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References: <20181012032431.32693-1-david@gibson.dropbear.id.au>
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From: Richard Henderson <richard.henderson@linaro.org>
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Date: Fri, 12 Oct 2018 10:41:33 -0700
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Subject: Re: [Qemu-devel] [RFC 1/5] virtio-balloon: Remove unnecessary
 MADV_WILLNEED on deflate
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To: David Gibson <david@gibson.dropbear.id.au>, dhildenb@redhat.com, imammedo@redhat.com, ehabkost@redhat.com
Cc: pbonzini@redhat.com, qemu-ppc@nongnu.org, qemu-devel@nongnu.org, mst@redhat.com

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?

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~