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

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Message-ID: <CAKTCnz=zTjYeqeTYZbnOMsT1Ccus4yW=jAws_OgXp3q4xmuSPA@mail.gmail.com>
Date:   Wed, 5 Jul 2017 13:18:18 +1000
From:   Balbir Singh <bsingharora@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Jérôme Glisse <jglisse@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>
Subject: Re: [PATCH 4/5] mm/memcontrol: allow to uncharge page without using
 page->lru field

On Tue, Jul 4, 2017 at 10:51 PM, Michal Hocko <mhocko@...nel.org> 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.

Balbir Singh.