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Message-ID: <1425311491.17007.165.camel@misato.fc.hp.com>
Subject: Re: [PATCH v2 0/7] Kernel huge I/O mapping support
From: Toshi Kani <toshi.kani@hp.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, hpa@zytor.com,
        tglx@linutronix.de, mingo@redhat.com, arnd@arndb.de,
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Date: Mon, 02 Mar 2015 08:51:31 -0700
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On Tue, 2015-02-24 at 09:09 +0100, Ingo Molnar wrote:
> * Andrew Morton <akpm@linux-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