linux-kernel - Re: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts in do_page

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Message-ID: <1496405473.2842.9.camel@kernel.crashing.org>
Date:   Fri, 02 Jun 2017 22:11:13 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Christophe LEROY <christophe.leroy@....fr>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts
 in do_page_fault()

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.