Re: [PATCH 2/2] mm: hugetlbfs: Close race during teardown ofhugetlbfs shared page tables

[Michal Hocko]

Just a nit

On Fri 27-07-12 11:46:05, Mel Gorman wrote:
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index fd1d530..8c6e5a5 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2429,6 +2429,25 @@ again:
>  	tlb_end_vma(tlb, vma);
>  }
>

I would welcome a comment here. Something like:
/*
 * Called when the VMA is on the way out and page tables will be freed
 * by free_pagetables.
 * i_mmap_mutex has to be held when calling this function
 */

> +void __unmap_hugepage_range_final(struct mmu_gather *tlb,
> +			  struct vm_area_struct *vma, unsigned long start,
> +			  unsigned long end, struct page *ref_page)
> +{
> +	__unmap_hugepage_range(tlb, vma, start, end, ref_page);
> +
> +	/*
> +	 * Clear this flag so that x86's huge_pmd_share page_table_shareable
> +	 * test will fail on a vma being torn down, and not grab a page table
> +	 * on its way out.  We're lucky that the flag has such an appropriate
> +	 * name, and can in fact be safely cleared here. We could clear it
> +	 * before the __unmap_hugepage_range above, but all that's necessary
> +	 * is to clear it before releasing the i_mmap_mutex. This works
> +	 * because in the context this is called, the VMA is about to be
> +	 * destroyed and the i_mmap_mutex is held.
> +	 */
> +	vma->vm_flags &= ~VM_MAYSHARE;
> +}
> +

-- 
Michal Hocko
SUSE Labs
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