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

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Date:   Mon, 24 Oct 2016 11:00:32 -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 10/24/2016 10:38 AM, Dave Hansen wrote:
> 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?

Urg, this is even worse than I suspected.

Does this handle shared pages (like the page cache mode you call out as
a requirement) where the "cdm" page is faulted into one process VMA, but
it was allocated against another?

Can't that give you a "cdm" page mapped into a non-VM_CDM VMA?  Or, a
VM_CDM VMA with no "cdm" pages in it?