linux-kernel - Re: [PATCH 0/6] prohibit pinning pages in ZONE

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Date:   Mon, 7 Dec 2020 16:12:50 +0900
From:   Joonsoo Kim <js1304@...il.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        mike.kravetz@...cle.com, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH 0/6] prohibit pinning pages in ZONE_MOVABLE

On Fri, Dec 04, 2020 at 12:50:56PM -0500, Pavel Tatashin wrote:
> > > Yes, this indeed could be a problem for some configurations. I will
> > > add your comment to the commit log of one of the patches.
> >
> > It sounds like there is some inherent tension here, breaking THP's
> > when doing pin_user_pages() is a really nasty thing to do. DMA
> > benefits greatly from THP.
> >
> > I know nothing about ZONE_MOVABLE, is this auto-setup or an admin
> > option? If the result of this patch is standard systems can no longer
> > pin > 80% of their memory I have some regression concerns..
> 
> ZONE_MOVABLE can be configured via kernel parameter, or when memory
> nodes are onlined after hot-add; so this is something that admins
> configure. ZONE_MOVABLE is designed to gurantee memory hot-plug

Just note, the origin of ZONE_MOVABLE is to provide availability of
huge page, especially, hugetlb page. AFAIK, not guarantee memory
hot-plug. See following commit that introduces the ZONE_MOVABLE.

2a1e274 Create the ZONE_MOVABLE zone

> functionality, and not availability of THP, however, I did not know
> about the use case where some admins might configure ZONE_MOVABLE to

The usecase is lightly mentioned in previous discussion.

http://lkml.kernel.org/r/alpine.DEB.2.23.453.2011221300100.2830030@chino.kir.corp.google.com

Anyway, I agree with your other arguments and this patchset.

Thanks.