linux-kernel - Re: [PATCH] mm,slab,vmscan: accumulate gradual pressure on small slabs

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Date:   Mon, 28 Jan 2019 15:34:03 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-team@...com, Johannes Weiner <hannes@...xchg.org>,
        Chris Mason <clm@...com>, Roman Gushchin <guro@...com>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm,slab,vmscan: accumulate gradual pressure on small
 slabs

On Mon, 2019-01-28 at 12:10 -0800, Andrew Morton wrote:
> On Mon, 28 Jan 2019 15:03:28 -0500 Rik van Riel <riel@...riel.com>
> 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@...riel.com
> > > >
> > > wrote:
> > > 
> > > > 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;

When delta is a non-zero number, we subtract (delta << priority)
from shrinker->small_scan.

That should happen every time delta >= (1<<priority), which is
4096 for DEF_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.
-- 
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