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Message-ID: <d4e9359fdb3817bbc2b2c7cea368f8fd0dc7da62.camel@surriel.com>
Subject: Re: [PATCH] mm,slab,vmscan: accumulate gradual pressure on small
 slabs
From:   Rik van Riel <riel@surriel.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        kernel-team@fb.com, Johannes Weiner <hannes@cmpxchg.org>,
        Chris Mason <clm@fb.com>, Roman Gushchin <guro@fb.com>,
        Michal Hocko <mhocko@suse.com>
Date:   Mon, 28 Jan 2019 15:34:03 -0500
In-Reply-To: <20190128121028.8ef4c19dd3fd1d60d2e3284c@linux-foundation.org>
References: <20190128143535.7767c397@imladris.surriel.com>
         <20190128115424.df3f4647023e9e43e75afe67@linux-foundation.org>
         <8ddf2ea674711f373062f4e056dd14fb81c5a2fe.camel@surriel.com>
         <20190128121028.8ef4c19dd3fd1d60d2e3284c@linux-foundation.org>
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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@surriel.com>
> wrote:
>=20
> > 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@surriel.com
> > > >
> > > wrote:
> > >=20
> > > > memory.
> > > >  	 */
> > > > -	delta =3D max_t(unsigned long long, delta, min(freeable,
> > > > batch_size));
> > > > +	if (!delta) {
> > > > +		shrinker->small_scan +=3D freeable;
> > > > +
> > > > +		delta =3D shrinker->small_scan >> priority;
> > > > +		shrinker->small_scan -=3D delta << priority;

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

That should happen every time delta >=3D (1<<priority), which is
4096 for DEF_PRIORITY.

> > > > +
> > > > +		delta *=3D 4;
> > > > +		do_div(delta, shrinker->seeks);
> > >=20
> > > What prevents shrinker->small_scan from over- or underflowing
> > > over
> > > time?
> >=20
> > We only go into this code path if
> > delta >> DEF_PRIORITY is zero.
> >=20
> > That is, freeable is smaller than 4096.
> >=20
>=20
> 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.
--=20
All Rights Reversed.

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