linux-kernel - Re: [PATCH -v2 1/2] mm, swap: Use kvzalloc to allocate some swap data structure

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Date:   Fri, 24 Mar 2017 11:15:49 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>
CC:     David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>, Shaohua Li <shli@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Aaron Lu <aaron.lu@...el.com>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Vegard Nossum <vegard.nossum@...cle.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -v2 1/2] mm, swap: Use kvzalloc to allocate some swap data
 structure

On 03/24/2017 09:52 AM, Tim Chen wrote:
> On Fri, 2017-03-24 at 06:56 -0700, Dave Hansen wrote:
>> On 03/24/2017 12:33 AM, John Hubbard wrote:
>>>
>>> There might be some additional information you are using to come up with
>>> that conclusion, that is not obvious to me. Any thoughts there? These
>>> calls use the same underlying page allocator (and I thought that both
>>> were subject to the same constraints on defragmentation, as a result of
>>> that). So I am not seeing any way that kmalloc could possibly be a
>>> less-fragmenting call than vmalloc.
>> You guys are having quite a discussion over a very small point.
>>
>> But, Ying is right.
>>
>> Let's say we have a two-page data structure.  vmalloc() takes two
>> effectively random order-0 pages, probably from two different 2M pages
>> and pins them.  That "kills" two 2M pages.
>>
>> kmalloc(), allocating two *contiguous* pages, is very unlikely to cross
>> a 2M boundary (it theoretically could).  That means it will only "kill"
>> the possibility of a single 2M page.  More 2M pages == less fragmentation.
>
> In vmalloc, it eventually calls __vmalloc_area_node that allocates the
> page one at a time.  There's no attempt there to make the pages contiguous
> if I am reading the code correctly.  So that will increase the memory
> fragmentation as we will be piecing together pages from all over the places.
>
> Tim

OK. Thanks everyone for spelling it out for me, before I started doing larger projects, with an 
incorrect way of looking at the fragmentation behavior. :)

--
thanks,
john h