linux-kernel - Re: [PATCH v2 2/2] virtio_balloon: replace oom notifier with shrinker

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Date:   Wed, 1 Aug 2018 13:34:44 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, linux-mm@...ck.org,
        mst@...hat.com, akpm@...ux-foundation.org
Subject: Re: [PATCH v2 2/2] virtio_balloon: replace oom notifier with shrinker

On Wed 01-08-18 19:12:25, Wei Wang wrote:
> On 07/30/2018 05:00 PM, Michal Hocko wrote:
> > On Fri 27-07-18 17:24:55, Wei Wang wrote:
> > > The OOM notifier is getting deprecated to use for the reasons mentioned
> > > here by Michal Hocko: https://lkml.org/lkml/2018/7/12/314
> > > 
> > > This patch replaces the virtio-balloon oom notifier with a shrinker
> > > to release balloon pages on memory pressure.
> > It would be great to document the replacement. This is not a small
> > change...
> 
> OK. I plan to document the following to the commit log:
> 
>   The OOM notifier is getting deprecated to use for the reasons:
>     - As a callout from the oom context, it is too subtle and easy to
>       generate bugs and corner cases which are hard to track;
>     - It is called too late (after the reclaiming has been performed).
>       Drivers with large amuont of reclaimable memory is expected to be
>       released them at an early age of memory pressure;
>     - The notifier callback isn't aware of the oom contrains;
>     Link: https://lkml.org/lkml/2018/7/12/314
> 
>     This patch replaces the virtio-balloon oom notifier with a shrinker
>     to release balloon pages on memory pressure. Users can set the amount of
>     memory pages to release each time a shrinker_scan is called via the
>     module parameter balloon_pages_to_shrink, and the default amount is 256
>     pages. Historically, the feature VIRTIO_BALLOON_F_DEFLATE_ON_OOM has
>     been used to release balloon pages on OOM. We continue to use this
>     feature bit for the shrinker, so the shrinker is only registered when
>     this feature bit has been negotiated with host.

Do you have any numbers for how does this work in practice? Let's say
you have a medium page cache workload which triggers kswapd to do a
light reclaim? Hardcoded shrinking sounds quite dubious to me but I have
no idea how people expect this to work. Shouldn't this be more
adaptive? How precious are those pages anyway?
-- 
Michal Hocko
SUSE Labs