linux-kernel - Re: [RFC 4/4] mm: Ignore cpuset enforcement when allocation flag has __GFP

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Date:   Tue, 29 Nov 2016 08:52:08 -0800
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, vbabka@...e.cz, mgorman@...e.de,
        minchan@...nel.org, aneesh.kumar@...ux.vnet.ibm.com,
        bsingharora@...il.com, srikar@...ux.vnet.ibm.com,
        haren@...ux.vnet.ibm.com, jglisse@...hat.com
Subject: Re: [RFC 4/4] mm: Ignore cpuset enforcement when allocation flag has
 __GFP_THISNODE

On 11/28/2016 10:51 PM, Anshuman Khandual wrote:
> On 11/29/2016 02:42 AM, Dave Hansen wrote:
>> > On 11/22/2016 06:19 AM, Anshuman Khandual wrote:
>>> >> --- a/mm/page_alloc.c
>>> >> +++ b/mm/page_alloc.c
>>> >> @@ -3715,7 +3715,7 @@ struct page *
>>> >>  		.migratetype = gfpflags_to_migratetype(gfp_mask),
>>> >>  	};
>>> >>  
>>> >> -	if (cpusets_enabled()) {
>>> >> +	if (cpusets_enabled() && !(alloc_mask & __GFP_THISNODE)) {
>>> >>  		alloc_mask |= __GFP_HARDWALL;
>>> >>  		alloc_flags |= ALLOC_CPUSET;
>>> >>  		if (!ac.nodemask)
>> > 
>> > This means now that any __GFP_THISNODE allocation can "escape" the
>> > cpuset.  That seems like a pretty major change to how cpusets works.  Do
>> > we know that *ALL* __GFP_THISNODE allocations are truly lacking in a
>> > cpuset context that can be enforced?
> Right, I know its a very blunt change. With the cpuset based isolation
> of coherent device node for the user space tasks leads to a side effect
> that a driver or even kernel cannot allocate memory from the coherent
...

Well, we have __GFP_HARDWALL:

	 * __GFP_HARDWALL enforces the cpuset memory allocation policy.

which you can clear in the places where you want to do an allocation but
want to ignore cpusets.  But, __cpuset_node_allowed() looks like it gets
a little funky if you do that since it would probably be falling back to
the root cpuset that also would not have the new node in mems_allowed.

What exactly are the kernel-internal places that need to allocate from
the coherent device node?  When would this be done out of the context of
an application *asking* for memory in the new node?