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

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Message-Id: <20190517145050.2b6b0afdaab5c3c69a4b153e@linux-foundation.org>
Date:   Fri, 17 May 2019 14:50:50 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
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 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.

So...  what (would be) the user-visible runtime effects of this change?