Re: [PATCH v2 0/7] Kernel huge I/O mapping support
From: Toshi Kani
Date: Mon Mar 02 2015 - 10:52:15 EST
On Tue, 2015-02-24 at 09:09 +0100, Ingo Molnar wrote:
> * Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> 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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