Re: [PATCH 4/5] mm/memcontrol: allow to uncharge page without using page->lru field

From: Michal Hocko
Date: Wed Jul 05 2017 - 02:38:44 EST

On Wed 05-07-17 13:18:18, Balbir Singh wrote:
> On Tue, Jul 4, 2017 at 10:51 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > On Mon 03-07-17 17:14:14, Jérôme Glisse wrote:
> >> HMM pages (private or public device pages) are ZONE_DEVICE page and
> >> thus you can not use page->lru fields of those pages. This patch
> >> re-arrange the uncharge to allow single page to be uncharge without
> >> modifying the lru field of the struct page.
> >>
> >> There is no change to memcontrol logic, it is the same as it was
> >> before this patch.
> >
> > What is the memcg semantic of the memory? Why is it even charged? AFAIR
> > this is not a reclaimable memory. If yes how are we going to deal with
> > memory limits? What should happen if go OOM? Does killing an process
> > actually help to release that memory? Isn't it pinned by a device?
> >
> > For the patch itself. It is quite ugly but I haven't spotted anything
> > obviously wrong with it. It is the memcg semantic with this class of
> > memory which makes me worried.
> >
> This is the HMM CDM case. Memory is normally malloc'd and then
> migrated to ZONE_DEVICE or vice-versa. One of the things we did
> discuss was seeing ZONE_DEVICE memory in user page tables.

This doesn't answer any of the above questions though.
Michal Hocko