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Date:   Tue, 29 Sep 2020 15:11:35 +0200
From:   Marco Elver <elver@google.com>
To:     Andrey Konovalov <andreyknvl@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
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        Andy Lutomirski <luto@kernel.org>,
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        Catalin Marinas <catalin.marinas@arm.com>,
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Subject: Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure
Message-ID: <20200929131135.GA2822082@elver.google.com>
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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.

[...]
> > +       /* Allocation and free stack information. */
> > +       int num_alloc_stack;
> > +       int num_free_stack;
> > +       unsigned long alloc_stack[KFENCE_STACK_DEPTH];
> > +       unsigned long free_stack[KFENCE_STACK_DEPTH];
> 
> It was a concious decision to not use stackdepot, right? Perhaps it
> makes sense to document the reason somewhere.

Yes; we want to avoid the dynamic allocations that stackdepot does.

[...]

Thanks,
-- Marco