linux-kernel - Re: Pinning ZONE

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Message-ID: <20201123171502.GX244516@ziepe.ca>
Date:   Mon, 23 Nov 2020 13:15:02 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     Michal Hocko <mhocko@...e.com>, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>,
        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>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, sthemmin@...rosoft.com
Subject: Re: Pinning ZONE_MOVABLE pages

On Mon, Nov 23, 2020 at 11:06:21AM -0500, Pavel Tatashin wrote:

> What I mean here is allowing users to guarantee that the page's PA is
> going to stay the same. Sort of a stronger mlock. Mlock only
> guarantees that the page is not swapped, but something like

You've just described get/pin_user_pages(), that is exactly what it is
for.

I agree with the other emails, ZONE_MOVABLE needs to be reconciled
with FOLL_LONGTERM - most likely by preventing ZONE_MOVABLE pages from
being returned. This will need migration like CMA does and the point
about faulting is only an optimization to prevent fault then immediate
migration.

Jason