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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>