Re: [PATCH v1 2/5] userfaultfd: introduce access-likely mode for common operations

[Nadav Amit <namit@vmware.com>]

On Jun 22, 2022, at 11:50 AM, Nadav Amit <nadav.amit@gmail.com> wrote:

> From: Nadav Amit <namit@vmware.com>
> 
> Using a PTE on x86 with cleared access-bit (aka young-bit)
> takes ~600 cycles more than when the access bit is set. At the same
> time, setting the access-bit for memory that is not used (e.g.,
> prefetched) can introduce greater overheads, as the prefetched memory is
> reclaimed later than it should be.
> 
> Userfaultfd currently does not set the access-bit (excluding the
> huge-pages case). Arguably, it is best to let the user control whether
> the access bit should be set or not. The expected use is to request
> userfaultfd to set the access-bit when the copy/wp operation is done to
> resolve a page-fault, and not to set the access-bit when the memory is
> prefetched.
> 
> Introduce UFFDIO_[op]_ACCESS_LIKELY to enable userspace to request the
> young bit to be set.

I reply to my own email, but this mostly addresses the concerns that Peter
has raised.

So I ran the test below on my Haswell (x86), which showed two things:

1. Accessing an address using a clean PTE or old PTE takes ~500 cycles
more than with dirty+young (depending on the access, of course: dirty
does not matter for read, dirty+young both matter for write).

2. I made a mistake in my implementation. PTEs are - at least on x86 -
created as young with mk_pte(). So the logic should be similar to
do_set_pte():

        if (prefault && arch_wants_old_prefaulted_pte())
                entry = pte_mkold(entry);
        else
                entry = pte_sw_mkyoung(entry);

Based on these results, I will send another version for both young and
dirty. Let me know if these results are not convincing.

I will add, as we discussed (well, I think I raised these things, so
hopefully you agree):

1. On x86, avoid flush if changing WP->RO and PTE is clean.

2. When write-unprotecting entry, if PTE is exclusive, set it as writable.
[ I considered not setting it as writable if write-hint is not provided, but
with the change in (1), it does not provide any real value. ]

---

#define _GNU_SOURCE
#include <sys/types.h>
#include <stdio.h>
#include <linux/userfaultfd.h>
#include <pthread.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <signal.h>
#include <poll.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <sys/ioctl.h>
#include <stdint.h>
#include <stdbool.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
		       } while (0)

static inline uint64_t rdtscp(void)
{
	uint64_t rax, rdx;
	uint32_t aux;
	asm volatile ("rdtscp" : "=a" (rax), "=d" (rdx), "=c" (aux):: "memory");
}

int main(int argc, char *argv[])
{
	long uffd;          /* userfaultfd file descriptor */
	char *addr;         /* Start of region handled by userfaultfd */
	unsigned long len;  /* Length of region handled by userfaultfd */
	pthread_t thr;      /* ID of thread that handles page faults */
	bool young, dirty, write;
	struct uffdio_api uffdio_api;
	struct uffdio_register uffdio_register;
	int l;
	static char *page = NULL;
	struct uffdio_copy uffdio_copy;
	ssize_t nread;
	int page_size;

	if (argc != 5) {
		fprintf(stderr, "Usage: %s [num-pages] [write] [young] [dirty]\n", argv[0]);
		exit(EXIT_FAILURE);
	}

	page_size = sysconf(_SC_PAGE_SIZE);
	len = strtoul(argv[1], NULL, 0) * page_size;
	write = !!strtoul(argv[2], NULL, 0);
	young = !!strtoul(argv[3], NULL, 0);
	dirty = !!strtoul(argv[4], NULL, 0);

	page = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
		   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

	if (page == MAP_FAILED)
		errExit("mmap");

	uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
	if (uffd == -1)
		errExit("userfaultfd");

	uffdio_api.api = UFFD_API;
	uffdio_api.features = (1<<11); //UFFD_FEATURE_EXACT_ADDRESS;
	if (ioctl(uffd, UFFDIO_API, &uffdio_api) == -1)
		errExit("ioctl-UFFDIO_API");

	addr = mmap(NULL, len, PROT_READ | PROT_WRITE,
		MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
	if (addr == MAP_FAILED)
		errExit("mmap");

	uffdio_register.range.start = (unsigned long) addr;
	uffdio_register.range.len = len;
	uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
	if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) == -1)
		errExit("ioctl-UFFDIO_REGISTER");

	uffdio_copy.src = (unsigned long) page;
	uffdio_copy.mode = 0;
	if (young)
		uffdio_copy.mode |= (1ul << 2);
	if (dirty)
		uffdio_copy.mode |= (1ul << 3);

	uffdio_copy.len = page_size;
	uffdio_copy.copy = 0;

	for (l = 0; l < len; l += page_size) {
		uffdio_copy.dst = (unsigned long)(&addr[l]);
		if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) == -1)
			errExit("ioctl-UFFDIO_COPY");
	}

	for (l = 0; l < len; l += page_size) {
		char c;
		uint64_t start;

		start = rdtscp();
		if (write)
			addr[l] = 5;
		else
			c = *(volatile char *)(&addr[l]);
		printf("%ld\n", rdtscp() - start);
	}

	exit(EXIT_SUCCESS);
}