linux-kernel - Re: [BUGFIX][PATCH -tip] x86: fix text

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Date:	Mon, 6 Apr 2009 13:58:07 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	systemtap-ml <systemtap@...rces.redhat.com>
Subject: Re: [BUGFIX][PATCH -tip] x86: fix text_poke to handle highmem pages

* Masami Hiramatsu (mhiramat@...hat.com) wrote:
> Mathieu Desnoyers wrote:
> > * Masami Hiramatsu (mhiramat@...hat.com) wrote:
> >> Fix a bug in text_poke to handle highmem pages, because module
> >> text pages are possible to be highmem pages on x86-32.
> >> In that case, since fixmap can't handle those pages, text_poke
> >> uses kmap_atomic.
> >>
> > 
> > Hrm, can you remind me what would be the downside of using kmap_atomic
> > in every scenarios (highmem and non-highmem) then ?
> 
> kmap_atomic can handle only highmem pages. If you passes lowmem pages,
> it returns just original vaddr of it. (because kmap is only for
> highmem support)
> 

OK, and if we are doing a second kmap_atomic() of a module text page
which is already mapped, does the second kmap_atomic return the vaddr of
the page originally mapped or is it creating a second mapping ?

Because if we ever decide to enforce read-only page mapping for module
text pages, touching highmem pages too, we will run into real trouble if
those happen to be the same page.

Mathieu

> > 
> > I would try to avoid "special cases" as much as possible, because they
> > just make problems harder to reproduce.
> 
> Actually, this bug is a special case because it happens only on PAE kernel.
> 
> > 
> > The idea would be to either add fixmap highmem support, or to simply use
> > kmap_atomic() for all cases until we add fixmap highmem support.
> > 
> 
> 
> Thank you,
> 
> -- 
> Masami Hiramatsu
> 
> Software Engineer
> Hitachi Computer Products (America) Inc.
> Software Solutions Division
> 
> e-mail: mhiramat@...hat.com
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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