Subject: Re: [patch 02/02] Debug option to write-protect rodata: the write
	protect logic and config option
From: Arjan van de Ven <arjan@infradead.org>
To: Coywolf Qi Hunt <coywolf@gmail.com>
Cc: Josh Boyer <jdub@us.ibm.com>, linux-kernel@vger.kernel.org, ak@suse.de,
       akpm@osdl.org
In-Reply-To: <2cd57c900511110139v221ed3f3m@mail.gmail.com>
References: <20051107105624.GA6531@infradead.org>
	 <20051107105807.GB6531@infradead.org>
	 <1131372374.23658.1.camel@windu.rchland.ibm.com>
	 <1131373248.2858.17.camel@laptopd505.fenrus.org>
	 <2cd57c900511110139v221ed3f3m@mail.gmail.com>
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Date: Fri, 11 Nov 2005 10:47:07 +0100
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> people objecting to that.
> >
> > (It's not clear cut: while the last bit of the kernel no longer is
> > covered by a 2Mb tlb, most intel cpus have very few of such tlbs in the
> > first place and this would free up one such tlb for other things (say
> > the stack data) or even the userspace database), so it's not all that
> > clear cut what the cost of this is)
> 
> I'm dumb. But how is "the last bit of the kernel no longer is covered
> by a 2Mb tlb"? Could you explain a bit more?

in memory it'll look something like this

0                 2                   4                         6
-- kernel text -- + -- kernel text -- + --- k. text-- rodata -- + --    

normally the range from 0 to 6 is covered with 2Mb tlb's.
Now to make rodata read only, the hugetlb entry covering 4-6 Mb range
needs to be split into 4Kb entries, so that the rodata portion can have
different permissions than the rest of that range.


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