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" [off-list ref] wrote:

quoted

quoted

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.

quoted

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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