linux-kernel - Re: [RESENDING PATCH] powerpc/wii: properly disable use of BATs when requested.

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Message-ID: <19fd4dd4-ece2-d508-1ed5-5a9ed2aa84fe@c-s.fr>
Date:   Thu, 17 Jan 2019 11:29:06 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Jonathan Neuschäfer <j.neuschaefer@....net>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        stable@...r.kernel.org
Subject: Re: [RESENDING PATCH] powerpc/wii: properly disable use of BATs when
 requested.

Le 17/01/2019 à 02:05, Jonathan Neuschäfer a écrit :
> Hi again,
> 
> On Tue, Jan 15, 2019 at 04:43:20PM +0000, Christophe Leroy wrote:
>> 'nobats' kernel parameter or some options like CONFIG_DEBUG_PAGEALLOC
>> deny the use of BATS for mapping memory.
>>
>> This patch makes sure that the specific wii RAM mapping function
>> takes it into account as well.
>>
>> Fixes: de32400dd26e ("wii: use both mem1 and mem2 as ram")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>> ---
> [...]
>>   	/* MEM2 64MB@...0000000 */
>>   	delta = wii_hole_start + wii_hole_size;
>> +	if (__map_without_bats)
>> +		return delta;
>> +
> 
> Nothing is visibly broken without this patch, even with
> CONFIG_DEBUG_PAGEALLOC (tested on top of v5.0-rc2), but the patch still
> looks correct.

Obviously, CONFIG_DEBUG_PAGEALLOC cannot work without this patch.
The purpose of CONFIG_DEBUG_PAGEALLOC is to unmap unused parts of memory 
so that any access to them will pagefault.

As this function inconditionnaly sets a BAT for the second block of RAM, 
any access to free area in the upper block will be granted without a fault.

I think you can test it by doing a kmalloc() then a kfree(), then try to 
read in that area (hopefully kmalloc() allocates memory from the top so 
it should go in the upper block). Maybe there is an LKDTM test for that.

> 
> I'd prefer the 'if' block to be before the whole delta/size calculation,
> to make the code slightly more readable because the delta and size
> calculations stay in one visual block. It doesn't need to happen after
> delta is calculated.

Euh ... the function has to return 'delta', so if I put the if block 
before the calculation of delta, it means we have to calculate delta twice:

	if (__map_without_bats)
		return wii_hole_start + wii_hole_size;

	delta = wii_hole_start + wii_hole_size;

My eyes don't really like it, so if we want to keep delta and size 
calculation together, the 'if' will go after calculation of size.

In anycase, this change is only really for fixing stable releases 
because this function will go away with my other serie.

Christophe

> 
> tentatively,
> Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@....net>
> 
> 
> Thanks,
> Jonathan
>