From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751318AbdE3Ibo (ORCPT ); Tue, 30 May 2017 04:31:44 -0400 Received: from foss.arm.com ([217.140.101.70]:54638 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751146AbdE3Ibm (ORCPT ); Tue, 30 May 2017 04:31:42 -0400 Subject: Re: [PATCH v1 00/11] mm/kasan: support per-page shadow memory to reduce memory consumption To: Dmitry Vyukov References: <1494897409-14408-1-git-send-email-iamjoonsoo.kim@lge.com> <20170516062318.GC16015@js1304-desktop> <20170524074539.GA9697@js1304-desktop> <20170525004104.GA21336@js1304-desktop> <1131ff71-eb7a-8396-9a72-211f7077e5ec@arm.com> Cc: Joonsoo Kim , Andrew Morton , Andrey Ryabinin , Alexander Potapenko , kasan-dev , "linux-mm@kvack.org" , LKML , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , kernel-team@lge.com From: Vladimir Murzin Message-ID: Date: Tue, 30 May 2017 09:31:38 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 30/05/17 09:15, Dmitry Vyukov wrote: > On Tue, May 30, 2017 at 9:58 AM, Vladimir Murzin > wrote: >> On 29/05/17 16:29, Dmitry Vyukov wrote: >>> I have an alternative proposal. It should be conceptually simpler and >>> also less arch-dependent. But I don't know if I miss something >>> important that will render it non working. >>> Namely, we add a pointer to shadow to the page struct. Then, create a >>> slab allocator for 512B shadow blocks. Then, attach/detach these >>> shadow blocks to page structs as necessary. It should lead to even >>> smaller memory consumption because we won't need a whole shadow page >>> when only 1 out of 8 corresponding kernel pages are used (we will need >>> just a single 512B block). I guess with some fragmentation we need >>> lots of excessive shadow with the current proposed patch. >>> This does not depend on TLB in any way and does not require hooking >>> into buddy allocator. >>> The main downside is that we will need to be careful to not assume >>> that shadow is continuous. In particular this means that this mode >>> will work only with outline instrumentation and will need some ifdefs. >>> Also it will be slower due to the additional indirection when >>> accessing shadow, but that's meant as "small but slow" mode as far as >>> I understand. >>> >>> But the main win as I see it is that that's basically complete support >>> for 32-bit arches. People do ask about arm32 support: >>> https://groups.google.com/d/msg/kasan-dev/Sk6BsSPMRRc/Gqh4oD_wAAAJ >>> https://groups.google.com/d/msg/kasan-dev/B22vOFp-QWg/EVJPbrsgAgAJ >>> and probably mips32 is relevant as well. >>> Such mode does not require a huge continuous address space range, has >>> minimal memory consumption and requires minimal arch-dependent code. >>> Works only with outline instrumentation, but I think that's a >>> reasonable compromise. >> >> .. or you can just keep shadow in page extension. It was suggested back in >> 2015 [1], but seems that lack of stack instrumentation was "no-way"... >> >> [1] https://lkml.org/lkml/2015/8/24/573 > > Right. It describes basically the same idea. > > How is page_ext better than adding data page struct? page_ext is already here along with some other debug options ;) > It seems that memory for all page_ext is preallocated along with page > structs; but just the lookup is slower. > Yup. Lookup would look like (based on v4.0): ... page_ext = lookup_page_ext_begin(virt_to_page(start)); do { page_ext->shadow[idx++] = value; } while (idx < bound); lookup_page_ext_end((void *)page_ext); ... Cheers Vladimir