linux-kernel - Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

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Date:   Tue, 19 Feb 2019 15:45:58 +0100
From:   David Hildenbrand <david@...hat.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>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, lcapitulino@...hat.com,
        pagupta@...hat.com, wei.w.wang@...el.com,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Rik van Riel <riel@...riel.com>, dodgen@...gle.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        dhildenb@...hat.com, Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

>>>>
>>>> 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