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From:   Alexander Potapenko <glider@google.com>
Date:   Mon, 5 Oct 2020 18:00:57 +0200
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Subject: Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Marco Elver <elver@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
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On Tue, Sep 29, 2020 at 5:06 PM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Tue, Sep 29, 2020 at 04:51:29PM +0200, Marco Elver wrote:
> > On Tue, 29 Sep 2020 at 16:24, Mark Rutland <mark.rutland@arm.com> wrote:
> > [...]
> > >
> > > From other sub-threads it sounds like these addresses are not part of
> > > the linear/direct map. Having kmalloc return addresses outside of the
> > > linear map is going to break anything that relies on virt<->phys
> > > conversions, and is liable to make DMA corrupt memory. There were
> > > problems of that sort with VMAP_STACK, and this is why kvmalloc() is
> > > separate from kmalloc().
> > >
> > > Have you tested with CONFIG_DEBUG_VIRTUAL? I'd expect that to scream.
> > >
> > > I strongly suspect this isn't going to be safe unless you always use an
> > > in-place carevout from the linear map (which could be the linear alias
> > > of a static carevout).
> >
> > That's an excellent point, thank you! Indeed, on arm64, a version with
> > naive static-pool screams with CONFIG_DEBUG_VIRTUAL.
> >
> > We'll try to put together an arm64 version using a carveout as you suggest.
>
> Great, thanks!
>
> Just to be clear, the concerns for DMA and virt<->phys conversions also
> apply to x86 (the x86 virt<->phys conversion behaviour is more forgiving
> in the common case, but still has cases that can go wrong).

To clarify, shouldn't kmalloc/kmem_cache allocations used with DMA be
allocated with explicit GFP_DMA?
If so, how practical would it be to just skip such allocations in
KFENCE allocator?