linux-kernel - Re: [PATCH RFT] arm64: kasan: Make KASAN work with 16K pages + 48 bit VA

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Date:	Fri, 27 Nov 2015 11:12:28 +0300
From:	Andrey Ryabinin <aryabinin@...tuozzo.com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
CC:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Yury <yury.norov@...il.com>,
	Alexey Klimov <klimov.linux@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Keitel <dkeitel@...eaurora.org>,
	Alexander Potapenko <glider@...gle.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	"Suzuki K. Poulose" <Suzuki.Poulose@....com>,
	Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH RFT] arm64: kasan: Make KASAN work with 16K pages + 48 bit
 VA

On 11/26/2015 07:40 PM, Ard Biesheuvel wrote:
> On 26 November 2015 at 14:14, Andrey Ryabinin <aryabinin@...tuozzo.com> 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 <Suzuki.Poulose@....com>
>> Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
> 
> 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.
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