Re: [PATCH 0/5] mm: rework hmm to use devm_memremap_pages

[Christoph Hellwig]

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.