On Wed, Oct 24, 2012 at 12:22:53PM -0700, Andrew Morton wrote: > On Wed, 24 Oct 2012 02:38:01 +0300 > "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote: > > > 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. > > I don't understand this. If reclaim is running continuously (which can > happen pretty easily: "dd if=/fast-disk/large-file") then the zero page > will be whipped away very shortly after its refcount has fallen to > zero. > > > 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. > > It would be useful if you could try to make this scenario happen. If > for some reason it doesn't happen then let's understand *why* it > doesn't happen. > > I'm thinking that such a workload would be the above dd in parallel > with a small app which touches the huge page and then exits, then gets > executed again. That "small app" sounds realistic to me. Obviously > one could exercise the zero page's refcount at higher frequency with a > tight map/touch/unmap loop, but that sounds less realistic. It's worth > trying that exercise as well though. > > Or do something else. But we should try to probe this code's > worst-case behaviour, get an understanding of its effects and then > decide whether any such workload is realisic enough to worry about. Okay, I'll try few memory pressure scenarios. Meanwhile, could you take patches 01-09? Patch 09 implements simpler allocation scheme. It would be nice to get all other code tested. Or do you see any other blocker? -- Kirill A. Shutemov
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