linux-kernel - Re: Pinning ZONE

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Date:   Mon, 23 Nov 2020 12:54:16 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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>,
        Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: Pinning ZONE_MOVABLE pages

On Mon, Nov 23, 2020 at 12:15 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> 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.

You are right. No need for the madvise() flag at all. (The slight
difference of being able to mark memory pinned prior to touching is
really insignificant).

>
> 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.

That is right, as the first step we could just do fault and immediate
migration, which is silly, but still better than what we have now.

>
> Jason