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

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Date:   Tue, 12 Sep 2023 18:33:34 +0800
From:   "zhaoyang.huang" <zhaoyang.huang@...soc.com>
To:     Russell King <linux@...linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Matthew Wilcox <willy@...radead.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: [PATCH] arch: arm: remove redundant clear_page when CONFIG_INIT_ON_ALLOC_DEFAULT_ON is on

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);

	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