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

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Date:   Wed, 5 Apr 2017 15:43:49 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.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>,
        Tim Chen <tim.c.chen@...ux.intel.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 02:56 PM, 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.

Sorry, I know I'm too late for this discussion, just wanted to clarify a
bit.

> 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).

If by "theoretically" you mean we switch kmalloc() from a buddy
allocator to something else, then yes. Otherwise, in the buddy
allocator, it cannot cross the 2M boundary by design.

> That means it will only "kill"
> the possibility of a single 2M page.  More 2M pages == less fragmentation.

IMHO John is right that kmalloc() will reduce the number of high-order
pages *in the short term*. But in the long term, vmalloc() will hurt us
more due to the scattering of unmovable pages as you describe. As this
is AFAIU a long-term allocation, kmalloc() should be preferred.

Vlastimil

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