linux-kernel - Re: [RFC PATCH 3/3] mm/map_contig: Add mmap(MAP

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Message-ID: <alpine.DEB.2.20.1710161058470.12436@nuc-kabylake>
Date:   Mon, 16 Oct 2017 11:00:19 -0500 (CDT)
From:   Christopher Lameter <cl@...ux.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Guy Shattah <sguy@...lanox.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Michal Nazarewicz <mina86@...a86.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Laura Abbott <labbott@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [RFC PATCH 3/3] mm/map_contig: Add mmap(MAP_CONTIG) support

On Mon, 16 Oct 2017, Michal Hocko wrote:

> But putting that aside. Pinning a lot of memory might cause many
> performance issues and misbehavior. There are still kernel users
> who need high order memory to work properly. On top of that you are
> basically allowing an untrusted user to deplete higher order pages very
> easily unless there is a clever way to enforce per user limit on this.

We already have that issue and have ways to control that by tracking
pinned and mlocked pages as well as limits on their allocations.

> That being said, the list is far from being complete, I am pretty sure
> more would pop out if I thought more thoroughly. The bottom line is that
> while I see many problems to actually implement this feature and
> maintain it longterm I simply do not see a large benefit outside of a
> very specific HW.

There is not much new here in terms of problems. The hardware that
needs this seems to become more and more plentiful. That is why we need a
generic implementation.