linux-kernel - Re: [PATCH v2 0/7] Kernel huge I/O mapping support

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Date:	Mon, 02 Mar 2015 08:51:31 -0700
From:	Toshi Kani <toshi.kani@...com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, hpa@...or.com,
	tglx@...utronix.de, mingo@...hat.com, arnd@...db.de,
	linux-mm@...ck.org, x86@...nel.org, linux-kernel@...r.kernel.org,
	Elliott@...com
Subject: Re: [PATCH v2 0/7] Kernel huge I/O mapping support

On Tue, 2015-02-24 at 09:09 +0100, Ingo Molnar wrote:
> * Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > <reads the code>
> > 
> > Oh.  We don't do any checking at all.  We're just telling 
> > userspace programmers "don't do that".  hrm.  What are 
> > your thoughts on adding the overlap checks to the kernel?
> 
> I have requested such sanity checking in previous review as 
> well, it has to be made fool-proof for this optimization to 
> be usable.
> 
> Another alternative would be to make this not a transparent 
> optimization, but a separate API: ioremap_hugepage() or so.
> 
> The devices and drivers dealing with GBs of remapped pages 
> is still relatively low, so they could make explicit use of 
> the API and opt in to it.
> 
> What I was arguing against was to make it a CONFIG_ option: 
> that achieves very little in practice, such APIs should be 
> uniformly available.

I was able to come up with simple changes that fall back to 4KB mappings
when a target range is covered by MTRRs.  So, with the changes, it is
now safe to enable huge page mappings to ioremap() transparently without
such restriction.  I will post updated patchset hopefully soon.

Thanks,
-Toshi

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