		if (pte_mapped2 == (char *)-1) {
+			perror("mremap failed");
+			exit(EXIT_FAILURE);
+		}
+	}
+
+	/* smap does not show THPs after mremap, use kpageflags instead */
+	thp_size = 0;
+	for (i = 0; i < pagesize * 4; i++)
+		if (i % pagesize == 0 &&
+		    is_backed_by_thp(&pte_mapped[i], pagemap_fd, kpageflags_fd))
+			thp_size++;
+
+	if (thp_size != 4) {
+		printf("Some THPs are missing during mremap\n");
+		exit(EXIT_FAILURE);
+	}
+
+	/* split all remapped THPs */
+	write_debugfs(getpid(), (uint64_t)pte_mapped,
+		      (uint64_t)pte_mapped + pagesize * 4);
+
+	/* smap does not show THPs after mremap, use kpageflags instead */
+	thp_size = 0;
+	for (i = 0; i < pagesize * 4; i++) {
+		if (pte_mapped[i] != (char)i) {
+			printf("%ld byte corrupted\n", i);
+			exit(EXIT_FAILURE);
+		}
+		if (i % pagesize == 0 &&
+		    is_backed_by_thp(&pte_mapped[i], pagemap_fd, kpageflags_fd))
+			thp_size++;
+	}
+
+	if (thp_size) {
+		printf("Still %ld THPs not split\n", thp_size);
+		exit(EXIT_FAILURE);
+	}
+
+	printf("Split PTE-mapped huge pages successful\n");
+	munmap(one_page, len);
+	close(pagemap_fd);
+	close(kpageflags_fd);
+}
+
+int main(int argc, char **argv)
+{
+	if (geteuid() != 0) {
+		printf("Please run the benchmark as root\n");
+		exit(EXIT_FAILURE);
+	}
+
+	pagesize = getpagesize();
+	pageshift = ffs(pagesize) - 1;
+	pmd_pagesize = read_pmd_pagesize();
+
+	split_pmd_thp();
+	split_pte_mapped_thp();
+
+	return 0;
+}