Re: [PATCH v4 5/5] hugetlb: add hugetlb demote page support

[Oscar Salvador <osalvador@suse.de>]

On Fri, Oct 08, 2021 at 01:57:48PM -0700, Mike Kravetz wrote:
> From 25e4dac59f4d203f3a7e86d3591d70c1e956d11c Mon Sep 17 00:00:00 2001
> From: Mike Kravetz <mike.kravetz@oracle.com>
> Date: Fri, 8 Oct 2021 13:21:21 -0700
> Subject: [PATCH v4 5/5] hugetlb: add hugetlb demote page support
> 
> Demote page functionality will split a huge page into a number of huge
> pages of a smaller size.  For example, on x86 a 1GB huge page can be
> demoted into 512 2M huge pages.  Demotion is done 'in place' by simply
> splitting the huge page.
> 
> Added '*_for_demote' wrappers for remove_hugetlb_page,
> destroy_compound_hugetlb_page and prep_compound_gigantic_page for use
> by demote code.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Reviewed-by: Oscar Salvador <osalvador@suse.de>

> ---
>  mm/hugetlb.c | 100 ++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 92 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 794e0c4c1b3c..e1883510309a 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1270,7 +1270,7 @@ static int hstate_next_node_to_free(struct hstate *h, nodemask_t *nodes_allowed)
>  		((node = hstate_next_node_to_free(hs, mask)) || 1);	\
>  		nr_nodes--)
>  
> -#ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE
> +/* used to demote non-gigantic_huge pages as well */
>  static void __destroy_compound_gigantic_page(struct page *page,
>  					unsigned int order, bool demote)
>  {
> @@ -1293,6 +1293,13 @@ static void __destroy_compound_gigantic_page(struct page *page,
>  	__ClearPageHead(page);
>  }
>  
> +static void destroy_compound_hugetlb_page_for_demote(struct page *page,
> +					unsigned int order)
> +{
> +	__destroy_compound_gigantic_page(page, order, true);
> +}
> +
> +#ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE
>  static void destroy_compound_gigantic_page(struct page *page,
>  					unsigned int order)
>  {
> @@ -1438,6 +1445,12 @@ static void remove_hugetlb_page(struct hstate *h, struct page *page,
>  	__remove_hugetlb_page(h, page, adjust_surplus, false);
>  }
>  
> +static void remove_hugetlb_page_for_demote(struct hstate *h, struct page *page,
> +							bool adjust_surplus)
> +{
> +	__remove_hugetlb_page(h, page, adjust_surplus, true);
> +}
> +
>  static void add_hugetlb_page(struct hstate *h, struct page *page,
>  			     bool adjust_surplus)
>  {
> @@ -1779,6 +1792,12 @@ static bool prep_compound_gigantic_page(struct page *page, unsigned int order)
>  	return __prep_compound_gigantic_page(page, order, false);
>  }
>  
> +static bool prep_compound_gigantic_page_for_demote(struct page *page,
> +							unsigned int order)
> +{
> +	return __prep_compound_gigantic_page(page, order, true);
> +}
> +
>  /*
>   * PageHuge() only returns true for hugetlbfs pages, but not for normal or
>   * transparent huge pages.  See the PageTransHuge() documentation for more
> @@ -3304,9 +3323,72 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>  	return 0;
>  }
>  
> +static int demote_free_huge_page(struct hstate *h, struct page *page)
> +{
> +	int i, nid = page_to_nid(page);
> +	struct hstate *target_hstate;
> +	int rc = 0;
> +
> +	target_hstate = size_to_hstate(PAGE_SIZE << h->demote_order);
> +
> +	remove_hugetlb_page_for_demote(h, page, false);
> +	spin_unlock_irq(&hugetlb_lock);
> +
> +	rc = alloc_huge_page_vmemmap(h, page);
> +	if (rc) {
> +		/* Allocation of vmemmmap failed, we can not demote page */
> +		spin_lock_irq(&hugetlb_lock);
> +		set_page_refcounted(page);
> +		add_hugetlb_page(h, page, false);
> +		return rc;
> +	}
> +
> +	/*
> +	 * Use destroy_compound_hugetlb_page_for_demote for all huge page
> +	 * sizes as it will not ref count pages.
> +	 */
> +	destroy_compound_hugetlb_page_for_demote(page, huge_page_order(h));
> +
> +	/*
> +	 * Taking target hstate mutex synchronizes with set_max_huge_pages.
> +	 * Without the mutex, pages added to target hstate could be marked
> +	 * as surplus.
> +	 *
> +	 * Note that we already hold h->resize_lock.  To prevent deadlock,
> +	 * use the convention of always taking larger size hstate mutex first.
> +	 */
> +	mutex_lock(&target_hstate->resize_lock);
> +	for (i = 0; i < pages_per_huge_page(h);
> +				i += pages_per_huge_page(target_hstate)) {
> +		if (hstate_is_gigantic(target_hstate))
> +			prep_compound_gigantic_page_for_demote(page + i,
> +							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);
> +	}
> +	mutex_unlock(&target_hstate->resize_lock);
> +
> +	spin_lock_irq(&hugetlb_lock);
> +
> +	/*
> +	 * Not absolutely necessary, but for consistency update max_huge_pages
> +	 * based on pool changes for the demoted page.
> +	 */
> +	h->max_huge_pages--;
> +	target_hstate->max_huge_pages += pages_per_huge_page(h);
> +
> +	return rc;
> +}
> +
>  static int demote_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
>  	__must_hold(&hugetlb_lock)
>  {
> +	int nr_nodes, node;
> +	struct page *page;
>  	int rc = 0;
>  
>  	lockdep_assert_held(&hugetlb_lock);
> @@ -3317,9 +3399,15 @@ static int demote_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
>  		return -EINVAL;		/* internal error */
>  	}
>  
> -	/*
> -	 * TODO - demote fucntionality will be added in subsequent patch
> -	 */
> +	for_each_node_mask_to_free(h, nr_nodes, node, nodes_allowed) {
> +		if (!list_empty(&h->hugepage_freelists[node])) {
> +			page = list_entry(h->hugepage_freelists[node].next,
> +					struct page, lru);
> +			rc = demote_free_huge_page(h, page);
> +			break;
> +		}
> +	}
> +
>  	return rc;
>  }
>  
> @@ -3554,10 +3642,6 @@ static ssize_t demote_store(struct kobject *kobj,
>  		/*
>  		 * Check for available pages to demote each time thorough the
>  		 * loop as demote_pool_huge_page will drop hugetlb_lock.
> -		 *
> -		 * NOTE: demote_pool_huge_page does not yet drop hugetlb_lock
> -		 * but will when full demote functionality is added in a later
> -		 * patch.
>  		 */
>  		if (nid != NUMA_NO_NODE)
>  			nr_available = h->free_huge_pages_node[nid];
> -- 
> 2.31.1
> 
> 

-- 
Oscar Salvador
SUSE Labs