uffd: Disable image deduplication after fork

[liusdu]

After a fork, both the child and parent processes may trigger a page fault (#PF) at the same virtual address, referencing the same position in the page image. If deduplication is enabled, the last process to trigger the page fault will fail.

Therefore, deduplication should be disabled after a fork to prevent this issue.

[rst0git]

If deduplication is enabled, the last process to trigger the page fault will fail.

Would it be possible to add a ZDTM test for this use-case?

[liusdu]

If deduplication is enabled, the last process to trigger the page fault will fail.

Would it be possible to add a ZDTM test for this use-case?

I will add a ZDTM testcase later. The follow test case can reproduce this issue.

• source code

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <string.h>
#include <signal.h>
#include <sched.h>


#define VALUE 0x1001
#define STACK_SIZE (1024 * 1024)

int validate_memory(void *arg)
{
    if  (*(int *)arg != VALUE) {
        printf("memory is corrupted\n");
        return -1;
    }
    printf("memory: %x\n", *(int *)arg);
    return 0;
}

int main() {
    int pid = 0;
    char *stack;
    int *buffer;;
     int flags = CLONE_VFORK;
    stack = malloc(STACK_SIZE + 4096);
    if (stack == NULL) {
        perror("malloc");
        exit(EXIT_FAILURE);
    }
    buffer = (int *)(stack + STACK_SIZE);
    buffer[0] = VALUE;

    kill(getpid(), 19);
    if (clone(validate_memory, buffer, flags, (void *)buffer) == -1) {
        perror("clone");
        free(stack);
        exit(EXIT_FAILURE);
    }
    validate_memory(buffer);

    return 0;
}

• steps

  gcc -o clone clone.c 
  setsid ./clone
  criu dump --shell-job  -vvvv --tree `pidof ./clone` -o dumo.log  --images-dir ./ckp 
  criu lazy-pages -vvvv -o lp.log  --auto-dedup --images-dir ckp/ &
  criu  restore -vvvv -o restore.log  --images-dir  ./ckp/ --lazy-pages -j --no-background-fetch  &
  kill -s SIGCONT `ps aux | grep '\[clone\]' | awk '{print $2}'`
 

[duguhaotian]

can you show some error message of this issuse?

[liusdu]

@rst0git @rppt I found it very difficult to disable background fetch. I cannot reliably reproduce this issue in the tests because the memory may be fetched by the background process before deduplication. However, I can reliably reproduce this issue with a large-memory process, which is not suitable for testing. Could you please consider merging this patch without the test?

[liusdu]

can you show some error message of this issuse?

Please refer to the above testcase.

[rst0git]

LGTM

@liusdu Thank you for working on this!