From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756263AbdKGHyd (ORCPT ); Tue, 7 Nov 2017 02:54:33 -0500 Received: from mx2.suse.de ([195.135.220.15]:41415 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754746AbdKGHyc (ORCPT ); Tue, 7 Nov 2017 02:54:32 -0500 Date: Tue, 7 Nov 2017 08:54:28 +0100 From: Michal Hocko To: Will Deacon , Wang Nan Cc: Bob Liu , Linux-MM , Linux-Kernel , Bob Liu , Andrew Morton , David Rientjes , Ingo Molnar , Roman Gushchin , Konstantin Khlebnikov , Andrea Arcangeli Subject: Re: [RFC PATCH] mm, oom_reaper: gather each vma to prevent leaking TLB entry Message-ID: <20171107075428.xy4inqifbz6ucwjw@dhcp22.suse.cz> References: <20171106033651.172368-1-wangnan0@huawei.com> <20171106085251.jwrpgne4dnl4gopy@dhcp22.suse.cz> <20171106122726.jwe2ecymlu7qclkk@dhcp22.suse.cz> <20171107005432.GB12761@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171107005432.GB12761@arm.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 07-11-17 00:54:32, Will Deacon wrote: > On Mon, Nov 06, 2017 at 01:27:26PM +0100, Michal Hocko wrote: > > On Mon 06-11-17 09:52:51, Michal Hocko wrote: > > > On Mon 06-11-17 15:04:40, Bob Liu wrote: > > > > On Mon, Nov 6, 2017 at 11:36 AM, Wang Nan wrote: > > > > > tlb_gather_mmu(&tlb, mm, 0, -1) means gathering all virtual memory space. > > > > > In this case, tlb->fullmm is true. Some archs like arm64 doesn't flush > > > > > TLB when tlb->fullmm is true: > > > > > > > > > > commit 5a7862e83000 ("arm64: tlbflush: avoid flushing when fullmm == 1"). > > > > > > > > > > > > > CC'ed Will Deacon. > > > > > > > > > Which makes leaking of tlb entries. For example, when oom_reaper > > > > > selects a task and reaps its virtual memory space, another thread > > > > > in this task group may still running on another core and access > > > > > these already freed memory through tlb entries. > > > > > > No threads should be running in userspace by the time the reaper gets to > > > unmap their address space. So the only potential case is they are > > > accessing the user memory from the kernel when we should fault and we > > > have MMF_UNSTABLE to cause a SIGBUS. > > > > I hope we have clarified that the tasks are not running in userspace at > > the time of reaping. I am still wondering whether this is real from the > > kernel space via copy_{from,to}_user. Is it possible we won't fault? > > I am not sure I understand what "Given that the ASID allocator will > > never re-allocate a dirty ASID" means exactly. Will, could you clarify > > please? > > Sure. Basically, we tag each address space with an ASID (PCID on x86) which > is resident in the TLB. This means we can elide TLB invalidation when > pulling down a full mm because we won't ever assign that ASID to another mm > without doing TLB invalidation elsewhere (which actually just nukes the > whole TLB). Thanks for the clarification! > I think that means that we could potentially not fault on a kernel uaccess, > because we could hit in the TLB. Perhaps a fix would be to set the force > variable in tlb_finish_mmu if MMF_UNSTABLE is set on the mm? OK, I suspect this is a more likely scenario than a race with the reschedule IPI discussed elsewhere in the email thread. Even though I have to admit I have never checked how are IPIs implemented on arm64, so my perception might be off. I think it would be best to simply do per VMA tlb gather like the original patch does. It would be great if the changelog absorbed the above two paragraphs. Wangnan could you resend with those clarifications please? -- Michal Hocko SUSE Labs From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f197.google.com (mail-wr0-f197.google.com [209.85.128.197]) by kanga.kvack.org (Postfix) with ESMTP id C0F356B0284 for ; Tue, 7 Nov 2017 02:54:33 -0500 (EST) Received: by mail-wr0-f197.google.com with SMTP id v88so7215890wrb.22 for ; Mon, 06 Nov 2017 23:54:33 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id j3si149711edb.372.2017.11.06.23.54.32 for (version=TLS1 cipher=AES128-SHA bits=128/128); Mon, 06 Nov 2017 23:54:32 -0800 (PST) Date: Tue, 7 Nov 2017 08:54:28 +0100 From: Michal Hocko Subject: Re: [RFC PATCH] mm, oom_reaper: gather each vma to prevent leaking TLB entry Message-ID: <20171107075428.xy4inqifbz6ucwjw@dhcp22.suse.cz> References: <20171106033651.172368-1-wangnan0@huawei.com> <20171106085251.jwrpgne4dnl4gopy@dhcp22.suse.cz> <20171106122726.jwe2ecymlu7qclkk@dhcp22.suse.cz> <20171107005432.GB12761@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171107005432.GB12761@arm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Will Deacon , Wang Nan Cc: Bob Liu , Linux-MM , Linux-Kernel , Bob Liu , Andrew Morton , David Rientjes , Ingo Molnar , Roman Gushchin , Konstantin Khlebnikov , Andrea Arcangeli On Tue 07-11-17 00:54:32, Will Deacon wrote: > On Mon, Nov 06, 2017 at 01:27:26PM +0100, Michal Hocko wrote: > > On Mon 06-11-17 09:52:51, Michal Hocko wrote: > > > On Mon 06-11-17 15:04:40, Bob Liu wrote: > > > > On Mon, Nov 6, 2017 at 11:36 AM, Wang Nan wrote: > > > > > tlb_gather_mmu(&tlb, mm, 0, -1) means gathering all virtual memory space. > > > > > In this case, tlb->fullmm is true. Some archs like arm64 doesn't flush > > > > > TLB when tlb->fullmm is true: > > > > > > > > > > commit 5a7862e83000 ("arm64: tlbflush: avoid flushing when fullmm == 1"). > > > > > > > > > > > > > CC'ed Will Deacon. > > > > > > > > > Which makes leaking of tlb entries. For example, when oom_reaper > > > > > selects a task and reaps its virtual memory space, another thread > > > > > in this task group may still running on another core and access > > > > > these already freed memory through tlb entries. > > > > > > No threads should be running in userspace by the time the reaper gets to > > > unmap their address space. So the only potential case is they are > > > accessing the user memory from the kernel when we should fault and we > > > have MMF_UNSTABLE to cause a SIGBUS. > > > > I hope we have clarified that the tasks are not running in userspace at > > the time of reaping. I am still wondering whether this is real from the > > kernel space via copy_{from,to}_user. Is it possible we won't fault? > > I am not sure I understand what "Given that the ASID allocator will > > never re-allocate a dirty ASID" means exactly. Will, could you clarify > > please? > > Sure. Basically, we tag each address space with an ASID (PCID on x86) which > is resident in the TLB. This means we can elide TLB invalidation when > pulling down a full mm because we won't ever assign that ASID to another mm > without doing TLB invalidation elsewhere (which actually just nukes the > whole TLB). Thanks for the clarification! > I think that means that we could potentially not fault on a kernel uaccess, > because we could hit in the TLB. Perhaps a fix would be to set the force > variable in tlb_finish_mmu if MMF_UNSTABLE is set on the mm? OK, I suspect this is a more likely scenario than a race with the reschedule IPI discussed elsewhere in the email thread. Even though I have to admit I have never checked how are IPIs implemented on arm64, so my perception might be off. I think it would be best to simply do per VMA tlb gather like the original patch does. It would be great if the changelog absorbed the above two paragraphs. Wangnan could you resend with those clarifications please? -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org