Re: [PATCH v2 07/13] mm: Remove HUGETLB_PAGE_DTOR

[Sidhartha Kumar <sidhartha.kumar@oracle.com>]

On 8/16/23 8:11 AM, Matthew Wilcox (Oracle) wrote:
> We can use a bit in page[1].flags to indicate that this folio belongs
> to hugetlb instead of using a value in page[1].dtors.  That lets
> folio_test_hugetlb() become an inline function like it should be.
> We can also get rid of NULL_COMPOUND_DTOR.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>   .../admin-guide/kdump/vmcoreinfo.rst          | 10 +---
>   include/linux/mm.h                            |  4 --
>   include/linux/page-flags.h                    | 43 ++++++++++++----
>   kernel/crash_core.c                           |  2 +-
>   mm/hugetlb.c                                  | 49 +++----------------
>   mm/page_alloc.c                               |  2 +-
>   6 files changed, 43 insertions(+), 67 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kdump/vmcoreinfo.rst b/Documentation/admin-guide/kdump/vmcoreinfo.rst
> index c18d94fa6470..baa1c355741d 100644
> --- a/Documentation/admin-guide/kdump/vmcoreinfo.rst
> +++ b/Documentation/admin-guide/kdump/vmcoreinfo.rst
> @@ -325,8 +325,8 @@ NR_FREE_PAGES
>   On linux-2.6.21 or later, the number of free pages is in
>   vm_stat[NR_FREE_PAGES]. Used to get the number of free pages.
>   
> -PG_lru|PG_private|PG_swapcache|PG_swapbacked|PG_slab|PG_hwpoision|PG_head_mask
> -------------------------------------------------------------------------------
> +PG_lru|PG_private|PG_swapcache|PG_swapbacked|PG_slab|PG_hwpoision|PG_head_mask|PG_hugetlb
> +-----------------------------------------------------------------------------------------
>   
>   Page attributes. These flags are used to filter various unnecessary for
>   dumping pages.
> @@ -338,12 +338,6 @@ More page attributes. These flags are used to filter various unnecessary for
>   dumping pages.
>   
>   
> -HUGETLB_PAGE_DTOR
> ------------------
> -
> -The HUGETLB_PAGE_DTOR flag denotes hugetlbfs pages. Makedumpfile
> -excludes these pages.
> -
>   x86_64
>   ======
>   
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7b800d1298dc..642f5fe5860e 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1268,11 +1268,7 @@ void folio_copy(struct folio *dst, struct folio *src);
>   unsigned long nr_free_buffer_pages(void);
>   
>   enum compound_dtor_id {
> -	NULL_COMPOUND_DTOR,
>   	COMPOUND_PAGE_DTOR,
> -#ifdef CONFIG_HUGETLB_PAGE
> -	HUGETLB_PAGE_DTOR,
> -#endif
>   	TRANSHUGE_PAGE_DTOR,
>   	NR_COMPOUND_DTORS,
>   };
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 92a2063a0a23..aeecf0cf1456 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -171,15 +171,6 @@ enum pageflags {
>   	/* Remapped by swiotlb-xen. */
>   	PG_xen_remapped = PG_owner_priv_1,
>   
> -#ifdef CONFIG_MEMORY_FAILURE
> -	/*
> -	 * Compound pages. Stored in first tail page's flags.
> -	 * Indicates that at least one subpage is hwpoisoned in the
> -	 * THP.
> -	 */
> -	PG_has_hwpoisoned = PG_error,
> -#endif
> -
>   	/* non-lru isolated movable page */
>   	PG_isolated = PG_reclaim,
>   
> @@ -190,6 +181,15 @@ enum pageflags {
>   	/* For self-hosted memmap pages */
>   	PG_vmemmap_self_hosted = PG_owner_priv_1,
>   #endif
> +
> +	/*
> +	 * Flags only valid for compound pages.  Stored in first tail page's
> +	 * flags word.
> +	 */
> +
> +	/* At least one page in this folio has the hwpoison flag set */
> +	PG_has_hwpoisoned = PG_error,
> +	PG_hugetlb = PG_active,
>   };
>   
>   #define PAGEFLAGS_MASK		((1UL << NR_PAGEFLAGS) - 1)
> @@ -812,7 +812,23 @@ static inline void ClearPageCompound(struct page *page)
>   
>   #ifdef CONFIG_HUGETLB_PAGE
>   int PageHuge(struct page *page);
> -bool folio_test_hugetlb(struct folio *folio);
> +SETPAGEFLAG(HugeTLB, hugetlb, PF_SECOND)
> +CLEARPAGEFLAG(HugeTLB, hugetlb, PF_SECOND)
> +
> +/**
> + * folio_test_hugetlb - Determine if the folio belongs to hugetlbfs
> + * @folio: The folio to test.
> + *
> + * Context: Any context.  Caller should have a reference on the folio to
> + * prevent it from being turned into a tail page.
> + * Return: True for hugetlbfs folios, false for anon folios or folios
> + * belonging to other filesystems.
> + */
> +static inline bool folio_test_hugetlb(struct folio *folio)
> +{
> +	return folio_test_large(folio) &&
> +		test_bit(PG_hugetlb, folio_flags(folio, 1));
> +}
>   #else
>   TESTPAGEFLAG_FALSE(Huge, hugetlb)
>   #endif
> @@ -1040,6 +1056,13 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
>   #define PAGE_FLAGS_CHECK_AT_PREP	\
>   	((PAGEFLAGS_MASK & ~__PG_HWPOISON) | LRU_GEN_MASK | LRU_REFS_MASK)
>   
> +/*
> + * Flags stored in the second page of a compound page.  They may overlap
> + * the CHECK_AT_FREE flags above, so need to be cleared.
> + */
> +#define PAGE_FLAGS_SECOND						\
> +	(1UL << PG_has_hwpoisoned	| 1UL << PG_hugetlb)
> +
>   #define PAGE_FLAGS_PRIVATE				\
>   	(1UL << PG_private | 1UL << PG_private_2)
>   /**
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 90ce1dfd591c..dd5f87047d06 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -490,7 +490,7 @@ static int __init crash_save_vmcoreinfo_init(void)
>   #define PAGE_BUDDY_MAPCOUNT_VALUE	(~PG_buddy)
>   	VMCOREINFO_NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE);
>   #ifdef CONFIG_HUGETLB_PAGE
> -	VMCOREINFO_NUMBER(HUGETLB_PAGE_DTOR);
> +	VMCOREINFO_NUMBER(PG_hugetlb);
>   #define PAGE_OFFLINE_MAPCOUNT_VALUE	(~PG_offline)
>   	VMCOREINFO_NUMBER(PAGE_OFFLINE_MAPCOUNT_VALUE);
>   #endif
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 086eb51bf845..389490f100b0 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1585,25 +1585,7 @@ static inline void __clear_hugetlb_destructor(struct hstate *h,
>   {
>   	lockdep_assert_held(&hugetlb_lock);
>   
> -	/*
> -	 * Very subtle
> -	 *
> -	 * For non-gigantic pages set the destructor to the normal compound
> -	 * page dtor.  This is needed in case someone takes an additional
> -	 * temporary ref to the page, and freeing is delayed until they drop
> -	 * their reference.
> -	 *
> -	 * For gigantic pages set the destructor to the null dtor.  This
> -	 * destructor will never be called.  Before freeing the gigantic
> -	 * page destroy_compound_gigantic_folio will turn the folio into a
> -	 * simple group of pages.  After this the destructor does not
> -	 * apply.
> -	 *
> -	 */
> -	if (hstate_is_gigantic(h))
> -		folio_set_compound_dtor(folio, NULL_COMPOUND_DTOR);
> -	else
> -		folio_set_compound_dtor(folio, COMPOUND_PAGE_DTOR);
> +	folio_clear_hugetlb(folio);
>   }
>   
>   /*
> @@ -1690,7 +1672,7 @@ static void add_hugetlb_folio(struct hstate *h, struct folio *folio,
>   		h->surplus_huge_pages_node[nid]++;
>   	}
>   
> -	folio_set_compound_dtor(folio, HUGETLB_PAGE_DTOR);
> +	folio_set_hugetlb(folio);
>   	folio_change_private(folio, NULL);
>   	/*
>   	 * We have to set hugetlb_vmemmap_optimized again as above
> @@ -1814,9 +1796,8 @@ static void free_hpage_workfn(struct work_struct *work)
>   		/*
>   		 * The VM_BUG_ON_FOLIO(!folio_test_hugetlb(folio), folio) in
>   		 * folio_hstate() is going to trigger because a previous call to
> -		 * remove_hugetlb_folio() will call folio_set_compound_dtor
> -		 * (folio, NULL_COMPOUND_DTOR), so do not use folio_hstate()
> -		 * directly.
> +		 * remove_hugetlb_folio() will clear the hugetlb bit, so do
> +		 * not use folio_hstate() directly.
>   		 */
>   		h = size_to_hstate(page_size(page));
>   
> @@ -1955,7 +1936,7 @@ static void __prep_new_hugetlb_folio(struct hstate *h, struct folio *folio)
>   {
>   	hugetlb_vmemmap_optimize(h, &folio->page);
>   	INIT_LIST_HEAD(&folio->lru);
> -	folio_set_compound_dtor(folio, HUGETLB_PAGE_DTOR);

