linux-kernel - Re: use of shrinker in virtio balloon free page hinting

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Message-ID: <9c4a36ab-cfcb-f06e-355a-f118f3fcdb62@redhat.com>
Date:   Wed, 17 Jul 2019 13:32:05 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     wei.w.wang@...el.com, Nitesh Narayan Lal <nitesh@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yang Zhang <yang.zhang.wz@...il.com>, pagupta@...hat.com,
        Rik van Riel <riel@...riel.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        lcapitulino@...hat.com, Andrea Arcangeli <aarcange@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>, dan.j.williams@...el.com,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Subject: Re: use of shrinker in virtio balloon free page hinting

On 17.07.19 13:20, Michael S. Tsirkin wrote:
> Wei, others,
> 
> ATM virtio_balloon_shrinker_scan will only get registered
> when deflate on oom feature bit is set.
> 
> Not sure whether that's intentional.  Assuming it is:
> 
> virtio_balloon_shrinker_scan will try to locate and free
> pages that are processed by host.
> The above seems broken in several ways:
> - count ignores the free page list completely
> - if free pages are being reported, pages freed
>   by shrinker will just get re-allocated again
> 
> I was unable to make this part of code behave in any reasonable
> way - was shrinker usage tested? What's a good way to test that?
> 
> Thanks!
> 

Some companies are using deflate-on-oom for some kind of "auto
ballooning" approach, although I don't think it's a good idea.

In these scenarios, the total ramsize (cat /proc/meminfo) will not
change on inflation/deflation. So from a system POV, inflated memory is
simply allocated memory without affecting the total memory.

VMs will automatically "reclaim" inflated memory when they need it -
which is usually not what hypervisors want (especially when talking
about using ballooning for memory hotunplug).

So yes, it makes perfect sense that the shrinker is only registered for
deflate-on-oom.

-- 

Thanks,

David / dhildenb