Re: [v2 1/6] mm: hugetlb: Skip prep of tail pages when HVO is enabled

[Muchun Song <muchun.song@linux.dev>]

On 2023/7/30 23:16, Usama Arif wrote:
> When vmemmap is optimizable, it will free all the duplicated tail
> pages in hugetlb_vmemmap_optimize while preparing the new hugepage.
> Hence, there is no need to prepare them.
>
> For 1G x86 hugepages, it avoids preparing
> 262144 - 64 = 262080 struct pages per hugepage.
>
> The indirection of using __prep_compound_gigantic_folio is also removed,
> as it just creates extra functions to indicate demote which can be done
> with the argument.
>
> Signed-off-by: Usama Arif <usama.arif@bytedance.com>
> ---
>   mm/hugetlb.c         | 32 ++++++++++++++------------------
>   mm/hugetlb_vmemmap.c |  2 +-
>   mm/hugetlb_vmemmap.h | 15 +++++++++++----
>   3 files changed, 26 insertions(+), 23 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 64a3239b6407..541c07b6d60f 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1942,14 +1942,23 @@ static void prep_new_hugetlb_folio(struct hstate *h, struct folio *folio, int ni
>   	spin_unlock_irq(&hugetlb_lock);
>   }
>   
> -static bool __prep_compound_gigantic_folio(struct folio *folio,
> -					unsigned int order, bool demote)
> +static bool prep_compound_gigantic_folio(struct folio *folio, struct hstate *h, bool demote)
>   {
>   	int i, j;
> +	int order = huge_page_order(h);
>   	int nr_pages = 1 << order;
>   	struct page *p;
>   
>   	__folio_clear_reserved(folio);
> +
> +	/*
> +	 * No need to prep pages that will be freed later by hugetlb_vmemmap_optimize.
> +	 * Hence, reduce nr_pages to the pages that will be kept.
> +	 */
> +	if (IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP) &&
> +			vmemmap_should_optimize(h, &folio->page))
> +		nr_pages = HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page);

We need to initialize the refcount to zero of tail pages (see the big
comment below in this function), given a situation that someone (maybe
GUP) could get a ref on the tail pages when the vmemmap is optimizing,
what prevent this from happening?

Thanks.

> +
>   	for (i = 0; i < nr_pages; i++) {
>   		p = folio_page(folio, i);
>   
> @@ -2019,18 +2028,6 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
>   	return false;
>   }
>   
> -static bool prep_compound_gigantic_folio(struct folio *folio,
> -							unsigned int order)
> -{
> -	return __prep_compound_gigantic_folio(folio, order, false);
> -}
> -
> -static bool prep_compound_gigantic_folio_for_demote(struct folio *folio,
> -							unsigned int order)
> -{
> -	return __prep_compound_gigantic_folio(folio, order, true);
> -}
> -
>   /*
>    * PageHuge() only returns true for hugetlbfs pages, but not for normal or
>    * transparent huge pages.  See the PageTransHuge() documentation for more
> @@ -2185,7 +2182,7 @@ static struct folio *alloc_fresh_hugetlb_folio(struct hstate *h,
>   	if (!folio)
>   		return NULL;
>   	if (hstate_is_gigantic(h)) {
> -		if (!prep_compound_gigantic_folio(folio, huge_page_order(h))) {
> +		if (!prep_compound_gigantic_folio(folio, h, false)) {
>   			/*
>   			 * Rare failure to convert pages to compound page.
>   			 * Free pages and try again - ONCE!
> @@ -3201,7 +3198,7 @@ static void __init gather_bootmem_prealloc(void)
>   
>   		VM_BUG_ON(!hstate_is_gigantic(h));
>   		WARN_ON(folio_ref_count(folio) != 1);
> -		if (prep_compound_gigantic_folio(folio, huge_page_order(h))) {
> +		if (prep_compound_gigantic_folio(folio, h, false)) {
>   			WARN_ON(folio_test_reserved(folio));
>   			prep_new_hugetlb_folio(h, folio, folio_nid(folio));
>   			free_huge_page(page); /* add to the hugepage allocator */
> @@ -3624,8 +3621,7 @@ static int demote_free_hugetlb_folio(struct hstate *h, struct folio *folio)
>   		subpage = folio_page(folio, i);
>   		inner_folio = page_folio(subpage);
>   		if (hstate_is_gigantic(target_hstate))
> -			prep_compound_gigantic_folio_for_demote(inner_folio,
> -							target_hstate->order);
> +			prep_compound_gigantic_folio(inner_folio, target_hstate, true);
>   		else
>   			prep_compound_page(subpage, target_hstate->order);
>   		folio_change_private(inner_folio, NULL);
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index c2007ef5e9b0..b721e87de2b3 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -486,7 +486,7 @@ int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head)
>   }
>   
>   /* Return true iff a HugeTLB whose vmemmap should and can be optimized. */
> -static bool vmemmap_should_optimize(const struct hstate *h, const struct page *head)
> +bool vmemmap_should_optimize(const struct hstate *h, const struct page *head)
>   {
>   	if (!READ_ONCE(vmemmap_optimize_enabled))
>   		return false;
> diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> index 25bd0e002431..3e7978a9af73 100644
> --- a/mm/hugetlb_vmemmap.h
> +++ b/mm/hugetlb_vmemmap.h
> @@ -10,16 +10,17 @@
>   #define _LINUX_HUGETLB_VMEMMAP_H
>   #include <linux/hugetlb.h>
>   
> -#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
> -int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head);
> -void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head);
> -
>   /*
>    * Reserve one vmemmap page, all vmemmap addresses are mapped to it. See
>    * Documentation/vm/vmemmap_dedup.rst.
>    */
>   #define HUGETLB_VMEMMAP_RESERVE_SIZE	PAGE_SIZE
>   
> +#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
> +int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head);
> +void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head);
> +bool vmemmap_should_optimize(const struct hstate *h, const struct page *head);
> +
>   static inline unsigned int hugetlb_vmemmap_size(const struct hstate *h)
>   {
>   	return pages_per_huge_page(h) * sizeof(struct page);
> @@ -51,6 +52,12 @@ static inline unsigned int hugetlb_vmemmap_optimizable_size(const struct hstate
>   {
>   	return 0;
>   }
> +
> +static inline bool vmemmap_should_optimize(const struct hstate *h, const struct page *head)
> +{
> +	return false;
> +}
> +
>   #endif /* CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP */
>   
>   static inline bool hugetlb_vmemmap_optimizable(const struct hstate *h)