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

[David Hildenbrand <david@redhat.com>]

On 16.10.19 17:31, Anshuman Khandual wrote:
> 
> 
> 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.

As long as it is documented that implicit alignment will happen, fine 
with me.

The thing about !is_power_of2() is that we usually don't need an 
alignment there (or instead an explicit one). And as I mentioned, the 
current function might fail easily to allocate a suitable range due to 
the way the search works (== check aligned blocks only). The search 
really only provides reliable results when size==alignment and it's a 
power of two IMHO. Not documenting that is in my opinion misleading - 
somebody who wants !is_power_of2() and has no alignment requirements 
should probably rework the function first.

So with some documentation regarding that

Acked-by: David Hildenbrand <david@redhat.com>

-- 

Thanks,

David / dhildenb