Re: [PATCH v4 10/10] thp: implement refcounting for huge zero page

From: Andrew Morton
Date: Tue Oct 23 2012 - 18:59:12 EST

On Tue, 23 Oct 2012 10:00:18 +0300
"Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:

> > Well, how hard is it to trigger the bad behavior? One can easily
> > create a situation in which that page's refcount frequently switches
> > from 0 to 1 and back again. And one can easily create a situation in
> > which the shrinkers are being called frequently. Run both at the same
> > time and what happens?
> If the goal is to trigger bad behavior then:
> 1. read from an area where a huge page can be mapped to get huge zero page
> mapped. hzp is allocated here. refcounter == 2.
> 2. write to the same page. refcounter == 1.
> 3. echo 3 > /proc/sys/vm/drop_caches. refcounter == 0 -> free the hzp.
> 4. goto 1.
> But it's unrealistic. /proc/sys/vm/drop_caches is only root-accessible.

Yes, drop_caches is uninteresting.

> We can trigger shrinker only under memory pressure. But in this, most
> likely we will get -ENOMEM on hzp allocation and will go to fallback path
> (4k zero page).

I disagree. If, for example, there is a large amount of clean
pagecache being generated then the shrinkers will be called frequently
and memory reclaim will be running at a 100% success rate. The
hugepage allocation will be successful in such a situation?

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