linux-kernel - Re: Thoughts on simple scanner approach for free page hinting

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Message-ID: <ba84ec1f-79f2-e15b-e225-7d2e2885a4c6@redhat.com>
Date:   Tue, 9 Apr 2019 09:05:16 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     Nitesh Narayan Lal <nitesh@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.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: Thoughts on simple scanner approach for free page hinting

On 09.04.19 04:44, Michael S. Tsirkin wrote:
> On Fri, Apr 05, 2019 at 05:09:45PM -0700, Alexander Duyck wrote:
>> In addition we will need some way to identify which pages have been
>> hinted on and which have not. The way I believe easiest to do this
>> would be to overload the PageType value so that we could essentially
>> have two values for "Buddy" pages. We would have our standard "Buddy"
>> pages, and "Buddy" pages that also have the "Offline" value set in the
>> PageType field. Tracking the Online vs Offline pages this way would
>> actually allow us to do this with almost no overhead as the mapcount
>> value is already being reset to clear the "Buddy" flag so adding a
>> "Offline" flag to this clearing should come at no additional cost.
> 
> It bothers me a bit that this doesn't scale to multiple hint types
> if we ever need them. Would it be better to have two
> free lists: hinted and non-hinted one?

That would imply having to change all places trying to allocate memory
to have a look at both lists? I think that could be factored out. I
assume keeping track of the amount of pages would feel more naturally
than having split counters for the existing lists.

I think I'd actually prefer something like that, avoiding mixing page
types and working with different types per list.

-- 

Thanks,

David / dhildenb