Re: [PATCH 16/18] mm, hugetlb: return a reserved page to a reserved pool if failed

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Mon, Jul 29, 2013 at 02:32:07PM +0900, Joonsoo Kim wrote:
> If we fail with a reserved page, just calling put_page() is not sufficient,
> because put_page() invoke free_huge_page() at last step and it doesn't
> know whether a page comes from a reserved pool or not. So it doesn't do
> anything related to reserved count. This makes reserve count lower
> than how we need, because reserve count already decrease in
> dequeue_huge_page_vma(). This patch fix this situation.

I think we could use a page flag (for example PG_reserve) on a hugepage
in order to record that the hugepage comes from the reserved pool.
Furthermore, the reserve flag would be set when dequeueing a free hugepage,
and cleared when hugepage_fault returns, whether it fails or not.
I think it's simpler than put_page variant approach, but doesn't it work
to solve your problem?

Thanks,
Naoya Horiguchi

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index bb8a45f..6a9ec69 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -649,6 +649,34 @@ struct hstate *size_to_hstate(unsigned long size)
>  	return NULL;
>  }
>  
> +static void put_huge_page(struct page *page, int use_reserve)
> +{
> +	struct hstate *h = page_hstate(page);
> +	struct hugepage_subpool *spool =
> +		(struct hugepage_subpool *)page_private(page);
> +
> +	if (!use_reserve) {
> +		put_page(page);
> +		return;
> +	}
> +
> +	if (!put_page_testzero(page))
> +		return;
> +
> +	set_page_private(page, 0);
> +	page->mapping = NULL;
> +	BUG_ON(page_count(page));
> +	BUG_ON(page_mapcount(page));
> +
> +	spin_lock(&hugetlb_lock);
> +	hugetlb_cgroup_uncharge_page(hstate_index(h),
> +				     pages_per_huge_page(h), page);
> +	enqueue_huge_page(h, page);
> +	h->resv_huge_pages++;
> +	spin_unlock(&hugetlb_lock);
> +	hugepage_subpool_put_pages(spool, 1);
> +}
> +
>  static void free_huge_page(struct page *page)
>  {
>  	/*
> @@ -2625,7 +2653,7 @@ retry_avoidcopy:
>  	spin_unlock(&mm->page_table_lock);
>  	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
>  
> -	page_cache_release(new_page);
> +	put_huge_page(new_page, use_reserve);
>  out_old_page:
>  	page_cache_release(old_page);
>  out_lock:
> @@ -2725,7 +2753,7 @@ retry:
>  
>  			err = add_to_page_cache(page, mapping, idx, GFP_KERNEL);
>  			if (err) {
> -				put_page(page);
> +				put_huge_page(page, use_reserve);
>  				if (err == -EEXIST)
>  					goto retry;
>  				goto out;
> @@ -2798,7 +2826,7 @@ backout:
>  	spin_unlock(&mm->page_table_lock);
>  backout_unlocked:
>  	unlock_page(page);
> -	put_page(page);
> +	put_huge_page(page, use_reserve);
>  	goto out;
>  }
>  
> -- 
> 1.7.9.5
> 
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