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

[Kirill A. Shutemov]

On Tue, Oct 23, 2012 at 03:59:15PM -0700, Andrew Morton wrote:
> 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?

Yes.

Shrinker callbacks are called from shrink_slab() which happens after page
cache reclaim, so on next reclaim round page cache will reclaim first and
we will avoid frequent alloc-free pattern.

One more thing we can do: increase shrinker->seeks to something like
DEFAULT_SEEKS * 4. In this case shrink_slab() will call our callback after
callbacks with DEFAULT_SEEKS.

-- 
 Kirill A. Shutemov
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