Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20190815001636.12235-1-dja@axtens.net> <20190815001636.12235-2-dja@axtens.net>
 <15c6110a-9e6e-495c-122e-acbde6e698d9@c-s.fr> <20190816170813.GA7417@lakrids.cambridge.arm.com>
 <87imqtu7pc.fsf@dja-thinkpad.axtens.net>
In-Reply-To: <87imqtu7pc.fsf@dja-thinkpad.axtens.net>
From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 19 Aug 2019 15:20:47 -0700
Message-ID: <CALCETrXnvofB_2KciRL6gZBemtjwTVg4-EKSJx-nz-BULF5aMg@mail.gmail.com>
Subject: Re: [PATCH v4 1/3] kasan: support backing vmalloc space with real
 shadow memory
To:     Daniel Axtens <dja@axtens.net>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux-MM <linux-mm@kvack.org>, X86 ML <x86@kernel.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Andrew Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Vasily Gorbik <gor@linux.ibm.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

> On Aug 18, 2019, at 8:58 PM, Daniel Axtens <dja@axtens.net> wrote:
>

>>> Each page of shadow memory represent 8 pages of real memory. Could we use
>>> page_ref to count how many pieces of a shadow page are used so that we can
>>> free it when the ref count decreases to 0.
>
> I'm not sure how much of a difference it will make, but I'll have a look.
>

There are a grand total of eight possible pages that could require a
given shadow page. I would suggest that, instead of reference
counting, you just check all eight pages.

Or, better yet, look at the actual vm_area_struct and are where prev
and next point. That should tell you exactly which range can be freed.