Re: [PATCH] mm/hugetlb: fix null ptr defer in hugetlb_vma_lock_write

Mike Kravetz
2023-11-03T02:37:57Z

On 11/02/23 19:24, Mike Kravetz wrote:
>
> In the specific case causing the null-ptr-deref, the resv_map pointer
> (vm_private_data) is NULL.

Hi Rik,

In commit bf4916922c60 hugetlbfs: extend hugetlb_vma_lock to private VMAs,
it correctly says:

Extend the locking scheme used to protect shared hugetlb mappings from
truncate vs page fault races, in order to protect private hugetlb mappings
(with resv_map) against MADV_DONTNEED.

That qualification '(with resv_map)' caught my attention originally, and
I thought about it again while looking into this. We now cover the common
cases, but there are still quite a few cases where resv_map is NULL for
private mappings. In such cases, the race between MADV_DONTNEED and page
fault still exists. Is that a concern?

With a bit more work we 'could' make sure every hugetlb vma has a lock
to participate in this scheme.

Any thhoughts?
--
Mike Kravetz