[PATCH 0/1] mm/khugepaged: map anonymous pte-mapped THPs by pmds

[Xu Yu <xuyu@xxxxxxxxxxxxxxxxx>]

I write a testcase[1] to demonstrate this scenario.

vanilla kernel (v6.6):
# ./test
[mmap rw    ] vaddr: 0x0x7f2000000000, pfn: 0x110000, is_thp: 1, is_thp_mapped: 1
[mprotect ro] vaddr: 0x0x7f2000000000, pfn: 0x110000, is_thp: 1, is_thp_mapped: 0
          ^^ __split_huge_pmd() is called in  change_protection()
[mprotect rw] vaddr: 0x0x7f2000000000, pfn: 0x110000, is_thp: 1, is_thp_mapped: 0
          ^^ vma_merge()
[khugepaged ] vaddr: 0x0x7f2000000000, pfn: 0x110000, is_thp: 1, is_thp_mapped: 0
[khugepaged ] vaddr: 0x0x7f2000000000, pfn: 0x110000, is_thp: 1, is_thp_mapped: 0
[khugepaged ] vaddr: 0x0x7f2000000000, pfn: 0x117a00, is_thp: 1, is_thp_mapped: 1
         ^^ hpage_collapse_scan_pmd         ^^^^^^^^ new hugepage is allocated
[khugepaged ] vaddr: 0x0x7f2000000000, pfn: 0x117a00, is_thp: 1, is_thp_mapped: 1

patched kernel:
# ./test
[mmap rw    ] vaddr: 0x0x7f2000000000, pfn: 0x128400, is_thp: 1, is_thp_mapped: 1
[mprotect ro] vaddr: 0x0x7f2000000000, pfn: 0x128400, is_thp: 1, is_thp_mapped: 0
[mprotect rw] vaddr: 0x0x7f2000000000, pfn: 0x128400, is_thp: 1, is_thp_mapped: 0
[khugepaged ] vaddr: 0x0x7f2000000000, pfn: 0x128400, is_thp: 1, is_thp_mapped: 0
[khugepaged ] vaddr: 0x0x7f2000000000, pfn: 0x128400, is_thp: 1, is_thp_mapped: 0
[khugepaged ] vaddr: 0x0x7f2000000000, pfn: 0x128400, is_thp: 1, is_thp_mapped: 1
         ^^ hpage_collapse_scan_pmd         ^^^^^^^^ old hugepage
[khugepaged ] vaddr: 0x0x7f2000000000, pfn: 0x128400, is_thp: 1, is_thp_mapped: 1

[1] testcase source code
#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif

#include <ctype.h>
#include <fcntl.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>
#include<signal.h>

#define PAGE_SHIFT	12
#define SIZE_2M		(2<<20)
#define ADDRESS		0x7f2000000000

#define PAGEMAP_PRESENT		63
#define PAGEMAP_PFN_MASK	0x007FFFFFFFFFFFFFUL    /* bits 54:0 */

#define KPF_SIZE	8
#define KPF_THP		22

static bool is_thp(unsigned long pfn)
{
	int kpageflags_fd;
	unsigned long flag;
	int ret;

	kpageflags_fd = open("/proc/kpageflags", O_RDONLY);
	ret = pread(kpageflags_fd, &flag, KPF_SIZE, KPF_SIZE * pfn);
	close(kpageflags_fd);

	if (ret != KPF_SIZE)
		return false;

	return flag & (1 << KPF_THP);
}

static bool is_thp_mapped(void *addr)
{
	char path[64], line[128], pattern[64];
	int pid;
	FILE *fp;
	int anon_huge_pages = 0;

	snprintf(pattern, 64, "%lx", (unsigned long)addr);
	pid = getpid();
	snprintf(path, 64, "/proc/%u/smaps", pid);
	fp = fopen(path, "r");

	while (fgets(line, 64, fp) != NULL) {
		if (strstr(line, pattern))
			break;
	}

	while (fgets(line, 64, fp) != NULL) {
		if (1 == sscanf(line, "AnonHugePages:      %d kB", &anon_huge_pages))
			break;
	}
	fclose(fp);

	return anon_huge_pages != 0;
}

static unsigned long get_pfn(unsigned long vaddr)
{
	char path[64];
	int pid, pagemap_fd;
	off64_t offset;
	unsigned long pfn = 0;
	int ret;

	offset = (vaddr >> PAGE_SHIFT) * sizeof(unsigned long);
	pid = getpid();
	snprintf(path, 64, "/proc/%u/pagemap", pid);

	pagemap_fd = open(path, O_RDONLY);
	lseek64(pagemap_fd, offset, SEEK_SET);
	ret = read(pagemap_fd, &pfn, sizeof(pfn));
	close(pagemap_fd);

	if (ret < 0)
		return -1;

	if (!(pfn & (1UL << PAGEMAP_PRESENT)))
		return 0;

	return (pfn & PAGEMAP_PFN_MASK);
}

static void show_addr_info(void *addr, char *msg)
{
	unsigned long pfn;

	pfn = get_pfn((unsigned long)addr);
	printf("[%s] vaddr: 0x%p, pfn: 0x%lx, is_thp: %d, is_thp_mapped: %d\n",
	       msg, addr, pfn, is_thp(pfn), is_thp_mapped(addr));
}

int main(int argc, char *argv[])
{
	void *addr;

	addr = mmap((void *)ADDRESS, SIZE_2M, PROT_READ | PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_POPULATE, -1, 0);
	if (addr == MAP_FAILED) {
		perror("mmap");
		return 1;
	}
	show_addr_info(addr, "mmap rw    ");

	mprotect(addr, SIZE_2M / 2, PROT_READ);
	show_addr_info(addr, "mprotect ro");

	mprotect(addr, SIZE_2M / 2, PROT_READ | PROT_WRITE);
	show_addr_info(addr, "mprotect rw");

	while (1) {
		show_addr_info(addr, "khugepaged ");
		sleep(5);
	}

	return 0;
}

Xu Yu (1):
  mm/khugepaged: map anonymous pte-mapped THPs by pmds

 mm/khugepaged.c | 187 ++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 180 insertions(+), 7 deletions(-)

-- 
2.37.1