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Re: [Qemu-ppc] [Qemu-devel] [RFC 1/5] virtio-balloon: Remove unnecessary

From: David Gibson
Subject: Re: [Qemu-ppc] [Qemu-devel] [RFC 1/5] virtio-balloon: Remove unnecessary MADV_WILLNEED on deflate
Date: Sat, 13 Oct 2018 17:23:51 +1100
User-agent: Mutt/1.10.1 (2018-07-13)

On Fri, Oct 12, 2018 at 10:41:33AM -0700, Richard Henderson wrote:
> On 10/11/18 8:24 PM, David Gibson wrote:
> > When the balloon is inflated, we discard memory place in it using madvise()
> > with MADV_DONTNEED.  And when we deflate it we use MADV_WILLNEED, which
> > sounds like it makes sense but is actually unnecessary.
> > 
> > The misleadingly named MADV_DONTNEED just discards the memory in question,
> > it doesn't set any persistent state on it in-kernel; all that's necessary
> > to bring the memory back is to touch it.
> Isn't the point of deflate to free up host memory, for use by other guests or
> whatever?

As Eric notes, I think you have inflate and deflate the wrong way around.

> If you do nothing upon deflate, then that doesn't actually happen.  It seems 
> to
> me that this is backward and you should use DONTNEED on deflate and then
> inflate should do nothing.
> r~

David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!

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