Re: [PATCH 1/3] hugetlb: fix vma lock handling during split vma and range unmapping

[Miaohe Lin <linmiaohe@huawei.com>]

Sorry for late respond. It's a really busy week. :)

On 2022/10/5 9:17, Mike Kravetz wrote:
> The hugetlb vma lock hangs off the vm_private_data field and is specific
> to the vma.  When vm_area_dup() is called as part of vma splitting,  the

Oh, I checked vm_area_dup() from callsite of copy_vma and dup_mmap but split_vma
is missed... And yes, vma splitting can occur but vma merging won't for hugetlb
vma. Thanks for catching this, Mike.

> vma lock pointer is copied to the new vma.  This will result in issues
> such as double freeing of the structure.  Update the hugetlb open vm_ops
> to allocate a new vma lock for the new vma.
> 
> The routine __unmap_hugepage_range_final unconditionally unset
> VM_MAYSHARE to prevent subsequent pmd sharing.  hugetlb_vma_lock_free
> attempted to anticipate this by checking both VM_MAYSHARE and VM_SHARED.
> However, if only VM_MAYSHARE was set we would miss the free.  With the
> introduction of the vma lock, a vma can not participate in pmd sharing
> if vm_private_data is NULL.  Instead of clearing VM_MAYSHARE in
> __unmap_hugepage_range_final, free the vma lock to prevent sharing.  Also,
> update the sharing code to make sure vma lock is indeed a condition for
> pmd sharing.  hugetlb_vma_lock_free can then key off VM_MAYSHARE and not
> miss any vmas.
> 
> Fixes: "hugetlb: add vma based lock for pmd sharing"
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>  mm/hugetlb.c | 43 +++++++++++++++++++++++++++----------------
>  mm/memory.c  |  4 ----
>  2 files changed, 27 insertions(+), 20 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4443e87e814b..0129d371800c 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4612,7 +4612,14 @@ static void hugetlb_vm_op_open(struct vm_area_struct *vma)
>  		kref_get(&resv->refs);
>  	}
>  
> -	hugetlb_vma_lock_alloc(vma);
> +	/*
> +	 * vma_lock structure for sharable mappings is vma specific.
> +	 * Clear old pointer (if copied via vm_area_dup) and create new.
> +	 */
> +	if (vma->vm_flags & VM_MAYSHARE) {
> +		vma->vm_private_data = NULL;
> +		hugetlb_vma_lock_alloc(vma);
> +	}

IMHO this would lead to memoryleak. Think about the below move_vma() flow:
move_vma
  copy_vma
    new_vma = vm_area_dup(vma);
    new_vma->vm_ops->open(new_vma); --> new_vma has its own vma lock.
  is_vm_hugetlb_page(vma)
    clear_vma_resv_huge_pages
      hugetlb_dup_vma_private --> vma->vm_private_data is set to NULL
      				  without put ref. So vma lock is *leaked*?

Other part looks good to me. Thanks for your work.

Thanks,
Miaohe Lin