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

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Date:   Fri, 4 Dec 2020 10:55:30 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     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>, Jason Gunthorpe <jgg@...pe.ca>,
        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 Thu, Dec 3, 2020 at 11:03 PM Joonsoo Kim <js1304@...il.com> wrote:
>
> Hello,
>
> On Wed, Dec 02, 2020 at 12:23:24AM -0500, Pavel Tatashin wrote:
> > When page is pinned it cannot be moved and its physical address stays
> > the same until pages is unpinned.
> >
> > This is useful functionality to allows userland to implementation DMA
> > access. For example, it is used by vfio in vfio_pin_pages().
> >
> > However, this functionality breaks memory hotplug/hotremove assumptions
> > that pages in ZONE_MOVABLE can always be migrated.
> >
> > This patch series fixes this issue by forcing new allocations during
> > page pinning to omit ZONE_MOVABLE, and also to migrate any existing
> > pages from ZONE_MOVABLE during pinning.
>
> I love what this patchset does, but, at least, it's better to consider
> the side-effect of this patchset and inform it in somewhere. IIUC,
> ZONE_MOVABLE exists for two purposes.
>
> 1) increasing availability of THP
> 2) memory hot-unplug
>
> Potential issue would come from the case 1). They uses ZONE_MOVABLE
> for THP availability and hard guarantee for migration isn't required
> until now. So, there would be a system with following congifuration.
>
> - memory layout: ZONE_NORMAL-512MB, ZONE_MOVABLE-512MB
> - memory usage: unmovable-256MB, movable pinned-256MB, movable
>   unpinned-512MB
>
> With this patchset, movable pinned should be placed in ZONE_NORMAL so
> 512MB is required for ZONE_NORMAL. ZONE_NORMAL would be exhausted and
> system performance would be highly afftect according to memory usage
> pattern.
>
> I'm not sure whether such configuration exists or not, but, at least,
> it's better to write down this risk on commit message or something
> else.

Yes, this indeed could be a problem for some configurations. I will
add your comment to the commit log of one of the patches.

Thank you,
Pasha