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

[Michal Hocko <mhocko@kernel.org>]

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.

-- 
Michal Hocko
SUSE Labs

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