linux-kernel - [RFC]x86: clearing access bit don't flush tlb

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20130107081213.GA21779@kernel.org>
Date:	Mon, 7 Jan 2013 16:12:13 +0800
From:	Shaohua Li <shli@...nel.org>
To:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:	akpm@...ux-foundation.org, mingo@...hat.com, hpa@...or.com,
	riel@...hat.com
Subject: [RFC]x86: clearing access bit don't flush tlb

We use access bit to age a page at page reclaim. When clearing pte access bit,
we could skip tlb flush for the virtual address. The side effect is if the pte
is in tlb and pte access bit is unset, when cpu access the page again, cpu will
not set pte's access bit. So next time page reclaim can reclaim hot pages
wrongly, but this doesn't corrupt anything. And according to intel manual, tlb
has less than 1k entries, which coverers < 4M memory. In today's system,
several giga byte memory is normal. After page reclaim clears pte access bit
and before cpu access the page again, it's quite unlikely this page's pte is
still in TLB. Skiping the tlb flush for this case sounds ok to me.

And in some workloads, TLB flush overhead is very heavy. In my simple
multithread app with a lot of swap to several pcie SSD, removing the tlb flush
gives about 20% ~ 30% swapout speedup.

Signed-off-by: Shaohua Li <shli@...ionio.com>
---
 arch/x86/mm/pgtable.c |    7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

Index: linux/arch/x86/mm/pgtable.c
===================================================================
--- linux.orig/arch/x86/mm/pgtable.c	2012-12-17 16:54:37.847770807 +0800
+++ linux/arch/x86/mm/pgtable.c	2013-01-07 14:59:40.898066357 +0800
@@ -376,13 +376,8 @@ int pmdp_test_and_clear_young(struct vm_
 int ptep_clear_flush_young(struct vm_area_struct *vma,
 			   unsigned long address, pte_t *ptep)
 {
-	int young;

-	young = ptep_test_and_clear_young(vma, address, ptep);
-	if (young)
-		flush_tlb_page(vma, address);
-
-	return young;
+	return ptep_test_and_clear_young(vma, address, ptep);
 }

 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/