Re: [PATCH V2 3/3] mm: Enable Buddy allocation isolation for CDM nodes

[Anshuman Khandual <khandual@linux.vnet.ibm.com>]

On 02/14/2017 01:58 PM, Vlastimil Babka wrote:
> On 02/10/2017 11:06 AM, Anshuman Khandual wrote:
>> This implements allocation isolation for CDM nodes in buddy allocator by
>> discarding CDM memory zones all the time except in the cases where the gfp
>> flag has got __GFP_THISNODE or the nodemask contains CDM nodes in cases
>> where it is non NULL (explicit allocation request in the kernel or user
>> process MPOL_BIND policy based requests).
>>
>> Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
>> ---
>>  mm/page_alloc.c | 16 ++++++++++++++++
>>  1 file changed, 16 insertions(+)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 84d61bb..392c24a 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -64,6 +64,7 @@
>>  #include <linux/page_owner.h>
>>  #include <linux/kthread.h>
>>  #include <linux/memcontrol.h>
>> +#include <linux/node.h>
>>  
>>  #include <asm/sections.h>
>>  #include <asm/tlbflush.h>
>> @@ -2908,6 +2909,21 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>>  		struct page *page;
>>  		unsigned long mark;
>>  
>> +		/*
>> +		 * CDM nodes get skipped if the requested gfp flag
>> +		 * does not have __GFP_THISNODE set or the nodemask
>> +		 * does not have any CDM nodes in case the nodemask
>> +		 * is non NULL (explicit allocation requests from
>> +		 * kernel or user process MPOL_BIND policy which has
>> +		 * CDM nodes).
>> +		 */
>> +		if (is_cdm_node(zone->zone_pgdat->node_id)) {
>> +			if (!(gfp_mask & __GFP_THISNODE)) {
>> +				if (!ac->nodemask)
>> +					continue;
>> +			}
>> +		}
> 
> With the current cpuset implementation, this will have a subtle corner
> case when allocating from a cpuset that allows the cdm node, and there
> is no (task or vma) mempolicy applied for the allocation. In the fast
> path (__alloc_pages_nodemask()) we'll set ac->nodemask to
> current->mems_allowed, so your code will wrongly assume that this
> ac->nodemask is a policy that allows the CDM node. Probably not what you
> want?

You are right, its a problem and not what we want. We can make the
function get_page_from_freelist() take another parameter "orig_nodemask"
which gets passed into __alloc_pages_nodemask() in the first place. So
inside zonelist iterator we can compare orig_nodemask with current
ac.nodemask to figure out if cpuset swapping of nodemask happened and
skip CDM node if necessary. Thats a viable solution IMHO.

> 
> This might change if we decide to fix the cpuset vs mempolicy issues [1]
> so your input on that topic with your recent experience with all the
> alternative CDM isolation implementations would be useful. Thanks.
> 
> [1] http://www.spinics.net/lists/linux-mm/msg121760.html

Sure, will look into the details.

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