Subject: Re: [PATCH] powerpc/mm: Implemented default_hugepagesz verification for powerpc

From: victora <[email-masked]>
Date: Wed, 05 Jul 2017 13:03:57 -0300

Em 2017-07-05 01:26, Aneesh Kumar K.V escreveu:
> On Tuesday 04 July 2017 01:35 AM, Victor Aoqui wrote:
>> Implemented default hugepage size verification (default_hugepagesz=)
>> in order to allow allocation of defined number of pages (hugepages=)
>> only for supported hugepage sizes.
>> 
>> Signed-off-by: Victor Aoqui <[email-masked]>
>> ---
>>   arch/powerpc/mm/hugetlbpage.c | 15 +++++++++++++++
>>   1 file changed, 15 insertions(+)
>> 
>> diff --git a/arch/powerpc/mm/hugetlbpage.c 
>> b/arch/powerpc/mm/hugetlbpage.c
>> index a4f33de..464e72e 100644
>> --- a/arch/powerpc/mm/hugetlbpage.c
>> +++ b/arch/powerpc/mm/hugetlbpage.c
>> @@ -797,6 +797,21 @@ static int __init hugepage_setup_sz(char *str)
>>   }
>>   __setup("hugepagesz=", hugepage_setup_sz);
>> 
>> +static int __init default_hugepage_setup_sz(char *str)
>> +{
>> +        unsigned long long size;
>> +
>> +        size = memparse(str, &str);
>> +
>> +        if (add_huge_page_size(size) != 0) {
>> +                hugetlb_bad_size();
>> +                pr_err("Invalid default huge page size 
>> specified(%llu)\n", size);
>> +        }
>> +
>> +        return 1;
>> +}
>> +__setup("default_hugepagesz=", default_hugepage_setup_sz);
> 
> isn't that a behavior change in what we have now ? . Right now if size
> specified is not supported, we fallback to HPAGE_SIZE.

Yes, it is. However, is this a correct behavior? If we specify an 
unsupported value, for example default_hugepagesz=1M and hugepages=1000, 
1M will be ignored and 1000 pages of 16M (arch default) will be 
allocated. This could lead to non-expected out of of memory/performance 
issue.

> 
> mm/hugetlb.c
> 
> if (!size_to_hstate(default_hstate_size)) {
> 	default_hstate_size = HPAGE_SIZE;
> 	if (!size_to_hstate(default_hstate_size))
> 		hugetlb_add_hstate(HUGETLB_PAGE_ORDER);
> }
> 
> 
>> +
>>   struct kmem_cache *hugepte_cache;
>>   static int __init hugetlbpage_init(void)
>>   {
>> 
> 
> Even if we want to do this, this should be done in generic code and
> should not be powerpc specific
> 

The verification of supported powerpc hugepage size (hugepagesz=) is 
being performed on add_huge_page_size(), which is currently defined in 
arch/powerpc/mm/hugetlbpage.c. I think it makes more sense to implement 
default_hugepagesz= verification on arch/powerpc, don't you think?

> -aneesh