[PATCH] mm: don't zero ballooned pages

Wei Wang
Mon, 31 Jul 2017 07:41:49 -0000

On 07/31/2017 02:55 PM, Michal Hocko wrote:
> On Mon 31-07-17 12:13:33, Wei Wang wrote:
>> Ballooned pages will be marked as MADV_DONTNEED by the hypervisor and
>> shouldn't be given to the host ksmd to scan.
> Could you point me where this MADV_DONTNEED is done, please?

Sure. It's done in the hypervisor when the balloon pages are received.

Please see line 40 at
https://github.com/qemu/qemu/blob/master/hw/virtio/virtio-balloon.c


>
>> Therefore, it is not
>> necessary to zero ballooned pages, which is very time consuming when
>> the page amount is large. The ongoing fast balloon tests show that the
>> time to balloon 7G pages is increased from ~491ms to 2.8 seconds with
>> __GFP_ZERO added. So, this patch removes the flag.
> Please make it obvious that this is a revert of bb01b64cfab7
> ("mm/balloon_compaction.c: enqueue zero page to balloon device").
>
>

Ok, will do.

Best,
Wei