[BUG] Re: [PATCH v3 10/13] mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()

[Jann Horn <jannh@xxxxxxxxxx>]

On Wed, Jul 12, 2023 at 6:42 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp().
> It does need mmap_read_lock(), but it does not need mmap_write_lock(),
> nor vma_start_write() nor i_mmap lock nor anon_vma lock.  All racing
> paths are relying on pte_offset_map_lock() and pmd_lock(), so use those.

We can still have a racing userfaultfd operation at the "/* step 4:
remove page table */" point that installs a new PTE before the page
table is removed.

To reproduce, patch a delay into the kernel like this:


diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 9a6e0d507759..27cc8dfbf3a7 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -20,6 +20,7 @@
 #include <linux/swapops.h>
 #include <linux/shmem_fs.h>
 #include <linux/ksm.h>
+#include <linux/delay.h>

 #include <asm/tlb.h>
 #include <asm/pgalloc.h>
@@ -1617,6 +1618,11 @@ int collapse_pte_mapped_thp(struct mm_struct
*mm, unsigned long addr,
        }

        /* step 4: remove page table */
+       if (strcmp(current->comm, "DELAYME") == 0) {
+               pr_warn("%s: BEGIN DELAY INJECTION\n", __func__);
+               mdelay(5000);
+               pr_warn("%s: END DELAY INJECTION\n", __func__);
+       }

        /* Huge page lock is still held, so page table must remain empty */
        pml = pmd_lock(mm, pmd);


And then run the attached reproducer against mm/mm-everything. You
should get this in dmesg:

[  206.578096] BUG: Bad rss-counter state mm:000000000942ebea
type:MM_ANONPAGES val:1

// compile with "gcc -o khugepaged-vs-uffd khugepaged-vs-uffd.c -pthread"
#define _GNU_SOURCE
#include <pthread.h>
#include <err.h>
#include <sched.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <signal.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <sys/stat.h>
#include <sys/prctl.h>
#include <sys/mount.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
#include <linux/userfaultfd.h>

#ifndef MADV_COLLAPSE
#define MADV_COLLAPSE 25
#endif

#ifndef UFFD_USER_MODE_ONLY
#define UFFD_USER_MODE_ONLY 1
#endif

#define SYSCHK(x) ({          \
  typeof(x) __res = (x);      \
  if (__res == (typeof(x))-1) \
    err(1, "SYSCHK(" #x ")"); \
  __res;                      \
})

static void write_file(char *name, char *buf) {
  int fd = SYSCHK(open(name, O_WRONLY));
  if (write(fd, buf, strlen(buf)) != strlen(buf))
    err(1, "write %s", name);
  close(fd);
}

static void write_map(char *name, int outer_id) {
  char buf[100];
  sprintf(buf, "0 %d 1", outer_id);
  write_file(name, buf);
}

static void *thread_fn(void *dummy) {
  system("head -n50 /proc/$PPID/smaps;echo;echo");
  SYSCHK(prctl(PR_SET_NAME, "DELAYME"));
  SYSCHK(madvise((void*)0x200000UL, 0x200000, MADV_COLLAPSE));
  SYSCHK(prctl(PR_SET_NAME, "thread"));
  system("head -n50 /proc/$PPID/smaps");
  return NULL;
}

int main(void) {
  int outer_uid = getuid();
  int outer_gid = getgid();
  SYSCHK(unshare(CLONE_NEWNS|CLONE_NEWUSER));
  SYSCHK(mount(NULL, "/", NULL, MS_PRIVATE|MS_REC, NULL));
  write_file("/proc/self/setgroups", "deny");
  write_map("/proc/self/uid_map", outer_uid);
  write_map("/proc/self/gid_map", outer_gid);

  SYSCHK(mount("none", "/tmp", "tmpfs", MS_NOSUID|MS_NODEV, "huge=always"));
  int fd = SYSCHK(open("/tmp/a", O_RDWR|O_CREAT, 0600));
  SYSCHK(ftruncate(fd, 0x200000));
  void *ptr = SYSCHK(mmap((void*)0x200000UL, 0x100000, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED_NOREPLACE, fd, 0));
  *(volatile char *)ptr;
  SYSCHK(mmap((void*)0x300000UL, 0x100000, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED_NOREPLACE, fd, 0x100000));
  for (int i=0; i<512; i++)
    *(volatile char *)(0x200000UL + 0x1000 * i);

  int uffd = SYSCHK(syscall(__NR_userfaultfd, UFFD_USER_MODE_ONLY));

  struct uffdio_api api = { .api = UFFD_API, .features = 0 };
  SYSCHK(ioctl(uffd, UFFDIO_API, &api));

  struct uffdio_register reg = {
    .range = { .start = 0x200000, .len = 0x200000 },
    .mode = UFFDIO_REGISTER_MODE_MISSING
  };
  SYSCHK(ioctl(uffd, UFFDIO_REGISTER, &reg));

  pthread_t thread;
  if (pthread_create(&thread, NULL, thread_fn, NULL))
    errx(1, "pthread_create");

  sleep(1);

  unsigned char dummy_page[0x1000] = {1};
  struct uffdio_copy copy = {
    .dst = 0x201000,
    .src = (unsigned long)dummy_page,
    .len = 0x1000,
    .mode = 0
  };
  SYSCHK(ioctl(uffd, UFFDIO_COPY, &copy));

  if (pthread_join(thread, NULL))
    errx(1, "pthread_join");

  //system("cat /proc/$PPID/smaps");
}