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

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Date:   Mon, 20 May 2019 11:07:38 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        dan.j.williams@...el.com, jglisse@...hat.com,
        ldufour@...ux.vnet.ibm.com
Subject: Re: [PATCH] mm/dev_pfn: Exclude MEMORY_DEVICE_PRIVATE while computing
 virtual address

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.