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

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Date:   Wed, 05 Apr 2017 08:49:13 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     "Huang\, Ying" <ying.huang@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>,
        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>,
        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

Michal Hocko <mhocko@...nel.org> writes:

> On Sat 01-04-17 12:47:56, Huang, Ying wrote:
>> Hi, Michal,
>> 
>> Michal Hocko <mhocko@...nel.org> writes:
>> 
>> > On Fri 24-03-17 06:56:10, 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.
>> >
>> > Yes I agree with this. And the patch is no brainer. kvmalloc makes sure
>> > to not try too hard on the kmalloc side so I really didn't get the
>> > objection about direct compaction and reclaim which initially started
>> > this discussion. Besides that the swapon path usually happens early
>> > during the boot where we should have those larger blocks available.
>> 
>> Could I add your Acked-by for this patch?
>
> Yes but please add the reasoning pointed out by Dave. As the patch
> doesn't give any numbers and it would be fairly hard to add some without
> artificial workloads we should at least document our current thinking
> so that we can revisit it later.
>
> Thanks!
>
> Acked-by: Michal Hocko <mhocko@...e.com>

Thanks, will add the reasoning.

Best Regards,
Huang, Ying