On Mon, 28 Jan 2019 15:03:28 -0500 Rik van Riel <riel@xxxxxxxxxxx> 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 <riel@xxxxxxxxxxx>
> > 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.
