Linux Virtualization - Jul 2018 - [PATCH v35 1/5] mm: support to get hints of free page blocks

On Wed, Jul 11, 2018 at 7:17 PM Wei Wang <wei.w.wang at intel.com>
wrote:
>
> Would it be better to remove __GFP_THISNODE? We actually want to get all
> the guest free pages (from all the nodes).
Maybe. Or maybe it would be better to have the memory balloon logic be
per-node? Maybe you don't want to remove too much memory from one
node? I think it's one of those "play with it" things.

I don't think that's the big issue, actually. I think the real issue
is how to react quickly and gracefully to "oops, I'm trying to give
memory away, but now the guest wants it back" while you're in the
middle of trying to create that 2TB list of pages.

IOW, I think the real work is in whatever tuning for the righ
tbehavior. But I'm just guessing.

             Linus

On 07/12/2018 10:30 AM, Linus Torvalds wrote:
> On Wed, Jul 11, 2018 at 7:17 PM Wei Wang <wei.w.wang at intel.com>
wrote:
>> Would it be better to remove __GFP_THISNODE? We actually want to get
all
>> the guest free pages (from all the nodes).
> Maybe. Or maybe it would be better to have the memory balloon logic be
> per-node? Maybe you don't want to remove too much memory from one
> node? I think it's one of those "play with it" things.
>
> I don't think that's the big issue, actually. I think the real
issue
> is how to react quickly and gracefully to "oops, I'm trying to
give
> memory away, but now the guest wants it back" while you're in the
> middle of trying to create that 2TB list of pages.
OK. virtio-balloon has already registered an oom notifier 
(virtballoon_oom_notify). I plan to add some control there. If oom happens,
- stop the page allocation;
- immediately give back the allocated pages to mm.

Best,
Wei

On Thu 12-07-18 10:52:08, Wei Wang wrote:
> On 07/12/2018 10:30 AM, Linus Torvalds wrote:
> > On Wed, Jul 11, 2018 at 7:17 PM Wei Wang <wei.w.wang at
intel.com> wrote:
> > > Would it be better to remove __GFP_THISNODE? We actually want to
get all
> > > the guest free pages (from all the nodes).
> > Maybe. Or maybe it would be better to have the memory balloon logic be
> > per-node? Maybe you don't want to remove too much memory from one
> > node? I think it's one of those "play with it" things.
> > 
> > I don't think that's the big issue, actually. I think the real
issue
> > is how to react quickly and gracefully to "oops, I'm trying
to give
> > memory away, but now the guest wants it back" while you're in
the
> > middle of trying to create that 2TB list of pages.
> 
> OK. virtio-balloon has already registered an oom notifier
> (virtballoon_oom_notify). I plan to add some control there. If oom happens,
> - stop the page allocation;
> - immediately give back the allocated pages to mm.
Please don't. Oom notifier is an absolutely hideous interface which
should go away sooner or later (I would much rather like the former) so
do not build a new logic on top of it. I would appreciate if you
actually remove the notifier much more.

You can give memory back from the standard shrinker interface. If we are
reaching low reclaim priorities then we are struggling to reclaim memory
and then you can start returning pages back.
-- 
Michal Hocko
SUSE Labs