Hi Matthew,

In __prep_compound_gigantic_folio() there is still the comment:
/* we rely on prep_new_hugetlb_folio to set the destructor */
should that be modified to reference the hugetlb flag?

Thanks,
Sid

> +	folio_set_hugetlb(folio);
>   	hugetlb_set_folio_subpool(folio, NULL);
>   	set_hugetlb_cgroup(folio, NULL);
>   	set_hugetlb_cgroup_rsvd(folio, NULL);
> @@ -2070,28 +2051,10 @@ int PageHuge(struct page *page)
>   	if (!PageCompound(page))
>   		return 0;
>   	folio = page_folio(page);
> -	return folio->_folio_dtor == HUGETLB_PAGE_DTOR;
> +	return folio_test_hugetlb(folio);
>   }
>   EXPORT_SYMBOL_GPL(PageHuge);
>   
> -/**
> - * folio_test_hugetlb - Determine if the folio belongs to hugetlbfs
> - * @folio: The folio to test.
> - *
> - * Context: Any context.  Caller should have a reference on the folio to
> - * prevent it from being turned into a tail page.
> - * Return: True for hugetlbfs folios, false for anon folios or folios
> - * belonging to other filesystems.
> - */
> -bool folio_test_hugetlb(struct folio *folio)
> -{
> -	if (!folio_test_large(folio))
> -		return false;
> -
> -	return folio->_folio_dtor == HUGETLB_PAGE_DTOR;
> -}
> -EXPORT_SYMBOL_GPL(folio_test_hugetlb);
> -
>   /*
>    * Find and lock address space (mapping) in write mode.
>    *
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9638fdddf065..f8e276de4fd5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1122,7 +1122,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
>   		VM_BUG_ON_PAGE(compound && compound_order(page) != order, page);
>   
>   		if (compound)
> -			ClearPageHasHWPoisoned(page);
> +			page[1].flags &= ~PAGE_FLAGS_SECOND;
>   		for (i = 1; i < (1 << order); i++) {
>   			if (compound)
>   				bad += free_tail_page_prepare(page, page + i);