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

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Message-ID: <2447359e-6d22-4fdc-c48a-3912bfbb69b7@nvidia.com>
Date:   Mon, 23 Oct 2017 10:25:07 -0500
From:   David Nellans <dnellans@...dia.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>
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 10/16/2017 12:42 PM, Michal Hocko wrote:
> On Mon 16-10-17 11:00:19, Cristopher Lameter wrote:
>> On Mon, 16 Oct 2017, Michal Hocko wrote:
>>> 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.
> It would really help to name that HW and other potential usecases
> independent on the HW because I am rather skeptical about the
> _plentiful_ part. And so I really do not see any foundation to claim
> the generic part. Because, fundamentally, it is the HW which requires
> the specific memory placement/physically contiguous range etc. So the
> generic implementation doesn't really make sense in such a context.
>

There are TLB's in AMD Xen that can take advantage of contig memory to
improve TLB coverage.  AFAIK contig is not functionally required, its
purely a performance optimization.  Current Xen TLB implementation
doesn't support arbitrary contig lengths, page sizes, etc, but its a
start.  This
type of TLB optimization can be handled on the back end by de-fragging
phys mem (when possible) now that both base and THPs can be easily
migrated; no need for up-front contig, but defrag isn't free either.