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

From: Vlastimil Babka <vbabka@suse.cz>
To: Dave Hansen <dave.hansen@linux.intel.com>,
	John Hubbard <jhubbard@nvidia.com>,
	"Huang, Ying" <ying.huang@intel.com>
Cc: David Rientjes <rientjes@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andi Kleen <ak@linux.intel.com>, Shaohua Li <shli@kernel.org>,
	Rik van Riel <riel@redhat.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Michal Hocko <mhocko@suse.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Aaron Lu <aaron.lu@intel.com>,
	Gerald Schaefer <gerald.schaefer@de.ibm.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Hugh Dickins <hughd@google.com>, Ingo Molnar <mingo@kernel.org>,
	Vegard Nossum <vegard.nossum@oracle.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH -v2 1/2] mm, swap: Use kvzalloc to allocate some swap data structure
Date: Wed, 5 Apr 2017 15:43:49 +0200	[thread overview]
Message-ID: <d7fd1c69-2e0e-39ec-dfd8-16269f0cb898@suse.cz> (raw)
In-Reply-To: <e79064f1-8594-bef2-fbd8-1579afb4aac3@linux.intel.com>

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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