linux-kernel - Dirty deleted files cause pointless I/O storms (unless truncated first)

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 20 Jan 2014 16:59:23 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Dirty deleted files cause pointless I/O storms (unless truncated first)

The code below runs quickly for a few iterations, and then it slows
down and the whole system becomes laggy for far too long.

Removing the sync_file_range call results in no I/O being performed at
all (which means that the kernel isn't totally screwing this up), and
changing "4096" to SIZE causes lots of I/O but without
the going-out-to-lunch bit (unsurprisingly).

Surprisingly, uncommenting the ftruncate call seems to fix the
problem.  This suggests that all the necessary infrastructure to avoid
wasting time writing to deleted files is there but that it's not
getting used.


#define _GNU_SOURCE
#include <sys/mman.h>
#include <err.h>
#include <fcntl.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>

#define SIZE (16 * 1048576)

static void hammer(const char *name)
{
  int fd = open(name, O_RDWR | O_CREAT | O_EXCL, 0600);
  if (fd == -1)
    err(1, "open");

  fallocate(fd, 0, 0, SIZE);

  void *addr = mmap(NULL, SIZE, PROT_WRITE, MAP_SHARED, fd, 0);
  if (addr == MAP_FAILED)
    err(1, "mmap");

  memset(addr, 0, SIZE);

  if (munmap(addr, SIZE) != 0)
    err(1, "munmap");

  if (sync_file_range(fd, 0, 4096,
              SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE |
              SYNC_FILE_RANGE_WAIT_AFTER) != 0)
    err(1, "sync_file_range");

  if (unlink(name) != 0)
    err(1, "unlink");

  //  if (ftruncate(fd, 0) != 0)
  //    err(1, "ftruncate");

  close(fd);
}

int main(int argc, char **argv)
{
  if (argc != 2) {
    printf("Usage: hammer_and_delete FILENAME\n");
    return 1;
  }

  while (true) {
    hammer(argv[1]);
    write(1, ".", 1);
  }
}
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/