Re: [Qemu-devel] [RFC 1/5] virtio-balloon: Remove unnecessary MADV_WILLNEED on deflate

[Eric Blake <eblake@redhat.com>]

On 10/12/18 12:41 PM, 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?

If I remember correctly, deflate says to make the balloon smaller (that 
is, the balloon gets out of the way so that the guest can use more 
memory); inflate says to make the balloon bigger (that is, the guest has 
less memory available to use because the inflated balloon is occupying 
that memory instead - where memory occupied by the balloon is really 
memory handed back to the host).

> 
> 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.

Or rather, it sounds like your definition of deflate is opposite the one 
I recall (you are trying to argue that deflate means the guest uses less 
memory and is therefore deflating in size; while I thought deflate meant 
that the balloon shrinks in size making more memory available to the guest).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org