Messages in this thread | | | Date | Sat, 28 Feb 2009 17:56:24 +0100 | From | Andi Kleen <> | Subject | Re: [PATCH 4/6] ftrace, x86: make kernel text writable only for conversions |
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On Fri, Feb 27, 2009 at 10:08:56PM +0100, Pavel Machek wrote: > On Sun 2009-02-22 18:50:00, Andi Kleen wrote: > > Steven Rostedt <rostedt@goodmis.org> writes: > > > > > From: Steven Rostedt <srostedt@redhat.com> > > > > > > Impact: keep kernel text read only > > > > > > Because dynamic ftrace converts the calls to mcount into and out of > > > nops at run time, we needed to always keep the kernel text writable. > > > > > > But this defeats the point of CONFIG_DEBUG_RODATA. This patch converts > > > the kernel code to writable before ftrace modifies the text, and converts > > > it back to read only afterward. > > > > > > The conversion is done via stop_machine and no IPIs may be executed > > > at that time. The kernel text is set to write just before calling > > > stop_machine and set to read only again right afterward. > > > > The very old text poke code I had for this just used a dynamic > > mapping elsewhere instead to modify the code. That's much less > > intrusive than changing the complete mappings. Any reason you can't use > > that too? > > Is it legal to have two mappings of same page with different > attributes? IIRC some processors did not like that...
If you mean PAT caching attributes: correct it is not legal in x86 and causes problems including data corruption. If you mean other attributes like large page vs small page: it's normally legal, with a few exceptions.
-Andi
-- ak@linux.intel.com -- Speaking for myself only.
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