Re: [PATCH v2 1/2] mm/hugetlb: fix hugetlb not supporting softdirty tracking

[Mike Kravetz]

On 08/11/22 12:34, David Hildenbrand wrote:
> Staring at hugetlb_wp(), one might wonder where all the logic for shared
> mappings is when stumbling over a write-protected page in a shared
> mapping. In fact, there is none, and so far we thought we could get
> away with that because e.g., mprotect() should always do the right thing
> and map all pages directly writable.
> 
> Looks like we were wrong:
> 
> --------------------------------------------------------------------------
>  #include <stdio.h>
>  #include <stdlib.h>
>  #include <string.h>
>  #include <fcntl.h>
>  #include <unistd.h>
>  #include <errno.h>
>  #include <sys/mman.h>
> 
>  #define HUGETLB_SIZE (2 * 1024 * 1024u)
> 
>  static void clear_softdirty(void)
>  {
>          int fd = open("/proc/self/clear_refs", O_WRONLY);
>          const char *ctrl = "4";
>          int ret;
> 
>          if (fd < 0) {
>                  fprintf(stderr, "open(clear_refs) failed\n");
>                  exit(1);
>          }
>          ret = write(fd, ctrl, strlen(ctrl));
>          if (ret != strlen(ctrl)) {
>                  fprintf(stderr, "write(clear_refs) failed\n");
>                  exit(1);
>          }
>          close(fd);
>  }
> 
>  int main(int argc, char **argv)
>  {
>          char *map;
>          int fd;
> 
>          fd = open("/dev/hugepages/tmp", O_RDWR | O_CREAT);
>          if (!fd) {
>                  fprintf(stderr, "open() failed\n");
>                  return -errno;
>          }
>          if (ftruncate(fd, HUGETLB_SIZE)) {
>                  fprintf(stderr, "ftruncate() failed\n");
>                  return -errno;
>          }
> 
>          map = mmap(NULL, HUGETLB_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
>          if (map == MAP_FAILED) {
>                  fprintf(stderr, "mmap() failed\n");
>                  return -errno;
>          }
> 
>          *map = 0;
> 
>          if (mprotect(map, HUGETLB_SIZE, PROT_READ)) {
>                  fprintf(stderr, "mmprotect() failed\n");
>                  return -errno;
>          }
> 
>          clear_softdirty();
> 
>          if (mprotect(map, HUGETLB_SIZE, PROT_READ|PROT_WRITE)) {
>                  fprintf(stderr, "mmprotect() failed\n");
>                  return -errno;
>          }
> 
>          *map = 0;
> 
>          return 0;
>  }
> --------------------------------------------------------------------------
> 
> Above test fails with SIGBUS when there is only a single free hugetlb page.
>  # echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
>  # ./test
>  Bus error (core dumped)
> 
> And worse, with sufficient free hugetlb pages it will map an anonymous page
> into a shared mapping, for example, messing up accounting during unmap
> and breaking MAP_SHARED semantics:
>  # echo 2 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
>  # ./test
>  # cat /proc/meminfo | grep HugePages_
>  HugePages_Total:       2
>  HugePages_Free:        1
>  HugePages_Rsvd:    18446744073709551615
>  HugePages_Surp:        0
> 
> Reason in this particular case is that vma_wants_writenotify() will
> return "true", removing VM_SHARED in vma_set_page_prot() to map pages
> write-protected. Let's teach vma_wants_writenotify() that hugetlb does not
> support softdirty tracking.
> 
> Fixes: 64e455079e1b ("mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared")
> Cc: <stable@xxxxxxxxxxxxxxx> # v3.18+
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  mm/mmap.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)

Thanks,

Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
-- 
Mike Kravetz

> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c035020d0c89..9d780f415be3 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1646,8 +1646,11 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
>  	    pgprot_val(vm_pgprot_modify(vm_page_prot, vm_flags)))
>  		return 0;
>  
> -	/* Do we need to track softdirty? */
> -	if (vma_soft_dirty_enabled(vma))
> +	/*
> +	 * Do we need to track softdirty? hugetlb does not support softdirty
> +	 * tracking yet.
> +	 */
> +	if (vma_soft_dirty_enabled(vma) && !is_vm_hugetlb_page(vma))
>  		return 1;
>  
>  	/* Specialty mapping? */
> -- 
> 2.35.3
> 
>