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

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  Re: [PATCH -v2 1/2] mm, swap: Use kvzalloc to allocate some swap data  structure Vlastimil Babka <vbabka@suse.cz> - 2017-04-05 15:50 +0200

#1616943 — 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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