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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Tue, 29 Sep 2020 15:48:27 +0200
Message-ID: <CAAeHK+y0aPAZ8zheD5vWFDR-9YCTR251i0F1pZ9QfXuiaW0r8w@mail.gmail.com>
Subject: Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure
To:     Marco Elver <elver@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
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        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christoph Lameter <cl@linux.com>,
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On Tue, Sep 29, 2020 at 3:11 PM Marco Elver <elver@google.com> wrote:
>
> On Tue, Sep 29, 2020 at 02:42PM +0200, Andrey Konovalov wrote:
> [...]
> > > +        */
> > > +       index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1;
> >
> > Why do we subtract 1 here? We do have the metadata entry reserved for something?
>
> Above the declaration of __kfence_pool it says:
>
>         * We allocate an even number of pages, as it simplifies calculations to map
>         * address to metadata indices; effectively, the very first page serves as an
>         * extended guard page, but otherwise has no special purpose.
>
> Hopefully that clarifies the `- 1` here.

So there are two guard pages at the beginning and only then a page
that holds an object?