Re: [PATCH v1 0/2] mm/userfaultfd: fix and cleanup for migration entries with uffd-wp

[David Hildenbrand]

On 05.04.23 17:17, Peter Xu wrote:
> On Wed, Apr 05, 2023 at 04:25:33PM +0200, David Hildenbrand wrote:
> > One fix (I have a simple reproducer but it's too long to paste it into
> > the commit)
>
> I hope the recent rework of the unit test can help having more of these
> unit tests into the kselftests directly.
>
> Currently it's still ugly even after the rework patchset - we'll need to
> reference e.g. area_dst in unit tests for the allocated memory region (with
> specified type of either MEM_ANON, MEM_SHMEM, ...), but there's plan to
> make it even better.

Yes, I refrained from messing with the selftest for now while you rework
Here is the hacky reproducer:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <stdint.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <sys/ioctl.h>
#include <linux/userfaultfd.h>
#include <linux/mempolicy.h>

static int uffd;
static int pagemap_fd;
static size_t pagesize;

static uint64_t pagemap_get_entry(int fd, char *start)
{
	const unsigned long pfn = (unsigned long)start / pagesize;
	uint64_t entry;
	int ret;

	ret = pread(fd, &entry, sizeof(entry), pfn * sizeof(entry));
	if (ret != sizeof(entry)) {
		fprintf(stderr, "pread() failed\n");
		exit(1);
	}

	return entry;
}

#define BIT_ULL(nr)                   (1ULL << (nr))
#define PM_UFFD_WP                    BIT_ULL(57)
#define PM_SWAP                       BIT_ULL(62)
#define PM_PRESENT                    BIT_ULL(63)

static bool pagemap_is_uffd_wp(int fd, char *start)
{
	uint64_t entry = pagemap_get_entry(fd, start);

	return entry & PM_UFFD_WP;
}

static bool pagemap_is_populated(int fd, char *start)
{
	uint64_t entry = pagemap_get_entry(fd, start);

	return entry & (PM_SWAP | PM_PRESENT);
}

static int setup_uffd(char *mem, size_t size)
{
	struct uffdio_api uffdio_api;
	struct uffdio_register uffdio_register;
	struct uffdio_range uffd_range;

	uffd = syscall(__NR_userfaultfd,
		       O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY);
	if (uffd < 0) {
		fprintf(stderr, "syscall() failed: %d\n", errno);
		return -errno;
	}

	uffdio_api.api = UFFD_API;
	uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP;
	if (ioctl(uffd, UFFDIO_API, &uffdio_api) < 0) {
		fprintf(stderr, "UFFDIO_API failed: %d\n", errno);
		return -errno;
	}

	if (!(uffdio_api.features & UFFD_FEATURE_PAGEFAULT_FLAG_WP)) {
		fprintf(stderr, "UFFD_FEATURE_PAGEFAULT_FLAG_WP missing\n");
		return -ENOSYS;
	}

	/* Register UFFD-WP */
	uffdio_register.range.start = (unsigned long) mem;
	uffdio_register.range.len = size;
	uffdio_register.mode = UFFDIO_REGISTER_MODE_WP;
	if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) < 0) {
		fprintf(stderr, "UFFDIO_REGISTER failed: %d\n", errno);
		return -errno;
	}

	return 0;
}

int main(int argc, char **argv)
{
	struct uffdio_writeprotect uffd_writeprotect;
	const size_t thpsize = 2 * 1024 * 1024;
	const size_t mmap_size = 2 * thpsize;
	char *mem;

	pagesize = getpagesize();
	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
	if (pagemap_fd < 0) {
		fprintf(stderr, "open() failed\n");
		exit(1);
	}

	mem = mmap(NULL, mmap_size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON,
		   -1, 0);
	if (mem == MAP_FAILED) {
		fprintf(stderr, "mmap() failed\n");
		return -errno;
	}
	mem = (char *)(((uintptr_t)mem + thpsize) & ~(thpsize - 1));

	if (madvise(mem, thpsize, MADV_HUGEPAGE)) {
		fprintf(stderr, "MADV_HUGEPAGE failed\n");
		return -errno;
	}

	/* Populate a THP. */
	memset(mem, 0, pagesize);

	if (!pagemap_is_populated(pagemap_fd, mem + thpsize - pagesize)) {
		fprintf(stderr, "Did not get a THP populated\n");
		return -EBUSY;
	}

	/* Verify uffd-wp is not set. */
	if (pagemap_is_uffd_wp(pagemap_fd, mem)) {
		fprintf(stderr, "uffd-wp unexpectedly set\n");
		return -1;
	}

	/* Setup UFFD and protect the page. */
	if (setup_uffd(mem, thpsize))
		return 1;
	uffd_writeprotect.range.start = (unsigned long) mem;
	uffd_writeprotect.range.len = thpsize;
	uffd_writeprotect.mode = UFFDIO_WRITEPROTECT_MODE_WP;
	if (ioctl(uffd, UFFDIO_WRITEPROTECT, &uffd_writeprotect)) {
		fprintf(stderr, "UFFDIO_WRITEPROTECT failed: %d\n", errno);
		return -errno;
	}

	/* Verify uffd-wp is set. */
	if (!pagemap_is_uffd_wp(pagemap_fd, mem)) {
		fprintf(stderr, "uffd-wp not set\n");
		return -1;
	}

	/* Migrate the THP. */
	if (syscall(__NR_mbind, mem, thpsize, MPOL_LOCAL, NULL, 0x7fful,
		    MPOL_MF_MOVE)) {
		fprintf(stderr, "mbind() failed\n");
		return -errno;
	}

	/* Verify uffd-wp is still set. */
	if (!pagemap_is_uffd_wp(pagemap_fd, mem)) {
		fprintf(stderr, "uffd-wp lost\n");
		return -1;
	}

	return 0;
}

--
Thanks,

David / dhildenb