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Subject: Re: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts
 in do_page_fault()
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Christophe LEROY <christophe.leroy@c-s.fr>, Michael Ellerman
 <mpe@ellerman.id.au>, Paul Mackerras <paulus@samba.org>, Scott Wood
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Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Date: Fri, 02 Jun 2017 22:11:13 +1000
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References: <cover.1492606297.git.christophe.leroy@c-s.fr>
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On Fri, 2017-06-02 at 11:39 +0200, Christophe LEROY wrote:
> The difference between get_user() and __get_user() is that get_user() 
> performs an access_ok() in addition.
> 
> Doesn't access_ok() only checks whether addr is below TASK_SIZE to 
> ensure it is a valid user address ?

Do you have a measurable improvement by skipping that check ? I agree
with your reasoning but I'm also paranoid and so I wouldn't change it
unless it's really worth it.

Cheers,
Ben.