From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f177.google.com (mail-ig0-f177.google.com [209.85.213.177]) by kanga.kvack.org (Postfix) with ESMTP id CF8CD6B0038 for ; Fri, 27 Nov 2015 05:39:09 -0500 (EST) Received: by igcmv3 with SMTP id mv3so29305614igc.0 for ; Fri, 27 Nov 2015 02:39:09 -0800 (PST) Received: from mail-ig0-x234.google.com (mail-ig0-x234.google.com. [2607:f8b0:4001:c05::234]) by mx.google.com with ESMTPS id f127si1647337ioe.59.2015.11.27.02.39.09 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 27 Nov 2015 02:39:09 -0800 (PST) Received: by igcph11 with SMTP id ph11so25093398igc.1 for ; Fri, 27 Nov 2015 02:39:09 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20151127100210.GB25781@arm.com> References: <1448543686-31869-1-git-send-email-aryabinin@virtuozzo.com> <5658106C.10207@virtuozzo.com> <20151127093529.GX3109@e104818-lin.cambridge.arm.com> <20151127100210.GB25781@arm.com> Date: Fri, 27 Nov 2015 11:39:09 +0100 Message-ID: Subject: Re: [PATCH RFT] arm64: kasan: Make KASAN work with 16K pages + 48 bit VA From: Ard Biesheuvel Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Will Deacon Cc: Catalin Marinas , Andrey Ryabinin , Mark Rutland , Yury , Arnd Bergmann , "linux-mm@kvack.org" , Linus Walleij , "Suzuki K. Poulose" , "linux-kernel@vger.kernel.org" , Alexey Klimov , Alexander Potapenko , Dmitry Vyukov , David Keitel , "linux-arm-kernel@lists.infradead.org" On 27 November 2015 at 11:02, Will Deacon wrote: > On Fri, Nov 27, 2015 at 09:35:29AM +0000, Catalin Marinas wrote: >> On Fri, Nov 27, 2015 at 11:12:28AM +0300, Andrey Ryabinin wrote: >> > On 11/26/2015 07:40 PM, Ard Biesheuvel wrote: >> > > On 26 November 2015 at 14:14, Andrey Ryabinin wrote: >> > >> Currently kasan assumes that shadow memory covers one or more entire PGDs. >> > >> That's not true for 16K pages + 48bit VA space, where PGDIR_SIZE is bigger >> > >> than the whole shadow memory. >> > >> >> > >> This patch tries to fix that case. >> > >> clear_page_tables() is a new replacement of clear_pgs(). Instead of always >> > >> clearing pgds it clears top level page table entries that entirely belongs >> > >> to shadow memory. >> > >> In addition to 'tmp_pg_dir' we now have 'tmp_pud' which is used to store >> > >> puds that now might be cleared by clear_page_tables. >> > >> >> > >> Reported-by: Suzuki K. Poulose >> > >> Signed-off-by: Andrey Ryabinin >> > > >> > > I would argue that the Kasan code is complicated enough, and we should >> > > avoid complicating it even further for a configuration that is highly >> > > theoretical in nature. >> > > >> > > In a 16k configuration, the 4th level only adds a single bit of VA >> > > space (which is, as I understand it, exactly the issue you need to >> > > address here since the top level page table has only 2 entries and >> > > hence does not divide by 8 cleanly), which means you are better off >> > > using 3 levels unless you *really* need more than 128 TB of VA space. >> > > >> > > So can't we just live with the limitation, and keep the current code? >> > >> > No objections from my side. Let's keep the current code. >> >> Ard had a good point, so fine by me as well. > > Ok, so obvious follow-up question: why do we even support 48-bit + 16k > pages in the kernel? Either it's useful, and we make things work with it, > or it's not and we can drop it (or, at least, hide it behind EXPERT like > we do for 36-bit). > So there's 10 kinds of features in the world, useful ones and !useful ones? :-) I think 48-bit/16k is somewhat useful, and I think we should support it. But I also think we should be pragmatic, and not go out of our way to support the combinatorial expansion of all niche features enabled together. I think it is perfectly fine to limit kasan support to configurations whose top level translation table divides by 8 cleanly (which only excludes 16k/48-bit anyway) However, I think it deserves being hidden behind CONFIG_EXPERT more than 36-bit/16k does. -- Ard. -- 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