linux-kernel - Re: [PATCH] x86 get_unmapped_area: Add PMD alignment for DAX PMD mmap

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Date:	Thu, 7 Apr 2016 13:41:11 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Toshi Kani <toshi.kani@....com>
Cc:	mingo@...nel.org, bp@...e.de, hpa@...or.com, tglx@...utronix.de,
	dan.j.williams@...el.com, kirill.shutemov@...ux.intel.com,
	linux-mm@...ck.org, x86@...nel.org, linux-nvdimm@...ts.01.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86 get_unmapped_area: Add PMD alignment for DAX PMD mmap

On Wed, Apr 06, 2016 at 11:44:32AM -0600, Toshi Kani wrote:
> > The NVML chooses appropriate addresses and gets a properly aligned
> > address without any kernel code.
> 
> An application like NVML can continue to specify a specific address to
> mmap().  Most existing applications, however, do not specify an address to
> mmap().  With this patch, specifying an address will remain optional.

The point is that this *can* be done in userspace.  You need to sell us
on the advantages of doing it in the kernel.

> > I think this is the wrong place for it, if we decide that this is the
> > right thing to do.  The filesystem has a get_unmapped_area() which
> > should be used instead.
> 
> Yes, I considered adding a filesystem entry point, but decided going this
> way because:
>  - arch_get_unmapped_area() and arch_get_unmapped_area_topdown() are arch-
> specific code.  Therefore, this filesystem entry point will need arch-
> specific implementation. 
>  - There is nothing filesystem specific about requesting PMD alignment.

See http://article.gmane.org/gmane.linux.kernel.mm/149227 for Hugh's
approach for shmem.  I strongly believe that if we're going to do this
i the kernel, we should build on this approach, and not hack something
into each architecture's generic get_unmapped_area.