[PATCH] mm: memory: fix /proc/meminfo reporting for MLOCK_ONFAULT

[Lucian Adrian Grijincu]

As pages are faulted in MLOCK_ONFAULT correctly updates
/proc/self/smaps, but doesn't update /proc/meminfo's Mlocked field.

- Before this /proc/meminfo fields didn't change as pages were faulted in:

```
= Start =
/proc/meminfo
Unevictable:       10128 kB
Mlocked:           10132 kB
= Creating testfile =

= after mlock2(MLOCK_ONFAULT) =
/proc/meminfo
Unevictable:       10128 kB
Mlocked:           10132 kB
/proc/self/smaps
7f8714000000-7f8754000000 rw-s 00000000 08:04 50857050   /root/testfile
Locked:                0 kB

= after reading half of the file =
/proc/meminfo
Unevictable:       10128 kB
Mlocked:           10132 kB
/proc/self/smaps
7f8714000000-7f8754000000 rw-s 00000000 08:04 50857050   /root/testfile
Locked:           524288 kB

= after reading the entire the file =
/proc/meminfo
Unevictable:       10128 kB
Mlocked:           10132 kB
/proc/self/smaps
7f8714000000-7f8754000000 rw-s 00000000 08:04 50857050   /root/testfile
Locked:          1048576 kB

= after munmap =
/proc/meminfo
Unevictable:       10128 kB
Mlocked:           10132 kB
/proc/self/smaps
```

- After: /proc/meminfo fields are properly updated as pages are touched:

```
= Start =
/proc/meminfo
Unevictable:          60 kB
Mlocked:              60 kB
= Creating testfile =

= after mlock2(MLOCK_ONFAULT) =
/proc/meminfo
Unevictable:          60 kB
Mlocked:              60 kB
/proc/self/smaps
7f2b9c600000-7f2bdc600000 rw-s 00000000 08:04 63045798   /root/testfile
Locked:                0 kB

= after reading half of the file =
/proc/meminfo
Unevictable:      524220 kB
Mlocked:          524220 kB
/proc/self/smaps
7f2b9c600000-7f2bdc600000 rw-s 00000000 08:04 63045798   /root/testfile
Locked:           524288 kB

= after reading the entire the file =
/proc/meminfo
Unevictable:     1048496 kB
Mlocked:         1048508 kB
/proc/self/smaps
7f2b9c600000-7f2bdc600000 rw-s 00000000 08:04 63045798   /root/testfile
Locked:          1048576 kB

= after munmap =
/proc/meminfo
Unevictable:         176 kB
Mlocked:              60 kB
/proc/self/smaps
```

Repro code.
---

int mlock2wrap(const void* addr, size_t len, int flags) {
  return syscall(SYS_mlock2, addr, len, flags);
}

void smaps() {
  char smapscmd[1000];
  snprintf(
      smapscmd,
      sizeof(smapscmd) - 1,
      "grep testfile -A 20 /proc/%d/smaps | grep -E '(testfile|Locked)'",
      getpid());
  printf("/proc/self/smaps\n");
  fflush(stdout);
  system(smapscmd);
}

void meminfo() {
  const char* meminfocmd = "grep -E '(Mlocked|Unevictable)' /proc/meminfo";
  printf("/proc/meminfo\n");
  fflush(stdout);
  system(meminfocmd);
}

  {                                                 \
    int rc = (call);                                \
    if (rc != 0) {                                  \
      printf("error %d %s\n", rc, strerror(errno)); \
      exit(1);                                      \
    }                                               \
  }
int main(int argc, char* argv[]) {
  printf("= Start =\n");
  meminfo();

  printf("= Creating testfile =\n");
  size_t size = 1 << 30; // 1 GiB
  int fd = open("testfile", O_CREAT | O_RDWR, 0666);
  {
    void* buf = malloc(size);
    write(fd, buf, size);
    free(buf);
  }
  int ret = 0;
  void* addr = NULL;
  addr = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

  if (argc > 1) {
    PCHECK(mlock2wrap(addr, size, MLOCK_ONFAULT));
    printf("= after mlock2(MLOCK_ONFAULT) =\n");
    meminfo();
    smaps();

    for (size_t i = 0; i < size / 2; i += 4096) {
      ret += ((char*)addr)[i];
    }
    printf("= after reading half of the file =\n");
    meminfo();
    smaps();

    for (size_t i = 0; i < size; i += 4096) {
      ret += ((char*)addr)[i];
    }
    printf("= after reading the entire the file =\n");
    meminfo();
    smaps();

  } else {
    PCHECK(mlock(addr, size));
    printf("= after mlock =\n");
    meminfo();
    smaps();
  }

  PCHECK(munmap(addr, size));
  printf("= after munmap =\n");
  meminfo();
  smaps();

  return ret;
}

---

Signed-off-by: Lucian Adrian Grijincu <lucian@xxxxxx>
---
 mm/memory.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index e0c232fe81d9..7e8dc3ed4e89 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3311,6 +3311,9 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
 	} else {
 		inc_mm_counter_fast(vma->vm_mm, mm_counter_file(page));
 		page_add_file_rmap(page, false);
+		if ((vma->vm_flags & (VM_LOCKED | VM_SPECIAL)) == VM_LOCKED &&
+				!PageTransCompound(page))
+			mlock_vma_page(page);
 	}
 	set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
 
-- 
2.17.1