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Date:   Fri, 30 Jun 2023 15:38:52 +0200
From:   Claudio Imbrenda <imbrenda@linux.ibm.com>
To:     Hugh Dickins <hughd@google.com>
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Subject: Re: [PATCH v2 07/12] s390: add pte_free_defer() for pgtables
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On Tue, 20 Jun 2023 00:51:19 -0700 (PDT)
Hugh Dickins <hughd@google.com> wrote:

[...]

> @@ -407,6 +429,77 @@ void __tlb_remove_table(void *_table)
>  	__free_page(page);
>  }
>  
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +static void pte_free_now0(struct rcu_head *head);
> +static void pte_free_now1(struct rcu_head *head);
> +
> +static void pte_free_pgste(struct rcu_head *head)
> +{
> +	unsigned long *table;
> +	struct page *page;
> +
> +	page = container_of(head, struct page, rcu_head);
> +	table = (unsigned long *)page_to_virt(page);
> +	table = (unsigned long *)((unsigned long)table | 0x03U);
> +	__tlb_remove_table(table);
> +}
> +
> +static void pte_free_half(struct rcu_head *head, unsigned int bit)
> +{
> +	unsigned long *table;
> +	struct page *page;
> +	unsigned int mask;
> +
> +	page = container_of(head, struct page, rcu_head);
> +	mask = atomic_xor_bits(&page->_refcount, 0x04U << (bit + 24));
> +
> +	table = (unsigned long *)page_to_virt(page);
> +	table += bit * PTRS_PER_PTE;
> +	table = (unsigned long *)((unsigned long)table | (0x01U << bit));
> +	__tlb_remove_table(table);
> +
> +	/* If pte_free_defer() of the other half came in, queue it now */
> +	if (mask & 0x0CU)
> +		call_rcu(&page->rcu_head, bit ? pte_free_now0 : pte_free_now1);
> +}
> +
> +static void pte_free_now0(struct rcu_head *head)
> +{
> +	pte_free_half(head, 0);
> +}
> +
> +static void pte_free_now1(struct rcu_head *head)
> +{
> +	pte_free_half(head, 1);
> +}
> +
> +void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable)
> +{
> +	unsigned int bit, mask;
> +	struct page *page;
> +
> +	page = virt_to_page(pgtable);
> +	if (mm_alloc_pgste(mm)) {
> +		call_rcu(&page->rcu_head, pte_free_pgste);

so is this now going to be used to free page tables
instead of page_table_free_rcu?

or will it be used instead of page_table_free?

this is actually quite important for KVM on s390

> +		return;
> +	}
> +	bit = ((unsigned long)pgtable & ~PAGE_MASK) /
> +			(PTRS_PER_PTE * sizeof(pte_t));
> +
> +	spin_lock_bh(&mm_pgtable_list_lock);
> +	mask = atomic_xor_bits(&page->_refcount, 0x15U << (bit + 24));
> +	mask >>= 24;
> +	/* Other half not allocated? Other half not already pending free? */
> +	if ((mask & 0x03U) == 0x00U && (mask & 0x30U) != 0x30U)
> +		list_del(&page->lru);
> +	spin_unlock_bh(&mm_pgtable_list_lock);
> +
> +	/* Do not relink on rcu_head if other half already linked on rcu_head */
> +	if ((mask & 0x0CU) != 0x0CU)
> +		call_rcu(&page->rcu_head, bit ? pte_free_now1 : pte_free_now0);
> +}
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> +
>  /*
>   * Base infrastructure required to generate basic asces, region, segment,
>   * and page tables that do not make use of enhanced features like EDAT1.
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 306a3d1a0fa6..1667a1bdb8a8 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -146,7 +146,7 @@ struct page {
>  			pgtable_t pmd_huge_pte; /* protected by page->ptl */
>  			unsigned long _pt_pad_2;	/* mapping */
>  			union {
> -				struct mm_struct *pt_mm; /* x86 pgds only */
> +				struct mm_struct *pt_mm; /* x86 pgd, s390 */
>  				atomic_t pt_frag_refcount; /* powerpc */
>  			};
>  #if ALLOC_SPLIT_PTLOCKS