Re: [PATCH] mm/hugetlb: correct demote page offset logic

[Anshuman Khandual]

On 9/15/22 00:39, Doug Berger wrote:
> With gigantic pages it may not be true that struct page structures
> are contiguous across the entire gigantic page. The nth_page macro
> is used here in place of direct pointer arithmetic to correct for
> this.
> 
> Fixes: 8531fc6f52f5 ("hugetlb: add hugetlb demote page support")
> Signed-off-by: Doug Berger <opendmb@xxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

Reviewed-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>

> ---
>  mm/hugetlb.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e070b8593b37..0bdfc7e1c933 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3420,6 +3420,7 @@ static int demote_free_huge_page(struct hstate *h, struct page *page)
>  {
>  	int i, nid = page_to_nid(page);
>  	struct hstate *target_hstate;
> +	struct page *subpage;
>  	int rc = 0;
>  
>  	target_hstate = size_to_hstate(PAGE_SIZE << h->demote_order);
> @@ -3453,15 +3454,16 @@ static int demote_free_huge_page(struct hstate *h, struct page *page)
>  	mutex_lock(&target_hstate->resize_lock);
>  	for (i = 0; i < pages_per_huge_page(h);
>  				i += pages_per_huge_page(target_hstate)) {
> +		subpage = nth_page(page, i);
>  		if (hstate_is_gigantic(target_hstate))
> -			prep_compound_gigantic_page_for_demote(page + i,
> +			prep_compound_gigantic_page_for_demote(subpage,
>  							target_hstate->order);
>  		else
> -			prep_compound_page(page + i, target_hstate->order);
> -		set_page_private(page + i, 0);
> -		set_page_refcounted(page + i);
> -		prep_new_huge_page(target_hstate, page + i, nid);
> -		put_page(page + i);
> +			prep_compound_page(subpage, target_hstate->order);
> +		set_page_private(subpage, 0);
> +		set_page_refcounted(subpage);
> +		prep_new_huge_page(target_hstate, subpage, nid);
> +		put_page(subpage);
>  	}
>  	mutex_unlock(&target_hstate->resize_lock);
>