From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762613AbYDXPUp (ORCPT ); Thu, 24 Apr 2008 11:20:45 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759568AbYDXPO4 (ORCPT ); Thu, 24 Apr 2008 11:14:56 -0400 Received: from smtp.polymtl.ca ([132.207.4.11]:56112 "EHLO smtp.polymtl.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759491AbYDXPOt (ORCPT ); Thu, 24 Apr 2008 11:14:49 -0400 Message-Id: <20080424151352.660545603@polymtl.ca> References: <20080424150324.802695381@polymtl.ca> User-Agent: quilt/0.46-1 Date: Thu, 24 Apr 2008 11:03:26 -0400 From: Mathieu Desnoyers To: akpm@linux-foundation.org, Ingo Molnar , linux-kernel@vger.kernel.org Cc: Mathieu Desnoyers , akpm@osdl.org, "H. Peter Anvin" , Jeremy Fitzhardinge , Steven Rostedt , "Frank Ch. Eigler" Subject: [patch 02/37] x86_64 page fault NMI-safe Content-Disposition: inline; filename=x86_64-page-fault-nmi-safe.patch X-Poly-FromMTA: (dijkstra.casi.polymtl.ca [132.207.72.10]) at Thu, 24 Apr 2008 15:13:53 +0000 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > I think you're vastly overestimating what is sane to do from an NMI > context. It is utterly and totally insane to assume vmalloc is available > in NMI. > > -hpa > Ok, please tell me where I am wrong then.. by looking into arch/x86/mm/fault.c, I see that vmalloc_sync_all() touches pgd_list entries while the pgd_lock spinlock is taken, with interrupts disabled. So it's protected against concurrent pgd_list modification from a - vmalloc_sync_all() on other CPUs b - local interrupts However, a completely normal interrupt can come on a remote CPU, run vmalloc_fault() and issue a set_pgd concurrently. Therefore I conclude this interrupt disable is not there to insure any kind of protection against concurrent updates. Also, we see that vmalloc_fault has comments such as : (for x86_32) * Do _not_ use "current" here. We might be inside * an interrupt in the middle of a task switch.. So it takes the pgd_addr from cr3, not from current. Using only the stack/registers makes this NMI-safe even if "current" is invalid when the NMI comes. This is caused by the fact that __switch_to will update the registers before updating current_task without disabling interrupts. You are right in that x86_64 does not seems to play as safely as x86_32 on this matter; it uses current->mm. Probably it shouldn't assume "current" is valid. Actually, I don't see where x86_64 disables interrupts around __switch_to, so this would seem to be a race condition. Or have I missed something ? (Ingo) > > the scheduler disables interrupts around __switch_to(). (x86 does > > not set __ARCH_WANT_INTERRUPTS_ON_CTXSW) > (Mathieu) > Ok, so I guess it's only useful to NMIs then. However, it makes me > wonder why this comment was there in the first place on x86_32 > vmalloc_fault() and why it uses read_cr3() : > > * Do _not_ use "current" here. We might be inside > * an interrupt in the middle of a task switch.. (Ingo) hm, i guess it's still useful to keep the __ARCH_WANT_INTERRUPTS_ON_CTXSW case working too. On -rt we used to enable it to squeeze a tiny bit more latency out of the system. Signed-off-by: Mathieu Desnoyers CC: akpm@osdl.org CC: mingo@elte.hu CC: "H. Peter Anvin" CC: Jeremy Fitzhardinge CC: Steven Rostedt CC: "Frank Ch. Eigler" --- arch/x86/mm/fault.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) Index: linux-2.6-sched-devel/arch/x86/mm/fault.c =================================================================== --- linux-2.6-sched-devel.orig/arch/x86/mm/fault.c 2008-04-22 20:04:02.000000000 -0400 +++ linux-2.6-sched-devel/arch/x86/mm/fault.c 2008-04-22 20:09:50.000000000 -0400 @@ -535,6 +535,7 @@ static int vmalloc_fault(struct pt_regs } return 0; #else + unsigned long pgd_paddr; pgd_t *pgd, *pgd_ref; pud_t *pud, *pud_ref; pmd_t *pmd, *pmd_ref; @@ -548,7 +549,8 @@ static int vmalloc_fault(struct pt_regs happen within a race in page table update. In the later case just flush. */ - pgd = pgd_offset(current->mm ?: &init_mm, address); + pgd_paddr = read_cr3(); + pgd = __va(pgd_paddr) + pgd_index(address); pgd_ref = pgd_offset_k(address); if (pgd_none(*pgd_ref)) return -1; -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68