From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752962Ab1IZTgG (ORCPT ); Mon, 26 Sep 2011 15:36:06 -0400 Received: from acsinet15.oracle.com ([141.146.126.227]:50105 "EHLO acsinet15.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752865Ab1IZTgE (ORCPT ); Mon, 26 Sep 2011 15:36:04 -0400 Date: Mon, 26 Sep 2011 15:34:53 -0400 From: Konrad Rzeszutek Wilk To: Jeremy Fitzhardinge Cc: linux-kernel@vger.kernel.org, xen-devel@lists.xensource.com, Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, Peter Zijlstra , Jeremy Fitzhardinge , stable@kernel.org Subject: Is: [PATCH] x86/paravirt: PTE updates in k(un)map_atomic need to be synchronous, regardless of lazy_mmu mode. Was: Re: [PATCH] x86/paravirt: Partially revert "remove lazy mode in interrupts" Message-ID: <20110926193453.GA9717@phenom.oracle.com> References: <1317042797-19975-1-git-send-email-konrad.wilk@oracle.com> <4E80A6BD.3070703@goop.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4E80A6BD.3070703@goop.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-Source-IP: rtcsinet22.oracle.com [66.248.204.30] X-CT-RefId: str=0001.0A090204.4E80D3F2.0046,ss=1,re=0.000,fgs=0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 26, 2011 at 09:22:21AM -0700, Jeremy Fitzhardinge wrote: > On 09/26/2011 06:13 AM, Konrad Rzeszutek Wilk wrote: > > which has git commit b8bcfe997e46150fedcc3f5b26b846400122fdd9. > > > > The unintended consequence of removing the flushing of MMU > > updates when doing kmap_atomic (or kunmap_atomic) is that we can > > hit a dereference bug when processing a "fork()" under a heavy loaded > > machine. Specifically we can hit: > > The patch is all OK, but I wouldn't have headlined it as a "partial > revert" - the important point is that the pte updates in k(un)map_atomic > need to be synchronous, regardless of whether we're in lazy_mmu mode. > > The fact that b8bcfe997e4 introduced the problem is interesting to note, > but only somewhat relevant to the analysis of what's being fixed here. Good point. How about >>From 09966678dd645b68a422c9bf0223b13e73387302 Mon Sep 17 00:00:00 2001 From: Konrad Rzeszutek Wilk Date: Fri, 23 Sep 2011 17:02:29 -0400 Subject: [PATCH] x86/paravirt: PTE updates in k(un)map_atomic need to be synchronous, regardless of lazy_mmu mode. This patch fixes an outstanding issue that has been reported since 2.6.37. Under a heavy loaded machine processing "fork()" calls could keepover with: BUG: unable to handle kernel paging request at f573fc8c IP: [] swap_count_continued+0x104/0x180 *pdpt = 000000002a3b9027 *pde = 0000000001bed067 *pte = 0000000000000000 Oops: 0000 [#1] SMP Modules linked in: Pid: 1638, comm: apache2 Not tainted 3.0.4-linode37 #1 EIP: 0061:[] EFLAGS: 00210246 CPU: 3 EIP is at swap_count_continued+0x104/0x180 .. snip.. Call Trace: [] ? __swap_duplicate+0xc2/0x160 [] ? pte_mfn_to_pfn+0x87/0xe0 [] ? swap_duplicate+0x14/0x40 [] ? copy_pte_range+0x45b/0x500 [] ? copy_page_range+0x195/0x200 [] ? dup_mmap+0x1c6/0x2c0 [] ? dup_mm+0xa8/0x130 [] ? copy_process+0x98a/0xb30 [] ? do_fork+0x4f/0x280 [] ? getnstimeofday+0x43/0x100 [] ? sys_clone+0x30/0x40 [] ? ptregs_clone+0x15/0x48 [] ? syscall_call+0x7/0xb The problem is that in copy_page_range we turn lazy mode on, and then in swap_entry_free we call swap_count_continued which ends up in: map = kmap_atomic(page, KM_USER0) + offset; and then later we touch *map. Since we are running in batched mode (lazy) we don't actually set up the PTE mappings and the kmap_atomic is not done synchronously and ends up trying to dereference a page that has not been set. Looking at kmap_atomic_prot_pfn, it uses 'arch_flush_lazy_mmu_mode' and doing the same in kmap_atomic_prot and __kunmap_atomic makes the problem go away. Interestingly, git commit b8bcfe997e46150fedcc3f5b26b846400122fdd9 removed part of this to fix an interrupt issue - but it went to far and did not consider this scenario. CC: Thomas Gleixner CC: Ingo Molnar CC: "H. Peter Anvin" CC: x86@kernel.org CC: Peter Zijlstra CC: Jeremy Fitzhardinge CC: stable@kernel.org [v1: Redid the commit description per Jeremy's apt suggestion] Signed-off-by: Konrad Rzeszutek Wilk --- arch/x86/mm/highmem_32.c | 2 ++ 1 files changed, 2 insertions(+), 0 deletions(-) diff --git a/arch/x86/mm/highmem_32.c b/arch/x86/mm/highmem_32.c index b499626..f4f29b1 100644 --- a/arch/x86/mm/highmem_32.c +++ b/arch/x86/mm/highmem_32.c @@ -45,6 +45,7 @@ void *kmap_atomic_prot(struct page *page, pgprot_t prot) vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx); BUG_ON(!pte_none(*(kmap_pte-idx))); set_pte(kmap_pte-idx, mk_pte(page, prot)); + arch_flush_lazy_mmu_mode(); return (void *)vaddr; } @@ -88,6 +89,7 @@ void __kunmap_atomic(void *kvaddr) */ kpte_clear_flush(kmap_pte-idx, vaddr); kmap_atomic_idx_pop(); + arch_flush_lazy_mmu_mode(); } #ifdef CONFIG_DEBUG_HIGHMEM else { -- 1.7.4.1