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

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Date:   Wed, 17 Jul 2019 10:34:15 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>, 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 Wed, Jul 17, 2019 at 04:10:47PM +0200, David Hildenbrand wrote:
> 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".

Right but it's possible we exhaust all memory, then
someone else asks for a single page and that invokes
the shrinker.

> 
> 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

Sorry I was unclear.  The question was about
VIRTIO_BALLOON_F_FREE_PAGE_HINT specifically.

-- 
MST