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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Tue, 29 Sep 2020 16:01:52 +0200
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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>,
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On Tue, Sep 29, 2020 at 3:49 PM Marco Elver <elver@google.com> wrote:
>
> On Tue, 29 Sep 2020 at 15:48, Andrey Konovalov <andreyknvl@google.com> wrote:
> > 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?
>
> Yes, correct.

OK, I see. This isn't directly clear from the comment though, at least for me :)