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

[Souptick Joarder]

On Fri, Sep 13, 2019 at 4:49 AM Lucian Adrian Grijincu <lucian@xxxxxx> wrote:
>
> 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))

Do we need to check against VM_SPECIAL ?

> +                       mlock_vma_page(page);
>         }
>         set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
>
> --
> 2.17.1
>
>