Re: [PATCH] mm: don't zero ballooned pages

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 31, 2017 at 10:37:24AM +0200, Michal Hocko wrote:
> On Mon 31-07-17 16:23:26, ZhenweiPi wrote:
> > On 07/31/2017 03:51 PM, Michal Hocko wrote:
> > 
> > >On Mon 31-07-17 15:41:49, Wei Wang wrote:
> > >>>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
> > >And one more thing. I am not familiar with ksm much. But how is
> > >MADV_DONTNEED even helping? This madvise is not sticky - aka it will
> > >unmap the range without leaving any note behind. AFAICS the only way
> > >to have vma scanned is to have VM_MERGEABLE and that is an opt in:
> > >See Documentation/vm/ksm.txt
> > >"
> > >KSM only operates on those areas of address space which an application
> > >has advised to be likely candidates for merging, by using the madvise(2)
> > >system call: int madvise(addr, length, MADV_MERGEABLE).
> > >"
> > >
> > >So what exactly is going on here? The original patch looks highly
> > >suspicious as well. If somebody wants to make that memory mergable then
> > >the user of that memory should zero them out.
> > 
> > Kernel starts a kthread named "ksmd". ksmd scans the VM_MERGEABLE
> > memory, and merge the same pages.(same page means memcmp(page1,
> > page2, PAGESIZE) == 0).
> > 
> > Guest can not use ballooned pages, and these pages will not be accessed
> > in a long time. Kswapd on host will swap these pages out and get more
> > free memory.
> > 
> > Rather than swapping, KSM has better performence.  Presently pages in
> > the balloon device have random value,  they usually cannot be merged.
> > So enqueue zero pages will resolve this problem.
> > 
> > Because MADV_DONTNEED depends on host os capability and hypervisor capability,
> > I prefer to enqueue zero pages to balloon device and made this patch.

I think you should have hypervisor zero them out if it wants to then. Seems cleaner.

> 
> So why exactly are we zeroying pages (and pay some cost for that) in
> guest when we do not know what host actually does with them?

I suspect this is some special hypervisor that somehow benefits from
this patch. It should just use a feature bit for its special needs
I think.

Michal is also exactly right that patches like this should come
with some performance numbers.
I'll post a patch adding virtio lists for mm/balloon_compaction.c
so that we notice when people tweak it like that.

> -- 
> Michal Hocko
> SUSE Labs