linux-kernel - Re: [PATCH 0/5] mm: rework hmm to use devm_memremap

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Date:   Wed, 6 Jun 2018 09:14:18 +0200
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jerome Glisse <jglisse@...hat.com>,
        Dave Airlie <airlied@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        Christoph Hellwig <hch@....de>, Michal Hocko <mhocko@...e.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/5] mm: rework hmm to use devm_memremap_pages

On Tue, Jun 05, 2018 at 06:33:04PM -0700, Dan Williams wrote:
> Unless the nouveau patches are using the entirety of what is already
> upstream for HMM, we should look to pare HMM back.
> 
> There is plenty of precedent of building a large capability
> out-of-tree and piecemeal merging it later, so I do not buy the
> "chicken-egg" argument. The change in the export is to make sure we
> don't repeat this backward "merge first, ask questions later" mistake
> in the future as devm_memremap_pages() is continuing to find new users
> like peer-to-peer DMA support and Linux is better off if that
> development is upstream. From a purely technical standpoint
> devm_memremap_pages() is EXPORT_SYMBOL_GPL because it hacks around
> several implementation details in the core kernel to achieve its goal,
> and it leaks new assumptions all over the kernel. It is strictly not a
> self contained interface.

Agreed with all of that.  And remember EXPORT_SYMBOL_GPL really just is
a clear expression of the authors they think these are internals.
The lack of it doesn't make it any less a derived work, we just remove
a very clear hint to users that they are poking very deeply into internals.

And with HMM they very clearly do.