linux-kernel - Re: [PATCH V2] mm/page_alloc: Add alloc_contig

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Message-ID: <fe8cae46-6bd8-88eb-d3fe-2740bb79ee58@redhat.com>
Date:   Wed, 16 Oct 2019 14:29:05 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-mm@...ck.org, Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] mm/page_alloc: Add alloc_contig_pages()

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.

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".

b) Eventually adding something like

if (WARN_ON_ONCE(!is_power_of_2(nr_pages)))
	return NULL;

Once we need other granularities we can think about the alignment and an 
optimized search-process.

-- 

Thanks,

David / dhildenb