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

[Huang\, Ying]

Michal Hocko <mhocko@xxxxxxxxxx> writes:

> On Sat 01-04-17 12:47:56, Huang, Ying wrote:
>> Hi, Michal,
>> 
>> Michal Hocko <mhocko@xxxxxxxxxx> 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@xxxxxxxx>

Thanks, will add the reasoning.

Best Regards,
Huang, Ying