Re: [PATCH] mm/hugetlb.c: fix unnecessary address expansion of pmd sharing

[Peter Xu <peterx@redhat.com>]

On Tue, Dec 29, 2020 at 12:21:25PM +0800, Li Xinhai wrote:
> The current code would unnecessarily expand the address range. Consider
> one example, (start, end) = (1G-2M, 3G+2M), and  (vm_start, vm_end) =
> (1G-4M, 3G+4M), the expected adjustment should be keep (1G-2M, 3G+2M)
> without expand. But the current result will be (1G-4M, 3G+4M). Actually,
> the range (1G-4M, 1G) and (3G, 3G+4M) would never been involved in pmd
> sharing.
> 
> After this patch, if pud aligned *start across vm_start, then we know the
> *start and vm_start are in same pud_index, and vm_start is not pud
> aligned, so don't adjust *start. Same logic applied to *end.
> 
> Fixes: commit 75802ca66354 ("mm/hugetlb: fix calculation of adjust_range_if_pmd_sharing_possible")
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Peter Xu <peterx@redhat.com>
> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
> ---
>  mm/hugetlb.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index cbf32d2824fd..d1e9ea55b7e6 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5249,11 +5249,16 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
>  	a_end = ALIGN(*end, PUD_SIZE);
>  
>  	/*
> -	 * Intersect the range with the vma range, since pmd sharing won't be
> -	 * across vma after all
> +	 * If the PUD aligned address across vma range, then it means the
> +	 * vm_start/vm_end is not PUD aligned. In that case, we must don't
> +	 * adjust range because pmd sharing is not possbile at the start and/or
> +	 * end part of vma.
>  	 */
> -	*start = max(vma->vm_start, a_start);
> -	*end = min(vma->vm_end, a_end);
> +	if (a_start >= vma->vm_start)
> +		*start = a_start;
> +
> +	if (a_end <= vma->vm_end)
> +		*end = a_end;
>  }

Looks correct, thanks.

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu