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Subject: Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting
From:   David Hildenbrand <david@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Alexander Duyck <alexander.duyck@gmail.com>,
        Nitesh Narayan Lal <nitesh@redhat.com>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>, lcapitulino@redhat.com,
        pagupta@redhat.com, wei.w.wang@intel.com,
        Yang Zhang <yang.zhang.wz@gmail.com>,
        Rik van Riel <riel@surriel.com>, dodgen@google.com,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        dhildenb@redhat.com, Andrea Arcangeli <aarcange@redhat.com>
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Date:   Tue, 19 Feb 2019 15:45:58 +0100
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>>>>
>>>> Also one reason why I am not a fan of working with anything less than
>>>> PMD order is because there have been issues in the past with false
>>>> memory leaks being created when hints were provided on THP pages that
>>>> essentially fragmented them. I guess hugepaged went through and
>>>> started trying to reassemble the huge pages and as a result there have
>>>> been apps that ended up consuming more memory than they would have
>>>> otherwise since they were using fragments of THP pages after doing an
>>>> MADV_DONTNEED on sections of the page.
>>>
>>> I understand your concerns, but we should not let bugs in the hypervisor
>>> dictate the design. Bugs are there to be fixed. Interesting read,
>>> though, thanks!
>>
>> Right but if we break up a huge page we are then creating
>> more work for hypervisor to reassemble it.
> 
> Yes, but the hypervisor can decide what to do. E.g. on s390x there are
> no THP, so nothing to break up.

To clarify as that might be confusing: No THP in a KVM guest mapping for
now.

> 
> It is all very complicated :)
> 


-- 

Thanks,

David / dhildenb