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

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Date:   Tue, 14 Feb 2017 09:28:14 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     mhocko@...e.com, 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, dave.hansen@...el.com, dan.j.williams@...el.com
Subject: Re: [PATCH V2 3/3] mm: Enable Buddy allocation isolation for CDM
 nodes

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@...ux.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?

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

>  		if (cpusets_enabled() &&
>  			(alloc_flags & ALLOC_CPUSET) &&
>  			!__cpuset_zone_allowed(zone, gfp_mask))
>