Re: [PATCH v2 05/12] powerpc: add pte_free_defer() for pgtables sharing page

[Jason Gunthorpe]

On Wed, Jun 21, 2023 at 07:36:11PM -0700, Hugh Dickins wrote:
> [PATCH v3 05/12] powerpc: add pte_free_defer() for pgtables sharing page
> 
> Add powerpc-specific pte_free_defer(), to free table page via call_rcu().
> pte_free_defer() will be called inside khugepaged's retract_page_tables()
> loop, where allocating extra memory cannot be relied upon.  This precedes
> the generic version to avoid build breakage from incompatible pgtable_t.
> 
> This is awkward because the struct page contains only one rcu_head, but
> that page may be shared between PTE_FRAG_NR pagetables, each wanting to
> use the rcu_head at the same time.  But powerpc never reuses a fragment
> once it has been freed: so mark the page Active in pte_free_defer(),
> before calling pte_fragment_free() directly; and there call_rcu() to
> pte_free_now() when last fragment is freed and the page is PageActive.
> 
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxx>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
>  arch/powerpc/include/asm/pgalloc.h |  4 ++++
>  arch/powerpc/mm/pgtable-frag.c     | 29 ++++++++++++++++++++++++++---
>  2 files changed, 30 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/pgalloc.h b/arch/powerpc/include/asm/pgalloc.h
> index 3360cad78ace..3a971e2a8c73 100644
> --- a/arch/powerpc/include/asm/pgalloc.h
> +++ b/arch/powerpc/include/asm/pgalloc.h
> @@ -45,6 +45,10 @@ static inline void pte_free(struct mm_struct *mm, pgtable_t ptepage)
>  	pte_fragment_free((unsigned long *)ptepage, 0);
>  }
>  
> +/* arch use pte_free_defer() implementation in arch/powerpc/mm/pgtable-frag.c */
> +#define pte_free_defer pte_free_defer
> +void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable);
> +
>  /*
>   * Functions that deal with pagetables that could be at any level of
>   * the table need to be passed an "index_size" so they know how to
> diff --git a/arch/powerpc/mm/pgtable-frag.c b/arch/powerpc/mm/pgtable-frag.c
> index 20652daa1d7e..0c6b68130025 100644
> --- a/arch/powerpc/mm/pgtable-frag.c
> +++ b/arch/powerpc/mm/pgtable-frag.c
> @@ -106,6 +106,15 @@ pte_t *pte_fragment_alloc(struct mm_struct *mm, int kernel)
>  	return __alloc_for_ptecache(mm, kernel);
>  }
>  
> +static void pte_free_now(struct rcu_head *head)
> +{
> +	struct page *page;
> +
> +	page = container_of(head, struct page, rcu_head);
> +	pgtable_pte_page_dtor(page);
> +	__free_page(page);
> +}
> +
>  void pte_fragment_free(unsigned long *table, int kernel)
>  {
>  	struct page *page = virt_to_page(table);
> @@ -115,8 +124,22 @@ void pte_fragment_free(unsigned long *table, int kernel)
>  
>  	BUG_ON(atomic_read(&page->pt_frag_refcount) <= 0);
>  	if (atomic_dec_and_test(&page->pt_frag_refcount)) {
> -		if (!kernel)
> -			pgtable_pte_page_dtor(page);
> -		__free_page(page);
> +		if (kernel)
> +			__free_page(page);
> +		else if (TestClearPageActive(page))
> +			call_rcu(&page->rcu_head, pte_free_now);
> +		else
> +			pte_free_now(&page->rcu_head);
>  	}
>  }
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable)
> +{
> +	struct page *page;
> +
> +	page = virt_to_page(pgtable);
> +	SetPageActive(page);
> +	pte_fragment_free((unsigned long *)pgtable, 0);
> +}
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */

Yes, this makes sense to me, very simple..

I always for get these details but atomic_dec_and_test() is a release?
So the SetPageActive is guarenteed to be visible in another thread
that reaches 0?

Thanks,
Jason