linux-kernel - Re: [PATCH] Fix display of Maximum Memory

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Message-ID: <eb6c4171-09b3-b8f0-6219-38757da6fc3b@linux.ibm.com>
Date:   Wed, 15 Jan 2020 08:30:02 -0600
From:   Michael Bringmann <mwb@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@....fr>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Cc:     Gustavo Walbon <gwalbon@...ux.ibm.com>,
        Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH] Fix display of Maximum Memory

On 1/14/20 11:41 PM, Christophe Leroy wrote:
> 
> 
> Le 14/01/2020 à 22:07, Michael Bringmann a écrit :
>> Correct overflow problem in calculation+display of Maximum Memory
>> value to syscfg where 32bits is insufficient.
>>
>> Signed-off-by: Michael Bringmann <mwb@...ux.ibm.com>
>> ---
>>   arch/powerpc/platforms/pseries/lparcfg.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/powerpc/platforms/pseries/lparcfg.c b/arch/powerpc/platforms/pseries/lparcfg.c
>> index 4ee2594..183aeb7 100644
>> --- a/arch/powerpc/platforms/pseries/lparcfg.c
>> +++ b/arch/powerpc/platforms/pseries/lparcfg.c
>> @@ -435,12 +435,12 @@ static void parse_em_data(struct seq_file *m)
>>
>>   static void maxmem_data(struct seq_file *m)
>>   {
>> -       unsigned long maxmem = 0;
>> +       unsigned long long maxmem = 0;
> 
> What about using u64 instead, for readability ?

Okay.
> 
>>
>> -       maxmem += drmem_info->n_lmbs * drmem_info->lmb_size;
>> -       maxmem += hugetlb_total_pages() * PAGE_SIZE;
>> +       maxmem += (unsigned long long)drmem_info->n_lmbs * (unsigned long long)drmem_info->lmb_size;
> 
> This line is likely too long. You only need to cast one of the two operants to force a 64 bits multiply. And using u64 would shorten the line.
> 
> Can both multiplications overflow ?

Yes.

> 
> Christophe
> 
>> +       maxmem += (unsigned long long)hugetlb_total_pages() * (unsigned long long)PAGE_SIZE;
>>
>> -       seq_printf(m, "MaxMem=%ld\n", maxmem);
>> +       seq_printf(m, "MaxMem=%llu\n", maxmem);
>>   }
>>
>>   static int pseries_lparcfg_data(struct seq_file *m, void *v)
>>
> 

Thanks.
-- 
Michael W. Bringmann
Linux Technology Center
IBM Corporation
Tie-Line  363-5196
External: (512) 286-5196
Cell:       (512) 466-0650
mwb@...ux.ibm.com