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Subject: Re: use of shrinker in virtio balloon free page hinting
To:     "Michael S. Tsirkin" <mst@redhat.com>,
        Alexander Duyck <alexander.duyck@gmail.com>
Cc:     wei.w.wang@intel.com, Nitesh Narayan Lal <nitesh@redhat.com>,
        kvm list <kvm@vger.kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Yang Zhang <yang.zhang.wz@gmail.com>, pagupta@redhat.com,
        Rik van Riel <riel@surriel.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        lcapitulino@redhat.com, Andrea Arcangeli <aarcange@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>, dan.j.williams@intel.com,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>
References: <20190717071332-mutt-send-email-mst@kernel.org>
From:   David Hildenbrand <david@redhat.com>
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Message-ID: <959237f9-22cc-1e57-e07d-b8dc3ddf9ed6@redhat.com>
Date:   Wed, 17 Jul 2019 16:10:47 +0200
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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

Trying to answer your questions (not sure if I fully understood what you
mean)

virtio_balloon_shrinker_scan() will not be called due to inflation
requests (balloon_page_alloc()). It will be called whenever the system
is OOM, e.g., when starting a new application.

I assume you were expecting the shrinker getting called due to
balloon_page_alloc(). however, that is not the case as we pass
"__GFP_NORETRY".


To test, something like:

1. Start a VM with

-device virtio-balloon-pci,deflate-on-oom=true

2. Inflate the balloon, e.g.,

QMP: balloon 1024
QMP: info balloon
-> 1024

See how "MemTotal" in /proc/meminfo in the guest won't change

3. Run a workload that exhausts memory in the guest (OOM).

See how the balloon was automatically deflated

QMP: info balloon
-> Something bigger than 1024


Not sure if it is broken, last time I played with it, it worked, but
that was ~1-2 years ago.

-- 

Thanks,

David / dhildenb