On Mon, 28 Jan 2019 15:03:28 -0500 Rik van Riel <[email protected]> wrote:
> On Mon, 2019-01-28 at 11:54 -0800, Andrew Morton wrote: > > On Mon, 28 Jan 2019 14:35:35 -0500 Rik van Riel <[email protected]> > > wrote: > > > > > /* > > > * Make sure we apply some minimal pressure on default priority > > > - * even on small cgroups. Stale objects are not only consuming > > > memory > > > + * even on small cgroups, by accumulating pressure across > > > multiple > > > + * slab shrinker runs. Stale objects are not only consuming > > > memory > > > * by themselves, but can also hold a reference to a dying > > > cgroup, > > > * preventing it from being reclaimed. A dying cgroup with all > > > * corresponding structures like per-cpu stats and kmem caches > > > * can be really big, so it may lead to a significant waste of > > > memory. > > > */ > > > - delta = max_t(unsigned long long, delta, min(freeable, > > > batch_size)); > > > + if (!delta) { > > > + shrinker->small_scan += freeable; > > > + > > > + delta = shrinker->small_scan >> priority; > > > + shrinker->small_scan -= delta << priority; > > > + > > > + delta *= 4; > > > + do_div(delta, shrinker->seeks); > > > > What prevents shrinker->small_scan from over- or underflowing over > > time? > > We only go into this code path if > delta >> DEF_PRIORITY is zero. > > That is, freeable is smaller than 4096. > I'm still not understanding. If `freeable' always has a value of (say) 1, we'll eventually overflow shrinker->small_scan? Or at least, it's unobvious why this cannot happen.
