linux-kernel - Re: [PATCH v4 1/3] kasan: support backing vmalloc space with real shadow memory

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Message-ID: <CALCETrXnvofB_2KciRL6gZBemtjwTVg4-EKSJx-nz-BULF5aMg@mail.gmail.com>
Date:   Mon, 19 Aug 2019 15:20:47 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Daniel Axtens <dja@...ens.net>
Cc:     Mark Rutland <mark.rutland@....com>,
        Christophe Leroy <christophe.leroy@....fr>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux-MM <linux-mm@...ck.org>, X86 ML <x86@...nel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v4 1/3] kasan: support backing vmalloc space with real
 shadow memory

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