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

From: Balbir Singh
Date: Tue Jul 04 2017 - 23:18:25 EST

On Tue, Jul 4, 2017 at 10:51 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Mon 03-07-17 17:14:14, JÃrÃme 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.

Balbir Singh.