linux-kernel - Re: [PATCH] arch: arm: remove redundant clear_page when CONFIG_INIT_ON_ALLOC_DEFAULT

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Date:   Tue, 12 Sep 2023 13:17:31 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "zhaoyang.huang" <zhaoyang.huang@...soc.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Zhaoyang Huang <huangzhaoyang@...il.com>, ke.wang@...soc.com
Subject: Re: [PATCH] arch: arm: remove redundant clear_page when
 CONFIG_INIT_ON_ALLOC_DEFAULT_ON is on

On Tue, Sep 12, 2023 at 06:33:34PM +0800, zhaoyang.huang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> 
> Double times of clear_page observed in an arm SOC(A55) when
> CONFIG_INIT_ON_ALLOC_DEFAULT_ON is on, which introduced by
> vma_alloc_zeroed_movable_folio within do_anonymous_pages.
> Since there is no D-cache operation within v6's clear_user_highpage,
> I would like to suggest to remove the redundant clear_page.
> 
> struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
> 				   unsigned long vaddr)
> {
> 	struct folio *folio;
> 
> //first clear_page invoked by vma_alloc_folio==>alloc_page==>post_alloc_hook
> 	folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, vaddr, false);
> 	if (folio)
> //second clear_page which is meaningless since it do nothing to D-cache in armv6
> 		clear_user_highpage(&folio->page, vaddr);

This is, of course, not the only place which calls clear_user_highpage().
Please explain why this patch is safe for all the _other_ places which
call clear_user_highpage().

> 	return folio;
> }
> 
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> ---
>  arch/arm/mm/copypage-v6.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/arch/arm/mm/copypage-v6.c b/arch/arm/mm/copypage-v6.c
> index a1a71f36d850..6f8bee1b3203 100644
> --- a/arch/arm/mm/copypage-v6.c
> +++ b/arch/arm/mm/copypage-v6.c
> @@ -9,6 +9,7 @@
>  #include <linux/mm.h>
>  #include <linux/highmem.h>
>  #include <linux/pagemap.h>
> +#include <linux/gfp.h>
>  
>  #include <asm/shmparam.h>
>  #include <asm/tlbflush.h>
> @@ -45,6 +46,13 @@ static void v6_copy_user_highpage_nonaliasing(struct page *to,
>   */
>  static void v6_clear_user_highpage_nonaliasing(struct page *page, unsigned long vaddr)
>  {
> +	/*
> +	 * This criteria only help bailing out when CONFIG_INIT_ON_ALLOC_DEFAULT_ON
> +	 * is on. The page has been memset to zero when it allocated and the
> +	 * bellowing clear_page will do it again.
> +	 */
> +	if (want_init_on_alloc(GFP_HIGHUSER_MOVABLE))
> +		return;
>  	void *kaddr = kmap_atomic(page);
>  	clear_page(kaddr);
>  	kunmap_atomic(kaddr);
> -- 
> 2.25.1
>