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

[Dave Hansen]

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.