Re: [PATCH] mm/filemap: let sequential faults restart mmap readahead

[Gopal Malaviya <gopalmalaviya53@gmail.com>]

On Wed, Nov 11, 2026 at 07:43:54AM +0000, Alireza Haddadi wrote:
> That would be great, thank you. Could you share the short-run test,
> So I can run the same here while working on it?

Sure, it is below. It writes the file (or reuses it with "keep"),
drops it from the page cache with POSIX_FADV_DONTNEED, touches 1% of
its pages through a MAP_PRIVATE mapping and prints the MiB read
(pgpgin in /proc/vmstat) and the major faults for each pattern:

  ./mmap-rand FILE               2600 MiB file, removed at the end
  ./mmap-rand FILE 20480 keep    20 GiB file, kept for the next run

The tables came from the second form in a 3 GiB guest, with
"echo 128 > /sys/block/vda/queue/read_ahead_kb" (or 8192) before each
run. "random 4-page recs" is the short-run case. Like the earlier
tests, the program was written with the help of an AI assistant.

Thanks,
Gopal

---8<--- mmap-rand.c
/*
 * mmap-rand.c: how much does the kernel read from disk for sparse
 * mmap access (random, and fixed stride) on a cold file?
 * Reports disk input (pgpgin from /proc/vmstat) and major faults.
 *
 *   ./mmap-rand FILE        (FILE must not exist; removed at the end)
 *   ./mmap-rand FILE SIZE_MIB keep   (reuse FILE if it exists, keep it)
 */
#define _GNU_SOURCE
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/resource.h>
#include <unistd.h>

#define MIB	(1UL << 20)
static unsigned long SIZE = 2600 * MIB;

static long pgpgin_kib(void)
{
	char name[64];
	long val;
	FILE *f = fopen("/proc/vmstat", "r");

	if (!f)
		err(1, "/proc/vmstat");
	while (fscanf(f, "%63s %ld", name, &val) == 2)
		if (!strcmp(name, "pgpgin")) {
			fclose(f);
			return val;
		}
	errx(1, "no pgpgin");
}

static long majflt(void)
{
	struct rusage ru;

	getrusage(RUSAGE_SELF, &ru);
	return ru.ru_majflt;
}

static void run(const char *path, const char *name, unsigned long stride,
		unsigned long rec)
{
	unsigned long page = sysconf(_SC_PAGESIZE), npages = SIZE / page;
	unsigned long n = npages / 100, i, idx;	/* touch 1% of the pages */
	long in0, mf0;
	volatile char sink;
	char *m;
	int fd;

	fd = open(path, O_RDONLY);
	if (fd < 0)
		err(1, "open");
	if (posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED))
		errx(1, "posix_fadvise");
	m = mmap(NULL, SIZE, PROT_READ, MAP_PRIVATE, fd, 0);
	if (m == MAP_FAILED)
		err(1, "mmap");

	srandom(1);
	in0 = pgpgin_kib();
	mf0 = majflt();
	for (i = 0; i < n; i += rec) {
		unsigned long j;

		idx = stride ? (i * stride) % npages : random() % (npages - rec);
		for (j = 0; j < rec; j++)	/* a record of rec adjacent pages */
			sink = m[(idx + j) * page];
	}
	printf("%-20s %6lu touches %8ld MiB read %6ld major faults\n", name, n,
	       (pgpgin_kib() - in0) / 1024, majflt() - mf0);

	(void)sink;
	munmap(m, SIZE);
	close(fd);
}

int main(int argc, char **argv)
{
	static char buf[MIB];
	unsigned long i;
	int fd;

	if (argc != 2 && argc != 4)
		errx(2, "usage: %s FILE [SIZE_MIB keep]", argv[0]);
	if (argc == 4)
		SIZE = strtoul(argv[2], NULL, 0) * MIB;
	fd = open(argv[1], O_WRONLY | O_CREAT | O_EXCL, 0600);
	if (fd >= 0) {
		memset(buf, 0x5a, sizeof(buf));
		for (i = 0; i < SIZE / MIB; i++)
			if (write(fd, buf, MIB) != (ssize_t)MIB)
				err(1, "write");
		if (fsync(fd))
			err(1, "fsync");
		close(fd);
	} else if (argc != 4) {
		err(1, "create");
	}

	setvbuf(stdout, NULL, _IOLBF, 0);
	run(argv[1], "random", 0, 1);
	run(argv[1], "stride 2053 pages", 2053, 1);
	run(argv[1], "random 2-page recs", 0, 2);
	run(argv[1], "random 4-page recs", 0, 4);
	if (argc != 4)
		unlink(argv[1]);
	return 0;
}