linux-kernel - Re: [PATCH] mm/dev_pfn: Exclude MEMORY_DEVICE

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Date:   Mon, 20 May 2019 15:27:21 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        dan.j.williams@...el.com, ldufour@...ux.vnet.ibm.com
Subject: Re: [PATCH] mm/dev_pfn: Exclude MEMORY_DEVICE_PRIVATE while
 computing virtual address

On Mon, May 20, 2019 at 11:07:38AM +0530, Anshuman Khandual wrote:
> On 05/18/2019 03:20 AM, Andrew Morton wrote:
> > On Fri, 17 May 2019 16:08:34 +0530 Anshuman Khandual <anshuman.khandual@....com> wrote:
> > 
> >> The presence of struct page does not guarantee linear mapping for the pfn
> >> physical range. Device private memory which is non-coherent is excluded
> >> from linear mapping during devm_memremap_pages() though they will still
> >> have struct page coverage. Just check for device private memory before
> >> giving out virtual address for a given pfn.
> > 
> > I was going to give my standard "what are the user-visible runtime
> > effects of this change?", but...
> > 
> >> All these helper functions are all pfn_t related but could not figure out
> >> another way of determining a private pfn without looking into it's struct
> >> page. pfn_t_to_virt() is not getting used any where in mainline kernel.Is
> >> it used by out of tree drivers ? Should we then drop it completely ?
> > 
> > Yeah, let's kill it.
> > 
> > But first, let's fix it so that if someone brings it back, they bring
> > back a non-buggy version.
> 
> Makes sense.
> 
> > 
> > So...  what (would be) the user-visible runtime effects of this change?
> 
> I am not very well aware about the user interaction with the drivers which
> hotplug and manage ZONE_DEVICE memory in general. Hence will not be able to
> comment on it's user visible runtime impact. I just figured this out from
> code audit while testing ZONE_DEVICE on arm64 platform. But the fix makes
> the function bit more expensive as it now involve some additional memory
> references.

A device private pfn can never leak outside code that does not understand it
So this change is useless for any existing users and i would like to keep the
existing behavior ie never leak device private pfn.

Cheers,
Jérôme