Subject: Re: [RFC 5/8] mm: Add new flag VM_CDM for coherent device memory
To: Anshuman Khandual <khandual@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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 <580E4704.1040104@intel.com>
Cc: mhocko@suse.com, js1304@gmail.com, vbabka@suse.cz, mgorman@suse.de,
        minchan@kernel.org, akpm@linux-foundation.org,
        aneesh.kumar@linux.vnet.ibm.com, bsingharora@gmail.com
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <580E4C40.50107@intel.com>
Date: Mon, 24 Oct 2016 11:00:32 -0700
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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?