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

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Date:   Tue, 25 Oct 2016 23:36:34 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        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
Subject: Re: [RFC 5/8] mm: Add new flag VM_CDM for coherent device memory



On 25/10/16 04:38, 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?
> 
> 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?
> 

Those are good review comments to improve the patchset.

> 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.
> 

The RFC explains the motivation, this is not fancy pants, it is regular
memory from the systems perspective, with some changes as described

Thanks for the review!
Balbir Singh