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

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Date:   Thu, 17 Oct 2019 09:21:24 +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 17.10.19 09:11, Michal Hocko wrote:
> On Thu 17-10-19 10:44:41, Anshuman Khandual wrote:
> [...]
>> Does this add-on documentation look okay ? Should we also mention about the
>> possible reduction in chances of success during pfn block search for the
>> non-power-of-two cases as the implicit alignment will probably turn out to
>> be bigger than nr_pages itself ?
>>
>>   * Requested nr_pages may or may not be power of two. The search for suitable
>>   * memory range in a zone happens in nr_pages aligned pfn blocks. But in case
>>   * when nr_pages is not power of two, an implicitly aligned pfn block search
>>   * will happen which in turn will impact allocated memory block's alignment.
>>   * In these cases, the size (i.e nr_pages) and the alignment of the allocated
>>   * memory will be different. This problem does not exist when nr_pages is power
>>   * of two where the size and the alignment of the allocated memory will always
>>   * be nr_pages.
> 
> I dunno, it sounds more complicated than really necessary IMHO. Callers
> shouldn't really be bothered by memory blocks and other really deep
> implementation details.. Wouldn't be the below sufficient?
> 
> The allocated memory is always aligned to a page boundary. If nr_pages
> is a power of two then the alignement is guaranteed to be to the given

s/alignement/alignment/

and "the PFN is guaranteed to be aligned to nr_pages" (the address is 
aligned to nr_pages*PAGE_SIZE)

> nr_pages (e.g. 1GB request would be aligned to 1GB).
> 

I'd probably add "This function will miss allocation opportunities if 
nr_pages is not a power of two (and the implicit alignment is bogus)."

-- 

Thanks,

David / dhildenb