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

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Date:   Thu, 18 Jul 2019 17:08:41 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     Alexander Duyck <alexander.duyck@...il.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>,
        "Hansen, Dave" <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" <pagupta@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        "lcapitulino@...hat.com" <lcapitulino@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Williams, Dan J" <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 07/18/2019 02:47 PM, Michael S. Tsirkin wrote:
> On Thu, Jul 18, 2019 at 02:30:01PM +0800, Wei Wang wrote:
>> On 07/18/2019 01:58 PM, Michael S. Tsirkin wrote:
>>> what if it does not fail?
>>>
>>>
>>>> Shrinker is called on system memory pressure. On memory pressure
>>>> get_free_page_and_send will fail memory allocation, so it stops allocating
>>>> more.
>>> Memory pressure could be triggered by an unrelated allocation
>>> e.g. from another driver.
>> As memory pressure is system-wide (no matter who triggers it), free page
>> hinting
>> will fail on memory pressure, same as other drivers.
> That would be good.  Except instead of failing it can hit a race
> condition where it will reallocate memory freed by shrinker. Not good.

OK..I could see this when another module does allocation, which triggers 
kswapd
to have balloon's shrinker release some memory, which could be eaten by 
balloon
quickly again before that module takes it, and this could happen repeatedly
in theory.

So add a vb->stop_free_page_report boolean, set it in shrinker_count, 
and clear it in
virtio_balloon_queue_free_page_work?

Best,
Wei