linux-kernel - Re: [PATCH v5 1/7] mm, devm_memremap_pages: Mark devm_memremap_pages() EXPORT_SYMBOL

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Date:   Thu, 13 Sep 2018 10:25:58 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Dan Williams <dan.j.williams@...el.com>, akpm@...ux-foundation.org
Cc:     Michal Hocko <mhocko@...e.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Christoph Hellwig <hch@....de>, alexander.h.duyck@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/7] mm, devm_memremap_pages: Mark
 devm_memremap_pages() EXPORT_SYMBOL_GPL



On 12/09/18 08:22 PM, Dan Williams wrote:
> devm_memremap_pages() is a facility that can create struct page entries
> for any arbitrary range and give drivers the ability to subvert core
> aspects of page management.
> 
> Specifically the facility is tightly integrated with the kernel's memory
> hotplug functionality. It injects an altmap argument deep into the
> architecture specific vmemmap implementation to allow allocating from
> specific reserved pages, and it has Linux specific assumptions about
> page structure reference counting relative to get_user_pages() and
> get_user_pages_fast(). It was an oversight and a mistake that this was
> not marked EXPORT_SYMBOL_GPL from the outset.
> 
> Again, devm_memremap_pagex() exposes and relies upon core kernel
> internal assumptions and will continue to evolve along with 'struct
> page', memory hotplug, and support for new memory types / topologies.
> Only an in-kernel GPL-only driver is expected to keep up with this
> ongoing evolution. This interface, and functionality derived from this
> interface, is not suitable for kernel-external drivers.
> 
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: "Jérôme Glisse" <jglisse@...hat.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>

Reviewed-by: Logan Gunthorpe <logang@...tatee.com>

Mostly to say that I agree with you and Christoph on this debate and
that the change to GPL does not affect my P2PDMA work.

Logan