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

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Message-ID: <f7290b3e-0f74-cf29-52fb-5644894e9702@c-s.fr>
Date:   Fri, 2 Jun 2017 14:31:47 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        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()



Le 02/06/2017 à 14:11, Benjamin Herrenschmidt a écrit :
> 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.
> 

No I don't have. Taking into account the patch following this serie 
which limits even more the calls to get_user(), it is probably not worth 
it anymore (see https://patchwork.ozlabs.org/patch/757564/)

I will then have to resubmit the entire serie (including that additional 
one), but there is no get_user_inatomic() so will have to either:
- do the access_ok() verification inside the function
- get back to v2 (https://patchwork.ozlabs.org/patch/756234/)
- implement an get_user_inatomic() function

What would be the best ?

Christophe