linux-kernel - Re: [RFC PATCH v1 00/17] ban the use of _PAGE_XXX flags outside platform specific code

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Message-ID: <fe0afb3c-8b6c-fff1-47ee-4fc25c0bc8a6@c-s.fr>
Date:   Wed, 12 Sep 2018 18:07:57 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, npiggin@...il.com,
        aneesh.kumar@...ux.vnet.ibm.com
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH v1 00/17] ban the use of _PAGE_XXX flags outside
 platform specific code



Le 06/09/2018 à 11:58, Aneesh Kumar K.V a écrit :
> Christophe Leroy <christophe.leroy@....fr> writes:
> 
>> Today flags like for instance _PAGE_RW or _PAGE_USER are used through
>> common parts of code.
>> Using those directly in common parts of code have proven to lead to
>> mistakes or misbehaviour, because their use is not always as trivial
>> as one could think.
>>
>> For instance, (flags & _PAGE_USER) == 0 isn't enough to tell
>> that a page is a kernel page, because some targets are using
>> _PAGE_PRIVILEDGED and not _PAGE_USER, so the test has to be
>> (flags & (_PAGE_USER | _PAGE_PRIVILEDGED)) == _PAGE_PRIVILEDGED
>> This has to (bad) consequences:
>>
>>   - All targets must define every bit, even the unsupported ones,
>>     leading to a lot of useless #define _PAGE_XXX 0
>>   - If someone forgets to take into account all possible _PAGE_XXX bits
>>     for the case, we can get unexpected behaviour on some targets.
>>
>> This becomes even more complex when we come to using _PAGE_RW.
>> Testing (flags & _PAGE_RW) is not enough to test whether a page
>> if writable or not, because:
>>
>>   - Some targets have _PAGE_RO instead, which has to be unset to tell
>>     a page is writable
>>   - Some targets have _PAGE_R and _PAGE_W, in which case
>>     _PAGE_RW = _PAGE_R | _PAGE_W
>>   - Even knowing whether a page is readable is not always trivial because:
>>     - Some targets requires to check that _PAGE_R is set to ensure page
>>     is readable
>>     - Some targets requires to check that _PAGE_NA is not set
>>     - Some targets requires to check that _PAGE_RO or _PAGE_RW is set
>>
>> Etc ....
>>
>> In order to work around all those issues and minimise the risks of errors,
>> this serie aims at removing all use of _PAGE_XXX flags from powerpc code
>> and always use pte_xxx() and pte_mkxxx() accessors instead. Those accessors
>> are then defined in target specific parts of the kernel code.
> 
> The series is really good. It also helps in code readability. Few things
> i am not sure there is a way to reduce the overhead
> 
> -		access = _PAGE_EXEC;
> +		access = pte_val(pte_mkexec(__pte(0)));
> 
> Considering we have multiple big endian to little endian coversion there
> for book3s 64.
> 
> Other thing is __ioremap_at where we do
> 
> +       pte_t pte = __pte(flags);
>   
>          /* Make sure we have the base flags */
> -       if ((flags & _PAGE_PRESENT) == 0)
> +       if (!pte_present(pte))
> 
> -               err = map_kernel_page(v+i, p+i, flags);
> +               err = map_kernel_page(v + i, p + i, pte_val(pte));
> 

Finally, for that I now ensure that all base flags are set by the 
callers and I have removed that hack which adds PAGE_KERNEL flags when 
_PAGE_PRESENT is not in the handedover flags.

> 
> But otherwise for the series.
> 
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
> 

Thanks, I added it to all unchanged patches of the serie. You are 
welcome to give a feedback on the new ones if you have time.

Christophe