linux-kernel - Re: [RFC 3/8] mm: Isolate coherent device memory nodes from HugeTLB allocation paths

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Date:   Tue, 25 Oct 2016 18:17:26 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        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
Subject: Re: [RFC 3/8] mm: Isolate coherent device memory nodes from HugeTLB
 allocation paths



On 25/10/16 15:15, Aneesh Kumar K.V wrote:
> Dave Hansen <dave.hansen@...el.com> writes:
> 
>> On 10/23/2016 09:31 PM, Anshuman Khandual wrote:
>>> This change is part of the isolation requiring coherent device memory nodes
>>> implementation.
>>>
>>> Isolation seeking coherent device memory node requires allocation isolation
>>> from implicit memory allocations from user space. Towards that effect, the
>>> memory should not be used for generic HugeTLB page pool allocations. This
>>> modifies relevant functions to skip all coherent memory nodes present on
>>> the system during allocation, freeing and auditing for HugeTLB pages.
>>
>> This seems really fragile.  You had to hit, what, 18 call sites?  What
>> are the odds that this is going to stay working?
> 
> 
> I guess a better approach is to introduce new node_states entry such
> that we have one that excludes coherent device memory numa nodes. One
> possibility is to add N_SYSTEM_MEMORY and N_MEMORY.
> 
> Current N_MEMORY becomes N_SYSTEM_MEMORY and N_MEMORY includes
> system and device/any other memory which is coherent.
> 

I thought of this as well, but I would rather see N_COHERENT_MEMORY
as a flag. The idea being that some device memory is a part of
N_MEMORY, but N_COHERENT_MEMORY gives it additional attributes

> All the isolation can then be achieved based on the nodemask_t used for
> allocation. So for allocations we want to avoid from coherent device we
> use N_SYSTEM_MEMORY mask or a derivative of that and where we are ok to
> allocate from CDM with fallbacks we use N_MEMORY.
> 

I suspect its going to be easier to exclude N_COHERENT_MEMORY.

> All nodes zonelist will have zones from the coherent device nodes but we
> will not end up allocating from coherent device node zone due to the
> node mask used.
> 
> 
> This will also make sure we end up allocating from the correct coherent
> device numa node in the presence of multiple of them based on the
> distance of the coherent device node from the current executing numa
> node.
> 

The idea is good overall, but I think its going to be good to document
the exclusions with the flags

Balbir Singh.