Re: [PATCH V2] mm/page_alloc: Add alloc_contig_pages()

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 10/16/2019 06:11 PM, Michal Hocko wrote:
> On Wed 16-10-19 14:29:05, David Hildenbrand wrote:
>> On 16.10.19 13:51, Michal Hocko wrote:
>>> On Wed 16-10-19 16:43:57, Anshuman Khandual wrote:
>>>>
>>>>
>>>> On 10/16/2019 04:39 PM, David Hildenbrand wrote:
>>> [...]
>>>>> Just to make sure, you ignored my comment regarding alignment
>>>>> although I explicitly mentioned it a second time? Thanks.
>>>>
>>>> I had asked Michal explicitly what to be included for the respin. Anyways
>>>> seems like the previous thread is active again. I am happy to incorporate
>>>> anything new getting agreed on there.
>>>
>>> Your patch is using the same alignment as the original code would do. If
>>> an explicit alignement is needed then this can be added on top, right?
>>>
>>
>> Again, the "issue" I see here is that we could now pass in numbers that are
>> not a power of two. For gigantic pages it was clear that we always have a
>> number of two. The alignment does not make any sense otherwise.

ALIGN() does expect nr_pages two be power of two otherwise the mask
value might not be correct, affecting start pfn value for a zone.

#define ALIGN(x, a)             	__ALIGN_KERNEL((x), (a))
#define __ALIGN_KERNEL(x, a)            __ALIGN_KERNEL_MASK(x, (typeof(x))(a) - 1)
#define __ALIGN_KERNEL_MASK(x, mask)    (((x) + (mask)) & ~(mask))

>>
>> What I'm asking for is
>>
>> a) Document "The resulting PFN is aligned to nr_pages" and "nr_pages should
>> be a power of two".
> 
> OK, this makes sense.
Sure, will add this to the alloc_contig_pages() helper description and
in the commit message as well.

> 
>> b) Eventually adding something like
>>
>> if (WARN_ON_ONCE(!is_power_of_2(nr_pages)))
>> 	return NULL;
> 
> I am not sure this is really needed.
> 
Just wondering why not ? Ideally if we are documenting that nr_pages should be
power of two, then we should abort erring allocation request with an warning. Is
it because allocation can still succeed for non-power-of-two requests despite
possible problem with mask and alignment ? Hence there is no need to abort it.