linux-kernel - Re: [RFC 5/8] mm: Add new flag VM

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Date:   Mon, 24 Oct 2016 10:38:12 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     mhocko@...e.com, js1304@...il.com, vbabka@...e.cz, mgorman@...e.de,
        minchan@...nel.org, akpm@...ux-foundation.org,
        aneesh.kumar@...ux.vnet.ibm.com, bsingharora@...il.com
Subject: Re: [RFC 5/8] mm: Add new flag VM_CDM for coherent device memory

On 10/23/2016 09:31 PM, Anshuman Khandual wrote:
> VMAs containing coherent device memory should be marked with VM_CDM. These
> VMAs need to be identified in various core kernel paths and this new flag
> will help in this regard.

... and it's sticky?  So if a VMA *ever* has one of these funky pages in
it, it's stuck being VM_CDM forever?  Never to be merged with other
VMAs?  Never to see the light of autonuma ever again?

What if a 100TB VMA has one page of fancy pants device memory, and the
rest normal vanilla memory?  Do we really want to consider the whole
thing fancy?

This whole patch set is looking really hackish.  If you want things to
be isolated from the VM, them it should probably *actually* be isolated
from the VM.  As Jerome mentioned, ZONE_DEVICE is probably a better
thing to use here than to try what you're attempting.